Category Archives: Aggression

Chimpanzee Violence and the Post-War Dream

Below is the text (more or less) from the TEDxUMN talk that I gave on 03 May 2015. A video of the talk is here.

I grew up in the Cold War.

The books, movies, and music were filled with fears of the end of the world.

One of my favorite albums was Pink Floyd’s The Final Cut: A Requiem for the Post-War Dream.

The album ended with the song: Two Suns in the Sunset.

The second sun being, of course, a thermonuclear explosion.

Looks like the human race is run.

War seemed inevitable. There was no way out.

But despite all the doom and gloom, it turned out these were actually the final years of the Cold War.

In 1989, the Berlin wall fell, and in 1991 the Soviet Union collapsed, not with a bang, but with a whimper.

St. Basil's Cathedral, Moscow. (August 1991)
St. Basil’s Cathedral, Moscow. (August 1991)

 

 

 

 

 

 

 

 

I was lucky enough to get a glimpse behind the Iron Curtain just as it was coming down.

I had a fellowship to study in England that came with money for summer travel in Europe.

I used that money to travel to Russia, then all the way to China on the Tran-Siberian Railroad. It turns out that this was the last summer that the Soviet Union existed.

And on the other side of Iron Curtain I found that people were pretty much just like us.

Soviet soldiers near Red Square, Moscow. (August 1991)
Soviet soldiers near Red Square, Moscow. (August 1991)

 

 

 

 

 

 

Walking around Moscow near Red Square, I saw Soviet soldiers – boys, really, about my age – playing around in a children’s park, taking pictures of each other.

This was an exciting time, with the Soviet Bloc and China opening up to the outside world.

It gave me hope that maybe war was something we can overcome.

I grew up worried about war but fascinated by apes.

http://eden.uktv.co.uk/gorilla-revisted-david-attenborough/gallery/photos-gorilla-revisted-david-attenborough/#4
http://eden.uktv.co.uk/gorilla-revisted-david-attenborough/gallery/photos-gorilla-revisted-david-attenborough/#4

 

 

 

 

 

 

As a kid, I saw Dian Fossey on TV with mountain gorillas and thought to myself: that’s what I want to be when I grow up.

It’s because of war, though, that I don’t study gorillas.

The mountain gorilla study is located in Rwanda, a tiny country in the heart of Africa. After college, when I wrote to the directors of the gorilla study asking if they needed research assistants, they wrote back saying they were closing camp down because Rwanda was descending into war.

The war continued for years, killing hundreds of thousands of people.

So instead of growing up to study gorillas, I study chimpanzees.

 You can’t always get what you want.

But if you try sometimes,

You might find,

You get what you need.

And what I needed was chimpanzees.

I needed chimpanzees because they are our closest living cousins.

Human family tree

 

 

 

 

 

 

Chimps and gorillas look a lot alike: hairy, knuckle-walking apes. The the two kinds of chimpanzees — “common chimpanzees” and bonobos — are more closely related to us than they are to gorillas.

And chimpanzees share a number of unusual traits in common with people.

They make and use tools – like this one here, who is using a stick to fish termites from their nest so she can eat them:

Golden fishing for termites.
Golden fishing for termites. (Photo by Michael Wilson)

 

 

 

 

 

 

Like humans, chimpanzees work in groups to hunter other animals.

And like humans, chimpanzees defend group territories, and sometimes gang up on members of other groups, attacking and killing their enemies.

It is this warlike behavior that I set out to study, looking for clues about how the origins and evolution of warfare in our own species.

So I went to study chimpanzees in Kibale Forest in western Uganda – much safer, I figured, than studying gorillas in war-torn Rwanda.

As it happened, though, the war in Rwanda spilled over into Congo, resulting in a huge war that eventually involved nine African nations and killed millions of people.

The first Congo war. http://en.wikipedia.org/wiki/First_Congo_War#/media/File:First_Congo_War_map_en.png
The first Congo war. http://en.wikipedia.org/wiki/First_Congo_War#/media/File:First_Congo_War_map_en.png

 

 

 

 

 

 

 

 

Kibale is just 30 miles from the border with Congo. On a clear day in Kibale you can see the snowcapped Ruwenzori Mountains that form that border.

And while the war raged in Congo, a Ugandan rebel group, the ADF, set up in those very mountains. Some of them were even rumored to be hiding in Kibale. The ADF bombed cafes and buses around Uganda, and attacked a school just 20 miles from us, burning 80 students alive.

At one point bandits attacked our village. They robbed and beat chimpanzee project field assistants, and shot and killed the brother of one of our employees.

And right around this time, one of the same field assistants who had been robbed found our chimpanzees beating on the freshly killed body of a male chimpanzee from another community.

The stranger’s body was covered in bites and other wounds, and his throat was torn out.

Why do chimpanzees do this?

Lots of other animals defend group territories, but in most species, they just chase their enemies away, rather than hunt them down and kill them. What’s going on?

Why do we kill?

One explanation for why we kill is the imbalance of power hypothesis, developed by Richard Wrangham and colleagues.

Behavioral ecologists think of aggression as the result of a cost-benefit calculation: animals use aggression as a strategy to get some benefit, when it looks like the benefit will be greater than the cost.

The benefits of aggression in chimpanzees are similar to those in other group territorial species: territory, food and mates.

But the costs of killing are low because of an unusual social structure that chimpanzees share with humans: fission fusion societies.

If we were gorillas, we would travel in a stable group all the time: a single male with his harem of females and kids.

But instead we live like chimpanzees: sometimes gathering in big groups, like we are now, and sometimes separating off into smaller groups.

For example, here are two neighboring territories: Blue and Red.

Slide11

 

 

 

 

 

Each territory has 10 males.

There’s not much food at the moment in the Blue territory, so the Blue males are traveling in small parties, mainly ones and twos.

But there’s more food in the Red territory. They can travel in bigger parties, including this one with six males.

 These six males have safety in numbers, so they go on a border patrol.

 Slide12

 

 

 

 

 There they find a single blue male off by himself.

 Slide13

 

 

 

 

Bad luck for the blue male! The red males surround him, gang up on him, and kill him.

Slide14

 

 

 

 

 

Now Blue only has 9 males.

Slide15 

 

 

 

 

With fewer males, Blue loses territory to Red. The Red males get the benefit of a larger territory with more food for themselves, their mates and their offspring.

The imbalance of power hypothesis makes some clear predictions.

For example:

Males should visit borders only when in larger groups.

Parties with more males should be more likely to approach strangers, to win fights, and to kill their enemies.

And winners should gain more territory.

I’ve spent much of the past 20 years testing these predictions.

I used playback experiments to test how males would respond to a stranger.

Slide17
Donor and John setting up the playback equipment. (Photo by Becky Sun)

 

 

 

 

 

 

 

In each experiment, I played back a single pant-hoot call from a foreign male.

A pant-hoot sounds like this:

 

(And this is what a professor imitating a pant-hooting chimpanzee looks like:)

Pant-hoot demonstration.
Pant-hoot demonstration.

 

 

 

Hearing a single stranger calling in the distance had a big effect on the chimpanzees.

Slide18
Rosa, Lope, Ipassa and Makoku looking towards an unexpected sound. (Photo by Becky Sun)

 

 

 

 

 

 

 

In parties with just one or two males, they looked towards the speaker, which was hidden some 300 m away. Sometimes they just stayed still, looking, but in about half the cases they slowly, cautiously walked towards the speaker.

In parties with three or more males, the response was totally different. They gave a loud “wraa!” response right after hearing the call, dropped down from their trees, and rapidly walked single file towards the speaker.

After each playback, we quickly packed up the speaker and carried it away, while one of us stayed at the speaker location to see what happened. Often that person was me.

I remember sitting there quietly in the undergrowth when suddenly I heard footsteps. I saw a line of males walking single file, their hair out, looking for someone to kill.

They glanced my way, but they weren’t interested in me. They knew who I was. They were looking for a stranger.

More recently, I’ve analyzed data from all the long-term study sites for chimpanzees.

Slide19

 

 

 

 

 

 

What I’ve found is that killing is widespread, and occurs at most study sites.

In cases of intergroup killing, the attackers had an average 8:1 advantage over the defenders.

Analysis of long-term data has found that groups with more males expand their territory and obtain more food for self, mates, and offspring.

Just like chimpanzees, people are sensitive to the costs and benefits of aggression, and they prefer low cost fights: unfair fights that they are likely to win.

In human warfare, numbers matter, but even more important is weapons. Whenever people have developed a military advantage they have used it to conquer.

The Mongols, for example, swept across Eurasia with their fast horses and mounted archers.

Slide20
http://www.manchuarchery.org/photographs-mongolian-archers

 

 

 

 

 

 

 

But conquest, in humans and chimpanzees, is a zero sum game. Any benefit for my group is a loss for yours.

This is a risky game to play. About 12% of chimpanzees die from violence – that’s about out of eight.

Slide21

 

 

 

 

 

 

The tables in this room seat about 8 people – so if were living in such a world, on average one person at each table would die from violence.

In human groups that live much like we did for most of our evolutionary history – as hunter gatherers and small scale tribal societies – the rate of death from violence is also about 12%.

For both chimpanzees and people, playing this zero sum game of group territorial behavior means a high risk of death by violence.

But unlike chimpanzees, people have found some ways out of the zero-sum trap and have learned to play positive-sum games.

This slide shows the risk of dying in battle from war in the 20th Century, for people worldwide:

Slide22

 

 

 

 

 

The two big spikes are the First and Second World Wars.

What’s really striking about this graph is that there haven’t been any more really big spikes since 1945.

Nuclear weapons have raised cost of war so much that there have been no great-power wars in 70 years

People have – so far—avoided the horrible costs of direct nuclear exchange.

People are also sensitive to the benefits of peace, and these have increased over time, as the world has gotten more interconnected through trade.

The Trans-Siberian Railway is one of many links in this international trade. It connects Russia with China, and now with Europe as well.

Slide23

 

 

 

 

 

I think back to my journey across Siberia. If I were a young male chimpanzee venturing so far from home, I would have been killed by the first group of foreign males I met.

But traveling deep into what had been enemy territory, I was never threatened. People can benefit from a stranger, if only by having someone to talk with, and share their vodka.

So what can chimpanzees tell us about war?

Slide24

 

 

 

 

 

 

War is natural, but it is not inevitable.

People, like chimpanzees, are sensitive to both costs and benefits.

We can reduce war by increasing its costs, and by increasing the benefits of peace.

And what gives me hope is that the people I’ve met traveling around the world – they don’t want mutually assured destruction. They want the simple things listed in the Pink Floyd song about in the Post-war Dream:

A place to stay.

Enough to eat.

Somewhere old heroes shuffle safely down the street.

You can relax on both sides of the tracks

And maniacs don’t blow holes in bandsmen by remote control.

And everyone has recourse to the law.

And no one kills the children anymore.

 

Evolution and Warfare Workshop at NIMBioS

I’m very excited to announce that NIMBIoS, the National Institute for Mathematical and Biological Synthesis, will be hosting a workshop, “Evolutionary approaches to the understanding of decentralized warfare,” September 16-18, 2015, at the University of Tennessee, Knoxville.

I am organizing this workshop along with Luke Glowacki (Harvard University), Anna Simons (Naval Postgraduate School) and Sergey Gavrilets (University of Tennessee).

Objectives: Warfare is a widespread and arguably universal practice of human societies. While war’s origins continue to be debated, warfare has occurred in most historically documented societies, from hunter-gatherers to nation-states. Its prehistoric traces can be found in fortifications, weapons, mass graves, and skeletal trauma. Given that it is a major source of mortality, warfare has potentially acted as a powerful source of selection for both biological and cultural evolution. In a reprise of the past – and in contrast to force-on-force, hierarchically organized wars among great powers – conflict today takes a number of decentralized forms, from urban gang fights to livestock raids, and from clan warfare to leaderless resistance and terrorism. Suggestive parallels exist between current patterns of warfare and those that typified subsistence-level societies. Consequently, examining psychological mechanisms that evolved under ancestral warfare conditions may shed important light on what motivates individuals during modern decentralized wars and why people resort to violence in intergroup conflicts. Given the availability of data from contemporary societies and the great damage caused by violent conflict, advancing our understanding of such conflict is both tractable and important. In this workshop, we will begin working toward an integrated approach to the study of warfare under decentralized or only loosely controlled conditions, combining empirical data, evolutionary theory, and mathematical models. We envision developing working hypotheses to answer several key questions about between-group conflict in general, and the nature of ‘decentralized warfare’ in particular. These hypotheses will be informed by evolutionary theory using data from diverse conflict settings. Our hypotheses will be articulated in a framework amenable to formal modeling that will point the way toward a multi-level predictive understanding of warfare.

Descriptive flyer (PDF)

Application deadline: May 17, 2015. To apply CLICK HERE.
Participation in the workshop is by application only. Individuals with a strong interest in the topic are encouraged to apply, and successful applicants will be notified within two weeks of the application deadline. If needed, financial support for travel, meals, and lodging is available for workshop attendees.

 

Human Impacts

Our recent paper on lethal aggression (Wilson et al., 2014) has attracted lots of attention, most of it positive. Not surprisingly, though, some critical responses have also emerged. Copied below is our response to one of these critiques.

John Horgan at Scientific American devoted a blog post to critiquing the paper — though this critique is not really focused on the paper, but on whether the paper’s findings support what Horgan calls the “deep roots theory of war.” Horgan later published a critique by Brian Ferguson, the main advocate of the Human Impacts Hypothesis. In introducing this critique, Horgan offered to post responses from any of the original paper’s authors. We therefore have written a response, which is now posted here. In case it might be useful, I’ve copied it here as well:

Human impacts are neither necessary nor sufficient to explain chimpanzee violence (or bonobo non-violence)

Michael L. Wilson, Christopher Boesch, Takeshi Furuichi, Ian C. Gilby, Chie Hashimoto, Catherine Hobaiter, Gottfried Hohmann, Kathelijne Koops, Tetsuro Matsuzawa, John C. Mitani, David Morgan, Martin N. Muller, Roger Mundry, Anne E. Pusey, Julia Riedel, Crickette Sanz, Anne M. Schel, Michel Waller, David P. Watts, Frances White, Roman M. Wittig, and Richard W. Wrangham[1]

In response to our recent paper (Wilson et al., 2014), Brian Ferguson (2014) critiques the methods we used to test whether chimpanzee violence is the result of human impacts. As Joan Silk notes in her commentary on our paper, “These results should finally put an end to the idea that lethal aggression in chimpanzees is a non-adaptive by-product of anthropogenic influences — but they will probably not be enough to convince everyone” (Silk, 2014: 321).

We expect that for the majority of primatologists, and among the wider community of animal behavior researchers, the results of our study are neither surprising nor controversial. But for those hostile to the idea that human violence relates in any way to biology or adaptive behavior, the Human Impacts Hypothesis (HIH) offers an out. Violence among our ape cousins is, in this view, the result of human contact, not the result of evolution favoring aggression as a strategy. The argument closely parallels Ferguson’s earlier argument that violence in tribal societies is mainly the result of contact with outsiders, especially European imperialists (Ferguson, 1990).

In contrast to some critics, Ferguson recognizes that “there is no question that chimpanzees have the capability to make war and have done so on occasion” (Ferguson, 2011: 249). Ferguson’s critiques thus represent a departure from what we might call the “strong anti-adaptationism” of previous proponents of the HIH. For example, Power (1991) argues that chimpanzee violence is a non-adaptive response to frustration caused by restrictive feeding methods at the first long-term study sites of chimpanzees, Gombe and Mahale. Ferguson embraces Power’s hypothesis that feeding chimpanzees made them more violent, but in contrast to Power, he argues that chimpanzee violence is mainly the result of resource competition, which is exacerbated by human activities such as feeding and deforestation. This argument differs little from arguments that behavioral ecologists regularly make to explain chimpanzee violence. For example, intercommunity violence in chimpanzees is strongly associated with competition over food resources. At Kanyawara, intercommunity encounters occurred most frequently when key food species were abundant in areas bordering neighboring communities (Wilson et al., 2012). Lethal aggression is strongly associated with territorial expansion at Ngogo (Mitani et al., 2010) and Gombe (Goodall, 1986), and by expanding territory chimpanzees increase the amount of food available to themselves, their mates and offspring (Williams et al., 2004; Pusey et al., 2005).

However, in his critique, Ferguson seems to conflate “resource competition” with “disturbance brought about by the actions of people.” Resource competition is not necessarily a “disturbance”, nor does it occur only as a consequence of “disturbance.” Instead, resource competition is a routine part of existence for living things. This is one of the central premises of evolution by natural selection, and evolutionary disciplines such as behavioral ecology. Ferguson therefore appears to agree with us that violence is an adaptive strategy for resource competition. And we agree that actions of people can sometimes affect resource competition in other animals, for example by adding a valuable, concentrated resource (Wrangham, 1974), or by removing key resources such as fruit trees, increasing competition for available land (Goodall, 1977). What we disagree on is whether the evidence indicates that human impacts are the main cause of chimpanzee violence.

Ferguson argues that “human impact must be approached in historical detail.” The 30 co-authors of our paper are deeply familiar with the historical details, with many of them having been involved in these long-term sites for decades. But historical awareness does not preclude a scientific approach. Clear predictions emerge from the hypothesis that human impacts cause chimpanzees to kill. We have sought to test those predictions systematically. We also appreciate that our study sites and study populations, and the human populations that now surround those sites, have complex histories that long pre-date our research projects, and that we should be aware of this fact despite the difficulties of reconstructing these histories. However, we are also concerned with history in another sense: the evolutionary history of chimpanzees. For the great majority of this history, chimpanzees and their immediate ancestors could not have been subject to “disturbance” of the kinds that Ferguson and other proponents of the HIH invoke.

When introducing Ferguson’s critique, John Horgan writes of confirmation bias, which is indeed a concern in any scientific endeavor. To counter this bias, scientists use tools such as collecting quantitative data and using statistical methods to test which models best explain the observed data. In our study, we sought to use methods that are objective and transparent, and which would provide well-substantiated answers, whether they agreed with our prior opinions or not.

In contrast, the “holistic” approach promoted by Ferguson is vulnerable to confirmation bias. Without clearly defining variables such as disturbance, and without using quantitative data and statistical methods designed to test whether a given set of results is likely given the available sample size, efforts to compare contrasting interpretations of a given set of data risk degenerating into cherry picking and special pleading.

Ferguson argues that “[t]he three measures Wilson et al. created to test for human impact are questionable.” However, he agrees in principle with each of these measures, and offers no quantifiable alternatives.

He states that our first measure, “artificial provisioning of food, is good, where it applies, though the impact of provisioning varies by how it is carried out and other conditions of food availability.” By this Ferguson presumably refers to the argument of Power (1991) that the “restricted” provisioning at Gombe and Mahale is what frustrated chimpanzees and (in Power’s view) fundamentally changed their behavior.

We do not dispute that providing food can change the behavior of chimpanzees and other animals. Wrangham (1974) found that at Gombe, chimpanzees and baboons were more aggressive in the feeding area than when foraging for natural foods away from the feeding area. What is clear from our study, however, is that provisioning is neither necessary nor sufficient for chimpanzees to kill.

We examined data on both chimpanzees (18 communities at 10 study sites) and bonobos (4 communities at 3 study sites). Of these sites, provisioning occurred at two chimpanzee sites (Gombe and Mahale) and one bonobo site (Wamba).

Evidence for lethal aggression has been found in seven of the of eight never-provisioned study sites (Budongo, Fongoli, Goualougo, Kalinzu, Kibale, Kyambura, Taï); the only exception is the small, isolated population at Bossou. Ferguson notes that “sites marked P for provisioning cluster (4 out of 7 cases) toward the high end of the killing distribution.” Fair enough, but the two highest rates of killing are for sites where chimpanzees were never provisioned. Moreover, at Mahale and Gombe, killings have continued to be observed long after provisioning ended (in 1987 and 2000, respectively). Killings by never-provisioned chimpanzees have also been reported at shorter-term study sites not included in our study, including Loango in Gabon (Boesch et al., 2007), as well as Conkouati-Douli National Park, Congo, where male wild-born, captive chimpanzees released into the wild were attacked by resident chimpanzees (Goossens et al., 2005).

Ferguson claims that “Provisioning’s statistical association with killing, however, is diluted by two other sites,” Mahale’s K-group and Wamba. “Diluted” is an odd choice of words here; if provisioning causes chimpanzees to become violent, then presumably every community provides relevant data, rather than dilution. Excluding K-group from the analysis would involve picking and choosing, excluding any data that doesn’t fit the expectation (in other words, guaranteeing confirmation bias).

Provisioning is thus clearly not necessary for chimpanzees to kill. Nor is it sufficient for killing to occur. Killings have been observed at both of the historically provisioned chimpanzee sites, but the observed and inferred killings by the Mitumba community at Gombe occurred only after provisioning ended there (Wilson et al., 2004). (We note, though, that one infanticide is suspected to have occurred at Mitumba during the time that Mitumba chimpanzees were provisioned (Pusey et al., 2008)).

Killings have not been observed at the provisioned bonobo site (Wamba). The one suspected bonobo killing took place at a non-provisioned site (Lomako). Ferguson notes that bonobos are a different species. Of course, this is true. But if provisioning causes chimpanzees to kill, why should it not cause other species to kill, especially closely related species?

Ferguson argues that Wamba should not be included in the analysis because bonobos are a different species. Fair enough; and indeed, in Table 3, we present results focused on just chimpanzees, excluding bonobos, and found that provisioning history did not explain variation in rates of killing among chimpanzee communities. But we also note that Ferguson has previously written that violence in chimpanzees is the result of social learning, proposing that bonobos would behave like chimpanzees if they experienced similar conditions (“What would happen if a bonobo were raised among chimpanzees or vice versa? I expect their behaviors would reflect the local custom” (Ferguson 2011: 255)). Following this line of logic seems to suggest to us that exposing bonobos to the same stimulus as chimpanzees (provisioned food) should result in a similar increase in aggressive behavior. But the one suspected case of killing by bonobos occurred at the never-provisioned site of Lomako, rather than the provisioned site of Wamba.

In our view, the much less frequent occurrence of violent aggression in bonobos compared to chimpanzees raises interesting questions about the evolution of non-violence as well as violence.

We note with interest, though, that Ferguson’s argument for provisioning is profoundly different from Power’s argument. Power argued that restricted provisioning fundamentally changed the behavior of chimpanzees at Gombe and Mahale, and that a wide range of chimpanzee behaviors reported by researchers there were the result of provisioning: male dominance hierarchies, despotic alpha males, possessive sexual behavior, closed membership of social groups, territorial behavior, female dispersal, hunting of monkeys, and intergroup killings. Studies of never-provisioned chimpanzees have found that all of these behaviors occur in the absence of provisioning. We can therefore reject the idea that chimpanzee behavior is fundamentally altered by provisioning. And indeed, instead of following Power’s argument that chimpanzee violence is maladaptive, Ferguson accepts that chimpanzee violence is an adaptive component of resource competition.

As a second measure of human impacts, we examined size of protected area. Ferguson notes that “some chimpanzee groups living within large protected areas have been heavily impacted.” Fair enough. That is why we conducted multivariate analyses considering several different variables. However, size of protected area is a measure that is readily quantified, and is likely important both for chimpanzee conservation and as a general measure of the degree to which chimpanzees have been affected by human activities. Like our other measures of human disturbance, however, size of the protected area did not have a consistent effect on rates of violence.

Our third measure of human impacts was an index of disturbance, based on a method developed by Naomi Bishop and colleagues for assessing the impacts of human activities on Hanuman langur monkeys in India (Bishop et al., 1981). Ferguson agrees that these measures “work well as a general index of over-all human impact,” but complains that “they do not work as predictors of intensified violence.” Similar complaints could be made of any effort to quantify human disturbance, because what exactly qualifies as disturbance is never clearly stated. However, it is worth pointing out that the disturbance index developed we used was originally developed to address a controversy over causes of infanticide (Bishop et al., 1981).

Importantly, for our disturbance rankings, each site director ranked their own site without prior knowledge of the rankings of other researchers. These rankings thus provide an independent estimate of disturbance made by the people who best know each of these sites.

Ferguson notes that Budongo has been exposed to various forms of lumber extraction, and other chimpanzee sites have experienced “islandization.” Fair enough. As primatologists actively involved in conservation efforts, we are deeply familiar with such issues at our sites. Capturing all of these different effects in a single variable, or a single index that combines several measures of disturbance (the approach we used), can never be wholly satisfactory. But we believe that the rankings that we did for our study do correlate reasonably well with the degree to which these different sites have been affected by humans. Goualougo is clearly the least affected chimpanzee site, and Bossou the most. Kanyawara, at the edge of Kibale National Park, has a higher disturbance rating than Ngogo, at the center of the park. We do not argue that our index is perfect, but we are not aware of a better one.

Ferguson points to many different possible impacts: provisioning, habitat clearance, timber extraction, hunting, and so on. Some of these could well have an effect on chimpanzee violence, by (for example) increasing the intensity of competition for resources. This is an adaptationist argument following standard theory in behavioral ecology, and as such, is an approach that we find reasonable. It is the fact that these are reasonable questions that motivated us to conduct our study. Given the possibility that human activities can affect rates of violence in chimpanzees, it is important to investigate the extent to which the observed patterns of violence reflect human impacts. This is what we have attempted to do, using data that are quantifiable, systematic, defined using the same criteria across study sites, and using a statistical approach that allows us to test predictions from multiple contrasting models. We found that human impacts did not explain the variation in rates of lethal aggression as well as other factors. Eastern chimpanzees killed more often than western chimpanzees, which killed more often than bonobos. Communities with more males and communities living in denser populations killed more frequently than communities with fewer males and sparser populations.

It is not at all clear why chimpanzees should react this way when exposed to humans, but not bonobos, baboons, or other species. For example, baboons have been studied at Gombe almost as long as chimpanzees and have been influenced by human activity. But they have not been observed to participate in coalitionary killing. What accounts for such differences between species when exposed to precisely the same human impacts? The absence of coalitionary killing in baboons makes sense from an evolutionary perspective (Wrangham, 1999), but is inexplicable from an anti-adaptationist perspective.

Moreover, chimpanzees and humans are far from the only species to engage in lethal aggression. Fatal fighting occurs widely among animals, and includes a broad range of examples, such as fatal fights among male spiders competing for mating opportunities (Leimar et al., 1991) and the killing and consumption of male spiders by female spiders, after (or sometimes during or even before) the males have mated (e.g., Andrade, 1996). Most such fatal fighting involves fights between individuals over a highly valuable resource. In contrast, coalitionary killing occurs in fewer species, and seems mainly limited to certain social insects, social carnivores such as wolves, lions and spotted hyenas, and a few primates, including humans and chimpanzees (Wrangham, 1999). Why coalitionary killing occurs in these species but not others is explicable, in principle, using the comparative method to develop testable hypotheses. An anti-adaptationist approach promises no such explanatory power.

Whether chimpanzee violence is adaptive or not, is a question for which we do not yet have a definitive answer. Answering this question in full requires information on reproduction and information on individual participation in violence, which is available for only a few sites and which has not yet been analyzed. Additionally, chimpanzees (like humans and other animals) may sometimes make mistakes, participating in killings that result in fitness (i.e. reproductive) costs. Whether a given behavioral strategy is adaptive depends on average effects of traits. Given these caveats, previous studies provide evidence in support of the view that chimpanzee violence provides fitness benefits to the attackers. Mitani et al. (2010) found that the intergroup killings by the Ngogo community were associated with substantial territorial expansion in the area where disproportionately many of the killings had taken place. Studies at Gombe provide evidence that larger territories provide important fitness benefits, including more food, as indicated by heavier individual body weights, controlling for age and reproductive condition (Pusey et al., 2005) and shorter inter-birth intervals for females (Williams et al., 2004). Males who enlarge their territory thus provide more food for their mates and offspring, enabling faster reproduction, and thus greater reproductive success for the aggressors.

In his critique, Ferguson mostly ignores the second focus of our paper: the pattern of attackers and victims. We found that most participants (92%) were male, as were most victims (73%). Most victims were members of other communities (63%). Intercommunity killings generally involved gang attacks, in which attackers outnumbered victims by a factor of 8:1. These patterns make sense when seen as adaptive strategies. Male chimpanzees defend group territories; eliminating members of rival communities enables males to increase the amount of food available to themselves, their mates, and offspring (Williams et al, 2004; Pusey et al., 2005; Mitani et al., 2010). Chimpanzees prefer to attack when the odds are in their favor (Wrangham 1999; Wilson et al., 2012). Viewing these behaviors solely as a non-adaptive response to human disturbance provides no insights into why attacks mainly involve males attacking members of other groups when the odds are in their favor.

The question of what, if anything, chimpanzee violence has to do with human warfare is one we did not address in our paper. We expect that among our 30 co-authors some diversity of opinion exists on this topic. We would all agree, though, that definitive claims about human behavior need to be based on data from humans.

Nonetheless, there are some important things we can learn from chimpanzee studies. Our study examines lethal aggression broadly, including infanticide and within-community violence. Despite this, the criticisms of Ferguson (as well as John Horgan’s earlier post) focus mainly on intercommunity killing, and its relevance to studies of human warfare. Ferguson, Horgan, and many others argue that warfare has a relatively recent origin (within the past 10,000 years (Ferguson, 2003)), due to some relatively new phenomenon, such as agriculture, or settled societies, or food storage, or property rights, or ideology, or new kinds of weapons. Chimpanzees have none of these things. They do sometimes use weapons (sticks and stones) but they don’t generally use them to kill each other. So the documentation of warlike behavior in chimpanzees shows that similar behavior could have occurred in humans long before the origin of agriculture and other evolutionarily recent innovations. It also raises the intriguing possibility that humans and chimpanzees share similar patterns of violence due to our shared evolutionary history; we may have inherited these patterns of behavior from our common ancestor.

As many have noted, however, and as we fully recognize, the existence of bonobos, with their much less violent societies, highlights the need to be cautious in how much we infer along these lines. It is possible that the lineages leading to humans and chimpanzees have both become more violent, or that the lineage leading to bonobos has become more peaceful over evolutionary time. We don’t yet know the answer to this question.

We heartily agree with Silk’s point that “Humans are not destined to be warlike because chimpanzees sometimes kill their neighbours” (Silk, 2014: 322). Variation in rates of warfare among countries today, and across historical time, clearly show that people can develop institutions and mechanisms that reduce the frequency and severity of warfare (Gat, 2006; Pinker, 2011). Chimpanzee communities also vary considerably in their rates of intercommunity violence. As we have found, this variation is better explained by differences among species, populations, and demography than by human impacts (Wilson et al., 2014).

References

Andrade, M. C. B. (1996). “Sexual selection for male sacrifice in the Australian redback spider.” Science 271(5245): 70-72.

Bishop, N., S. B. Hrdy, J. Teas and J. Moore (1981). “Measures of human influence in habitats of South Asian monkeys.” International Journal of Primatology 2(2): 153-167.

Boesch, C., J. Head, N. Tagg, M. Arandjelovic, L. Vigilant and M. M. Robbins (2007). “Fatal chimpanzee attack in Loango National Park, Gabon.” International Journal of Primatology 28: 1025-1034.

Ferguson, R. B. (1990). “Blood of the Leviathan: Western contact and warfare in Amazonia.” American Ethnologist 17(2): 237-257.

Ferguson, R. B. (2003). “The birth of war.” Natural History 112(6): 28-35.

Ferguson, R. B. (2011). Born to Live: Challenging Killer Myths. Origins of Altruism and Cooperation. R. W. Sussman and C. R. Cloninger: 249-270.

Ferguson, R. B. (2014). “Anthropologist Brian Ferguson challenges claim that chimp violence is adaptive.” Retrieved 19 September 2014, from http://blogs.scientificamerican.com/cross-check/2014/09/18/anthropologist-brian-ferguson-challenges-claim-that-chimp-violence-is-adaptive/.

Gat, A. (2006). War in Human Civilization. Oxford, Oxford University Press.

Goodall, J. (1977). “Infant killing and cannibalism in free-living chimpanzees.” Folia Primatol 22: 259-282.

Goodall, J. (1986). The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, Massachusetts, Belknap Press.

Goossens, B., J. M. Setchell, E. Tchidongo, E. Dilambaka, C. Vidal, M. Ancrenaz and A. Jamart (2005). “Survival, interactions with wild conspecifics and reproduction in wild-born orphan chimpanzees following release into Conkouati-Douli National Park, Republic of Congo.” Biological Conservation 123: 461-475.

Leimar, O., S. Austad and M. Enquist (1991). “A test of the sequential assessment game: fighting in the bowl and doily spider Frontinella pyramitela.” Evolution 45(4): 862-874.

Mitani, J. C., D. P. Watts and S. J. Amsler (2010). “Lethal intergroup aggression leads to territorial expansion in wild chimpanzees.” Current Biology 20(12): R507-R508.

Pinker, S. (2011). The Better Angels of Our Nature: Why Violence Has Declined, Viking.

Power, M. (1991). The Egalitarians—Human and Chimpanzee: An Anthropological View of Social Organization. Cambridge, Cambridge University Press.

Pusey, A. E., G. W. Oehlert, J. M. Williams and J. Goodall (2005). “The influence of ecological and social factors on body mass of wild chimpanzees.” International Journal of Primatology 26: 3-31.

Pusey, A. E., C. Murray, W. R. Wallauer, M. L. Wilson, E. Wroblewski and J. Goodall (2008). “Severe aggression among female chimpanzees at Gombe National Park, Tanzania.” International Journal of Primatology 29(4): 949-973. get pdf

Silk, J. B. (2014). “Animal behaviour: The evolutionary roots of lethal conflict.” Nature 513(7518): 321-322.

Williams, J. M., G. Oehlert, J. Carlis and A. E. Pusey (2004). “Why do male chimpanzees defend a group range? Reassessing male territoriality.” Animal Behaviour 68(3): 523-532.

Wilson, M. L. (2012). Long-term studies of the Gombe chimpanzees. Long-term Field Studies of Primates. P. Kappeler and D. P. Watts. Heidelberg, Springer-Verlag: 357-384. get pdf

Wilson, M. L., C. Boesch, B. Fruth, T. Furuichi, I. C. Gilby, C. Hashimoto, C. Hobaiter, G. Hohman, N. Itoh, K. Koops, J. Lloyd, T. Matsuzawa, J. C. Mitani, D. C. Mjungu, D. Morgan, R. Mundry, M. N. Muller, M. Nakamura, J. D. Pruetz, A. E. Pusey, J. Riedel, C. Sanz, A. M. Schel, N. Simmons, M. Waller, D. P. Watts, F. J. White, R. M. Wittig, K. Zuberbühler and R. W. Wrangham (2014). “Lethal aggression in Pan is better explained by adaptive strategies than human impacts.” Nature 513: 414-417.

Wilson, M. L., S. M. Kahlenberg, M. T. Wells and R. W. Wrangham (2012). “Ecological and social factors affect the occurrence and outcomes of intergroup encounters in chimpanzees.” Animal Behaviour 83(1): 277-291. get pdf

Wilson, M. L., W. Wallauer and A. E. Pusey (2004). “New cases of intergroup violence among chimpanzees in Gombe National Park, Tanzania.” International Journal of Primatology 25(3): 523-549. get pdf

Wrangham, R. (1974). “Artificial feeding of chimpanzees and baboons in their natural habitat.” Animal Behaviour 22: 83-93.

Wrangham, R. W. (1999). “The evolution of coalitionary killing.” Yearbook of Physical Anthropology 42: 1-30.

[1] We invited all co-authors of our original paper to contribute to this response. All of those who responded to our invitation provided feedback and asked to be included as co-authors. Most (or perhaps all) of those who have not yet responded to this request are currently in the field with limited access to email. Not being included on this list, therefore, does not necessarily imply any disagreement with the contents of this response.

Chimpanzee violence

Are chimpanzees naturally violent? Or is chimpanzee violence the result of human interference, such as artificial feeding or habitat loss? Along with 29 co-authors, I  examine this question in a paper published this week in Nature.

Jane Goodall writes in her magnum opus, The Chimpanzees of Gombe:

Early field studies of chimpanzees (including my own) gave rise to the myth of the gentle, peace-loving ape. As more data on chimpanzee behavior have been collected over the years, this myth has gradually been dispelled. (Goodall 1986: 313)

Despite observations of violence in chimpanzees by Goodall and many others, a few  people still cling to the myth that chimpanzees are peace-loving apes at heart, moved to violence only by human impacts. This view was most fully developed by Margaret Power in her 1991 book, The Egalitarians—Human and Chimpanzee.

As far as I know, Power never studied chimpanzees in the wild. Instead, she based her arguments on reading the literature, especially Goodall’s early work, and also work by other people who had conducted shorter studies of chimpanzees at other sites, such as Vernon and Frankie Reynolds’ study of chimpanzees at Budongo Forest, Uganda, and Michael Ghiglieri’s study of chimpanzees at Ngogo, in Kibale Forest, also in Uganda.

Power distinguished “naturalistic” studies of chimpanzees from “provisioning” studies, in which chimpanzees were given food by researchers. This distinction follows a long tradition of researchers who have argued that Gombe chimpanzees were no longer truly “wild” after Goodall started feeding them bananas (e.g., Reynolds, 1975).

Power noted that in a 1974 paper,  Richard Wrangham showed that Gombe chimpanzees behaved more aggressively in “camp,” where researches fed them bananas,  than in the rest of their range, where they fed on naturally occurring foods. While Wrangham explained this change in behavior as a natural response to competing over an especially rich and concentrated source of food, Power developed a more elaborate argument based on psychological frustration theory, arguing that chimpanzee behavior was fundamentally changed by frustrations encountered at the feeding station.

Power argued that all sorts of chimpanzee behavior described by Goodall and colleagues, such as territorial behavior, dominance hierarchies, intense competition for mating opportunities, bullying by the alpha male, hunting of monkeys, and lethal aggression, were not natural behavior, but were instead the result of frustration caused by restrictive feeding of chimpanzees. She made the same arguments for Mahale, the study site established by Toshisada Nishida shortly after Goodall began her studies at Gombe. Nishida and his team used sugar cane to attract chimpanzees to an observation area. Power argued that this fundamentally changed their behavior, just as it had for Gombe chimpanzees. Only the earlier, “naturalistic” observations of  chimpanzees could be trusted. Power discounts all later observations from these sites, even though at both Gombe and Mahale, as chimpanzees became better habituated, researchers increasingly followed chimpanzees throughout their forest range, rather than focusing on observations at the feeding stations.

When Power published her book in 1991, chimpanzees had been studied at many different sites across Africa, but understanding of chimpanzee behavior in the wild was still very much dominated by studies from Gombe and Mahale. Nonetheless, new long-term studies were already underway at places including Taï Forest in Côte d’Ivoire and Kibale Forest in Uganda. These studies have continued, and new studies have been established at other sites. Researchers eventually stopped feeding chimpanzees at both Mahale and Gombe, and none of the newer study sites used artificial feeding to observe chimpanzees. Many of the new sites were in large, relatively undisturbed protected areas. And yet chimpanzees at all these sites demonstrated patterns of behavior that Power argued were the result of provisioning, including dominance hierarchies, bullying by alpha males, intense competition among males for mating opportunities, hunting of monkeys, and territorial behavior. These observations soundly refuted Power’s hypothesis that the behavior of provisioned chimpanzees was fundamentally different from that of unprovisioned chimpanzees.

Nonetheless, when I started studying chimpanzees in the mid-1990s, the number of detailed observations of lethal aggression in chimpanzees was still small. The most detailed accounts of killing were those from Gombe and Mahale. It seemed at the time reasonable to wonder whether those killings were the result of something unusual about those sites, such as the artificial feeding that occurred there. Or perhaps something else was responsible, such as the ecology of these sites, both located near the southeastern limits of the range of the species.

When I started graduate school, I was mainly interested  in language evolution. I wanted to do playback experiments with chimpanzees in order to test whether they had symbolic communication, like Dorothy Cheney and Robert Seyfarth had found with vervet monkeys. I went to Harvard to work with Marc Hauser, who had been a student of Cheney and Seyfarth, and Richard Wrangham, who had established a new long-term study of the Kanyawara community in Kibale.

Although my main focus was language evolution and communication, I was also inspired by two papers published in Current Anthropology:  “The human community as a primate society,” by Lars Rodseth, Richard Wrangham, and Barb Smuts, and “Intergroup aggression in chimpanzees and humans,” by Joe Manson and Richard Wrangham. The approach of these papers, seeking to explain human behavior through comparative study of other primates, seemed exactly the sort of thing we should be doing to gain a proper understanding of our species.

In 1996, I started my first field season at Kanyawara, doing a pilot study of playback experiments. These turned out to be the first successful playback experiments to wild chimpanzees — and they also ended up shifting my focus from language evolution to intergroup aggression.

We knew it would be hard to do playback experiments with chimpanzees. They are smart, and fast. We worried that playing soft calls at close range wouldn’t work easily, as they would find the speaker and catch on that something wasn’t right. So we needed to work with loud calls that would let us set up the speaker far away from the chimps. We also needed to focus on an experimental question that could be answered with relatively few trials, since we figured chimpanzees would habituate quickly to the experimental situation if we did the same thing over and over. So we settled on simulating intergroup events, playing a single pant-hoot call from a male stranger, using calls that John Mitani had recorded from chimpanzees at Mahale. This would enable us to test whether chimpanzees could assess the relative numbers of their opponents, much as Karen McComb, Craig Packer and Anne Pusey had recently shown with lions (McComb et al., 1994). As a result of choosing to do these particular experiments, I ended up shaping the rest of my dissertation research around questions of intergroup aggression.

In the fall after I had finished my first round of experiments at Kibale, Richard Wrangham and Dale Peterson published Demonic Males. This book inspired a closer look at intergroup aggression in chimpanzees. It also attracted criticism, particularly I think from people responding to the title of the book, rather than its contents. Among the more vocal critics have been Bob Sussman (1999, 2013) and Brian Ferguson (2011). Both Sussman and Ferguson have resurrected Margaret Power’s arguments that chimpanzee violence is not natural, but somehow the fault of humans.

At the same time, the evidence for chimpanzee violence continued to accumulate, not just at Gombe, but at sites across Africa. Many cases of violence have been reported from sites that were never provisioned. Critics such as Sussman and Ferguson have therefore shifted the focus away from provisioning and more towards other forms of disturbance: habitat loss, deaths from poaching and disease, and so forth.

In 2001, I finished my PhD and started working as a post-doc with Anne Pusey at the University of Minnesota. My main goal as a post-doc with Anne was to look at intergroup aggression data from Gombe, one of the very few sites where neighboring habituated communities could be studied. This provided a rare opportunity to examine intergroup aggression from both sides of the interaction.

Working at Gombe also increased my awareness of issues related to human impacts.  I ended up spending three years based at Gombe full-time, working for the Jane Goodall Institute. Conservation issues are important at every ape research site, but are particularly prominent at Gombe, given that it is a relatively small park exposed to substantial human impacts, especially deforestation outside the park. And throughout the time I have been working at Gombe, chimpanzees have, from time to time, attacked and killed one another.

In thinking about human impacts and chimpanzee violence, there are really two major issues to consider. One is whether human impacts, such as provisioning and habitat destruction, affect rates of violence. The other question is whether violence is mainly adaptive behavior or not. For example, Wrangham (1974) clearly showed that rates of aggression were higher at the feeding station than in the forest. But he argued this was a natural, adaptive response to a highly concentrated, high quality food source. Individuals who competed aggressively for bananas would get to feast on soft, easily digested fruits, rich in sugar and starch. Individuals who stayed out of the fray would go hungry, or have to go searching long distances in the forest for natural foods.

Likewise, during the decades when forests adjacent to Gombe were being cleared for farmland, it seems entirely plausible that this could lead to higher rates of violence, as chimpanzees retreated into the remaining protected area inside the park. If the number of chimpanzees in the area stayed the same, but the available habitat shrank, this could lead to increased aggression. In this case, though, increased aggression might well be a strategy by which individuals increased their reproductive success. Individuals that simply retreated, rather than defending their land, would be forced out of the good areas, while the aggressive victors would enjoy the spoils.

Critics such as Sussman and Ferguson seem mainly interested in arguing that aggressive behavior is maladaptive. They don’t like the idea that aggressors might benefit from violent behavior, and seem mainly worried about the consequences of such arguments. For example, if we argue that violent behavior is favored by natural selection, does that mean that we must then excuse violent behavior, and accept it as natural?

I think that such concerns are unjustified, however. Just because something is “natural” doesn’t mean it is desirable, or inevitable. Smallpox virus is natural, but deeply undesirable, and humans have intentionally and with great effort eradicated this virus. We have likewise made great strides towards reducing rates of warfare and other violence.

Nonetheless, whether chimpanzee violence is natural, or the result of human impacts, is an important question to get right. I have spent a number of years trying to answer it. I first presented a version of this study in 2004, in a talk at the International Society for Research on Aggression meetings in Santorini, Greece, titled: “Is chimpanzee intergroup violence the result of human disturbance?”

In this earlier effort, based mainly on the published literature, I didn’t find any strong link between human impacts and chimpanzee violence. By this time killings had been reported from several sites that had never been provisioned, including Ngogo and Kanyawara in Kibale Forest, Uganda, and Budongo, also in Uganda. But I had other things on my plate, and ended up putting this study on the back burner for several years.

It wasn’t until 2011 that I started working on the project again in earnest. Once I got started I realized that to really get a handle  on this question, published data wouldn’t be enough. I wanted to make sure that the data from each site was accurate, and that variables demographic and ranging data were all coded the same way. I wanted independent ratings of disturbance from each site. And I knew from my own experience that it can take years from the time that killings are observed to when detailed descriptions are written up and published. A study using just the published cases would result in a potentially severe underestimation of rates of violence. So I started contacting researchers at other sites and asked if they would be interested in participating. In the end the study included all the main long-term studies of chimpanzees and bonobos, and a long list of co-authors.

The main take home message of our study is that chimpanzee violence is natural behavior, not the result of human impacts. We have two main lines of evidence for this.

First, if we look across study sites, the degree of human impacts doesn’t explain the variation in rates of killing that we see. The site with the highest rate of killing, Ngogo, is in a forest with relatively low human impacts, and these chimpanzees were never artificially fed by researchers. In contrast, the site with the highest human disturbance rating, Bossou, is a site where killing has never been observed, despite many years of observation. Instead, overall killing rates are better explained by differences among species (chimpanzees kill more often than bonobos), and differences in demography (groups with more males, and that live at higher population densities, have more killing). Moreover, high population density appears to reflect good habitat quality, rather than human disturbance.

Second, if we look at the detailed patterns of who is killing whom, we see patterns that make sense from an evolutionary viewpoint, but which are hard to explain otherwise. Attackers did not kill at random. Instead, they mainly killed members of other groups (63% of killings). They mainly killed when they had an overwhelming numerical advantage (median 8:1 ratio of attackers to victims in intergroup killings). Attackers were much more often male than female (92% of participants in attacks) and they mainly killed males (73% of victims). They mainly killed when it was easy to kill victims, either because of a strong numerical advantage, or because the victim was weak (such as infants).

People often ask what the implications of this study are for human
behavior. I would say that definitive claims about human behavior need to be based on data from humans. But there are some important things we can learn from chimpanzee studies. One is that we can get much more detail on the contexts of killings in chimpanzees than is normally possible in human studies. We can watch them do everything in their daily lives, including killing — something we can’t do easily, ethically, or legally with humans. So we can collect lots of data that is useful for testing hypotheses about the biology of violence that apply to humans as well as other species. Another thing we have learned relates especially to the origins of warfare. Some people argue that warfare has a recent origin, due to some relatively new phenomenon, such as agriculture, or settled societies, or food storage, or property rights, or ideology, or new kinds of weapons, and so on. Chimpanzees have none of these things. They do sometimes use weapons (sticks and stones) but they don’t generally use them to kill each other. So the documentation of warlike behavior in chimpanzees shows that similar behavior could have occurred in humans long before the origin of agriculture and other evolutionarily recent innovations. It also raises the intriguing possibility that humans and chimpanzees share similar patterns of violence due to our shared evolutionary history; we may have inherited these patterns of behavior from our common ancestor.

The existence of bonobos, however, with their much less violent societies, highlights the need to be cautious in how much we infer along these lines. It is possible that the lineages leading to humans and chimpanzees have both become more violent, or that the lineage leading to bonobos has become more peaceful over evolutionary time. We don’t yet know the answer to this question.

 

References

Ferguson, R. B. (2011). “Born to Live: Challenging Killer Myths.” Origins of Altruism and Cooperation. R. W. Sussman and C. R. Cloninger, Eds. Springer New York. 36: 249-270.

Goodall, J. (1986). The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, Massachusetts, Belknap Press.

Manson, J. H. and R. W. Wrangham (1991). “Intergroup aggression in chimpanzees and humans.” Current Anthropology 32(4): 369-390.

McComb, K., C. Packer and A. Pusey (1994). “Roaring and numerical assessment in contests between groups of female lions, Panthera leo.” Animal Behaviour 47: 379-387.

Power, M. (1991). The Egalitarians—Human and Chimpanzee: An Anthropological View of Social Organization. Cambridge, Cambridge University Press.

Reynolds, V. (1975). “How wild are Gombe chimpanzees.” Man 10(1): 123-125.

Rodseth, L., R. W. Wrangham, A. M. Harrigan and B. B. Smuts (1991). “The human community as a primate society.” Current Anthropology 32(3): 221-254.

Sussman, R. W. (1999). “The myth of man the hunter, man the killer and the evolution of human morality (evolutionary and religious perspectives on morality).” Zygon 34(3): 453-472.

Sussman, R. W. (2013). Why the legend of the killer ape never dies: The enduring power of cultural beliefs to distort our view of human nature. War, Peace, and Human Nature: The Convergence of Evolutionary and Cultural Views. D. P. Fry. Oxford, England, Oxford University Press: 97-111.

Wilson, M. L., R. W. Wrangham and A. E. Pusey (2004). “Is chimpanzee intergroup violence the result of human disturbance?” XVI World Meeting of the International Society for Research on Aggression, Fira, Santorini, Greece.

Wilson, M. L., C. Boesch, B. Fruth, T. Furuichi, I. C. Gilby, C. Hashimoto, C. Hobaiter, G. Hohman, N. Itoh, K. Koops, J. Lloyd, T. Matsuzawa, J. C. Mitani, D. C. Mjungu, D. Morgan, R. Mundry, M. N. Muller, M. Nakamura, J. D. Pruetz, A. E. Pusey, J. Riedel, C. Sanz, A. M. Schel, N. Simmons, M. Waller, D. P. Watts, F. J. White, R. M. Wittig, K. Zuberbühler and R. W. Wrangham (2014). “Lethal aggression in Pan is better explained by adaptive strategies than human impacts.” Nature 513: 414-417.

Wrangham, R. (1974). “Artificial feeding of chimpanzees and baboons in their natural habitat.” Animal Behaviour 22: 83-93.

Wrangham, R. W. and D. Peterson (1996). Demonic Males: Apes and the Origins of Human Violence. Boston, Houghton Mifflin.

Video Games

Do playing violent video games and watching violent television shows make people act violently? That was one of many questions discussed at the XXIst World Meeting of the International Society for Research on Aggression (ISRA), which met this July in Atlanta, Georgia.

ISRA was founded in 1972, “In the shadow of the Cold War, the Vietnam War, and social unrest throughout the United States.” Since that beginning, a goal of the society has been to use research to find ways to “reduce harmful aggression.” This is a goal that I share. But during the course of the meetings, I couldn’t help but wonder: after all these years of research, what have aggression researchers learned that has been of practical use to help people reduce harmful aggression in the real world?

Some of the ISRA talks focused on the neurobiology of aggression. This is a scientifically important topic, definitely worth pursuing. But I suppose that the practical benefits of such research are largely limited to a fairly narrow range of specific circumstances, such as developing drugs to help reduce violent behavior in people with certain mental illnesses. Will a better understanding of neurobiology really help to reduce rates of warfare, or civil strife, or violent crime?

Many of the other talks and posters focused on the social psychology of aggression. A particularly popular topic was the effect of violent media on aggressive behavior. The most prominent of these was the John Paul Scott Award talk by Professor Rowell Huesmann. This award “recognizes a lifetime or substantial contribution to aggression research,” so this is clearly a topic close to the heart of the society.

Professor Huesmann spoke on aggression and the media. He described aggression as a contagious disease, which is contracted through exposure to violence, whether real or fictional. Watching violence on television, or playing violent video games, gives children “scripts” about how they should act. Because people imitate, they imitate the violence they see on TV and experience in videogames and act aggressively in real life.

Professor Huesmann is an esteemed scholar with a long list of publications, honors and grants. His findings are based on studies with large sample sizes and careful statistics that show consistent, positive correlations between exposure to violent media and violent real life behavior. He also seemed like a very nice guy, earnestly devoted to reducing problems of violence in the real world.

Despite all this, I think the Media Violence Hypothesis doesn’t explain a great deal of violent behavior, even though it is intuitively appealing on several levels.

The argument that exposure to media violence causes real world violence has a certain plausibility to it. Kids imitate things they see on TV. In my high school, certain guys were constantly imitating pro wrestling moves with their buddies (though they always did so in comic slow motion). Teachers tell me they see their kids imitating moves they see on TV, such as Teenage Mutant Ninja Turtle attacks.

Another appealing part of the argument is that as video games become increasingly realistic, the violence becomes increasingly graphic and disturbing. Back in the 1990s, I played a game on a friend’s computer that must have been some version of Wolfenstein — a first person shooter that involved walking around a castle and shooting at Nazis. After playing for a while, I found the whole first person shooter aspect rather disturbing, particularly the blood pouring from the bullet wounds of the dead computer Nazis. I wouldn’t want my kids playing this game, and can imagine how spending many hours playing such games might affect a person.

So I can see how the Violent Media Hypothesis is appealing, especially to parents and educators. This hypothesis also suggests an obvious intervention: increased control over the media. If the hypothesis is true, then limiting violence in the media should provide strong practical benefits for reducing rates of violence in the real world — though of course this raises all sorts of problems for anyone interested in protecting First Amendment rights.

Despite the intuitive appeal of the hypothesis, though, and quite apart from concerns about media censorship, there are a number of reasons to think that that Media Violence Hypothesis doesn’t really hold up.

First of all, exposure to violent media isn’t a necessary precursor to violence. Neither ants nor chimpanzees nor any other animals require exposure to violent media to commit acts of violence. Numerous acts of violence occurred in human history, and prehistory, long before the invention of Pac-Man and Space Invaders.

This is, admittedly, a weak objection, given that the argument isn’t just about the media; it’s about the violence depicted in the media. Movies, TV and video games just provide novel means of displaying violence; new ways of spreading the contagion of violence. Long before violent media existed, people saw acts of violence being committed, which according to this hypothesis, would have resulted in further imitative acts of violence, which in turn would spawn yet more acts of violence.

There are, however, other reasons to think this hypothesis is implausible. One of these is my own personal experience. I don’t particularly seek out violent media. I have never played Grand Theft Auto or similar video games, and I don’t seek out slasher films. All the same, over a lifetime of watching TV and movies, and playing various kinds of games, I have been exposed to an enormous amount of simulated violence: murder, mayhem, rape, pillaging, decapitation, dismemberment, and all manner of horrible things. I’ve watched the entire population of the planet of Alderaan being destroyed by the Death Star. As a teenager playing Dungeons and Dragons, I fought countless virtual battles, throwing spears and whiskey bombs at orcs, hobgoblins and various other enemies. Playing Risk, I have commanded armies that slew many thousands of soldiers (though these were represented in the innocuous, non-bloody form of Roman numerals). I watched all six seasons of the Sopranos, in which many horrible murders were graphically depicted.

One would think that, according to the Media Violence Hypothesis, I would be deeply infected with the contagion of violence, and would have acted out these violent scripts repeatedly in my life. And yet, I have enjoyed a quite peaceful existence,  apart from a few rough wrestling bouts and fist fights in grade school.

I don’t think I’m unusual in this respect. Lots of people watch lots of violence on TV and movies, and experience simulated violence in video games, fantasy role playing games, and other forms, and yet have never committed a single act of violence.

Given all this, it would seem that violent media are neither necessary nor sufficient for violence to occur. Nor does it explain one of the most consistent findings in aggressive studies: men are consistently more violent than women. Men commit the great majority of homicides, and commit far more atrocities of war than women. If both sexes are routinely exposed to the same violent media, why should men be more violent than women?

Additionally, the Violent Media Hypothesis doesn’t  fit with patterns of violence over time, or across geographic space.

As Manuel Eisner described in his excellent plenary talk, rates of homicide have declined over the past thousand years or so in Europe. If the Media Violence Hypothesis were correct, we should see an enormous increase in violence following the invention and widespread dissemination of movies, television, and video games in the 20th Century. But what we see instead is a marked decrease in many different kinds of violence, including warfare and homicide. The horrors of WWI occurred when the film industry was still in its infancy. Television barely existed at the start of WWII. Since these cataclysmic events, the 20th Century has seen a steady decline in per capita rates of death from war, and decreases in homicide rates across Europe and North America. According to Eisner, there was an uptick in homicide in the 1960s through the early 1990s, but homicide rates declined thereafter, despite the release of a series of increasingly violent video games. (Grand Theft Auto, for example, was first released in 1997, when homicide rates in the US were on their way down.)

Considering geography, one would predict that the parts of the world most plagued by violence would be those most intensively saturated by television, films, internet and video games, such as the United States, Europe, and Japan. But while the US has higher rates of violence than Europe and Japan, its homicide rates are far below those of many countries where access to violent media is much more restricted. In many war-torn countries of Africa and the Middle East, per capita incomes are much lower and many people lack electricity, much less Xboxes.

So on a broad scale, the data don’t seem to support the Media Violence Hypothesis. It cannot explain why the latter half of the 20th Century was more peaceful than the first half, or why Japan is more peaceful than, say, Afghanistan.

The hypothesis seems even more doubtful from a theoretical perspective, especially when considered from an evolutionary perspective. In particular, why should evolution design people, or any other organism, to imitate acts of violence?

In evolutionary game theory, violence is a strategy that animals use to obtain key resources for survival and reproduction. If I am attacked by a predator, or a hostile member of my own species, I will fight back. Male chimpanzees fight for status, and for opportunities to mate with fertile females, and to defend and expand their territory.

Under many circumstances, imitating violent behavior would be maladaptive. For example, in chimpanzee societies, the alpha male frequently conducts charge displays, charging at, chasing, and sometimes hitting other community members. If a young, low-ranking male started imitating the alpha male by charging around and attacking other chimpanzees, he would get beat up in a hurry.

Professor Huesmann described studies showing that both Palestinian and Israeli children exposed to more media violence were more aggressive. Fair enough. But does anyone really think that the ongoing conflict between Israelis and Palestinians is the result of too much violent television? Or that Russia’s invasion of Crimea and ongoing support for separatists in Eastern Ukraine is the result of, say, Vladimir Putin watching too many Steven Seagal movies?

I would like to think that research on aggression can, and should, contribute to reducing harmful aggression. But I think that to find practical and effective solutions, we need to focus on the right level of analysis, guided by theoretically plausible hypotheses. Evolutionary theory provides critically important guidelines for what sorts of hypotheses are likely to be plausible. But a complete understanding of violent behavior also needs to look at a broad range of factors, including history, politics, and demography. A few disturbed people may be inspired to violent acts by violent video games, but it seems likely that the great majority of violent acts are best explained by conflicts over limited resources,  such as land, oil, and status.

http://gamemoir.files.wordpress.com/2014/02/stephen-colbert-on-video-game-violence.jpg
http://gamemoir.files.wordpress.com/2014/02/stephen-colbert-on-video-game-violence.jpg

 

 

 

 

 

 

 

 

 

Planet of the Apes

28 July 2014

The latest Planet of the Apes movie raises interesting many interesting questions, such as: what would it take for other apes to replace humans as the planet’s ruling primates?

Spoiler Alert: if you haven’t seen the movie yet, you might not want to read any further until you have. I try to steer clear of plot details, but if you’re the kind of person who likes to know as little as possible about a movie before seeing it, consider yourself warned.

I grew up watching the original Planet of the Apes movies. I am sure seeing movies of a world ruled by apes fueled my interest in our hairy cousins. It was a rich time for anyone interested in apes. The first movie came out in 1968, the same year that the site where Jane Goodall studied chimpanzees, Gombe, was upgraded from a game reserve to a National Park. We watched films of Jane Goodall and the chimpanzees of Gombe in elementary school. New discoveries about the apes were reported regularly in the glossy pages of National Geographic. Studies of sign-language using apes like Washoe and Koko suggested apes were on the brink of human intelligence. Movies like King Kong and the Planet of the Apes franchise presented apes as both dangerous and fascinating, blurring the boundary between human and animal.

I had a special interest this latest Planet of the Apes movie as I contributed some recordings of chimpanzee vocalizations. As a result my name shows up on the big screen for a few seconds, after Ape Extras but before Editorial Assistant, New Orleans. The Chicago Sun-Times even noticed.

(The name  Michael Wilson also shows up in the credits for the original movie, as writer of the screenplay  — though that was of course somebody else!)

I thought they did a good job with ape vocalizations in the movie. One of my complaints in general about animals in movies is that they make much more noise than animals do in real life. Movie predators, whether lions or dinosaurs, always seem to roar right before attacking their prey – something real predators would never do, as they seek to catch their prey by surprise. Roars are for warning members of your own species to stay away (and/or for attracting mates), not for chasing away your prey!

Chimpanzees can be extremely noisy, but most of the time they are very quiet. So one of my recommendations to the sound editors was to avoid extraneous calls. I was very pleased to see that for many scenes, the apes were indeed fairly quiet.

And when the apes did vocalize, I enjoyed hearing real ape calls, and different calls for each species.  I particularly liked one scene where the apes give a massive round of pant-grunts to Caesar. This is a call that chimpanzees use to show submission, and they used it in the right context for this film.

I liked that the apes mainly used sign language, and that when they did speak, they had rough, breathy voices, much like Viki the chimpanzee did when being trained to say words like “cup” and “up.”

In general, I thought the film did an excellent job building the characters and story. The main ape and human characters are complex, with understandable motives, and aren’t depicted as being either simply good or evil.

I think this might be the best movie yet in the franchise, and well worth seeing.

As an ape ecologist, though, I can’t help thinking about certain things.

For example, Muir Woods seems like a pretty rotten place for apes to live. It has trees, sure, but they are mainly redwoods and other conifers that produce no ape-friendly food. Apes are specialists in ripe fruit, which is in pretty short supply in a redwood forest. According to the Muir Woods website:

“Life in a redwood forest is determined by the low light conditions that restrict growth of plant species producing flowers, nuts, or berries. In addition, coast redwood trees contain an abundance of tannin (or tannic acid), a chemical compound that deters the presence of insects. Taken together, these conditions create an environment that is relatively low in the resources that typically form the base of a food web.”

So while it’s really cool to see apes swinging from the branches of redwoods, that forest is pretty grim habitat for apes. The gorillas might be able to subsist on herbs growing in the understory, but these are largely ferns and not very palatable. The chimps and orangutans would be pretty hungry there. They might use the forest as a temporary refuge, but would quickly move on to more suitable habitat, such as the overgrown gardens and city parks of post-apocalyptic suburbs.

If ordinary chimpanzees, gorillas and orangutans were released into the California wilderness, they would probably go their separate ways. The orangutans would forage alone. The male gorillas would compete over the female gorillas, until each silverback had a small group of females for himself. Each gorilla group would then forage separately. The chimps might start off as a single community but over time they would probably fission into several mutually hostile communities, each defending their own territory. It’s not clear why these different ape species stick together, or why they live in a village instead of sleeping up in the trees like real apes do. But of course these are retrovirus-mutated, hyper-intelligent talking apes, so they behave differently.

The film is surprisingly conservative in depicting ape romantic relationships, in that Caesar at least seems to be in a monogamous marriage with Cornelia. I suppose showing Caesar as a loyal family man makes him more appealing to viewers. However, a normal alpha male chimpanzee would try to monopolize matings with all the fertile females; and these females would try to mate with multiple males, even against the wishes of the alpha male. But perhaps the mutating retrovirus also makes apes monogamous.

But a big question relates to the film’s fundamental premise: what would it take to destroy human civilization, and clear the way for the world to be ruled by another kind of ape? (Or, in this case, a triumvirate of three different ape species.)

As Ruben Bolling points out,the Rise of the Planet Ape is a true story, and we are living it: we are the apes that have taken over the whole planet. But is human domination of the planet inevitable? How hard would humanity have to be hit to make way for other apes?

In this movie, humans are very nearly wiped out by a genetically engineered retrovirus, ALZ-113,  that makes nonhuman apes super intelligent but kills humans. (This has interesting parallels with SIVcpz, a naturally occurring retrovirus, which was transmitted from chimpanzees to humans, probably by people hunting and butchering chimpanzees for food. When contracted by people, the virus is called HIV-1 and causes the disease AIDS, which has killed many millions of people around the world. SIVcpz doesn’t make apes super intelligent, of course, and we have learned that it is also fatal to chimpanzees (Keele et al., 2009)).

According to newscasts in the movie, almost everyone who contracts the virus dies; only 1 in 500 survive. Since the virus is highly contagious and transmitted by sneezing, this leads to a much more devastating result than even the AIDS pandemic.

There are about 7 billion people on the planet today. So if 1 in 500 people died, there would still be 14 million people on the planet. Such a rapid and catastrophic epidemic would have huge impacts on the survivors, though, as food distribution systems and everything else collapsed. Say only 1 in 10 of people who survived the virus would survive the aftermath of collapsing civilization. That would bring the total population of people on the planet down to 1.4 million (which is still four to five times the total number of chimpanzees living on the planet today). This is probably a low figure, given that many people on the planet are subsistence farmers and herders of livestock.  Many people living in rural Africa, for example, would be able to survive the collapse of industrialized civilization, because they mainly live off the land without access to electricity, plumbing or fossil fuels.

In the San Francisco Bay Area, though, most people have no idea how to farm, herd livestock, or live off the land. Collapse would hurt people hard. So starting from a Bay Area population of about 7.44 million, if 1 in 500 die from disease, that leaves around 14,880 survivors. If 90% of those survivors died from starvation and post-apocalyptic fighting and such, then only around 1,488 people would be left in the Bay Area. That seems in line with the number of people crowded into the refuge of San Francisco (though as my wife noticed, the virus seems to have selectively killed all the Asians).

(Though why are they living in the middle of the city? I would think any survivors would mainly live on isolated rural farmsteads, where they can grow their own food, rather than crowding into the city center. How do these people eat? But it does look cool and dystopian to have everyone crowded together in the post-apocalypse city — maybe more so than setting it, say, on the outskirts of post-apocalypse Fresno.)

(Some other quibbles: Ten years post-apocalypse, I’m not sure anyone would still have usable manufactured clothing, eyeglasses or electronics anymore.  Even in my own family, after a year living abroad, with easy access to clothing and other supplies, the clothes we brought with us are ragged, the kids need new eyeglasses, and my son and I both need new shoes. Life post-apocalypse would certainly be much harder on such supplies. Moreover, there would certainly be no birth control or antibiotics. Sexually active women would be pregnant or nursing — which would have huge impacts on society. Weirdly, almost no young  human children, or women with nursing babies, were shown in this film.)

ANYWAY…

Based on the number of apes living in Muir Woods, they must have been reproducing at a really high rate compared to normal apes. This wiki states that there are 2,000 apes living in the ape village. Now that’s a lot of apes. Currently there are only about 2,000 captive chimpanzees in the United States. The starting population in Muir Woods must have been a lot less than that, since they started with apes escaping from just two captive colonies, and it would be hard for apes from other parts of the country to find out about the Muir Woods population, much less travel there.

Is it realistic to have 2,000 apes in ape village just 10 years after the ape revolution?

One key to the success of humans is demography. We can reproduce much faster than other apes. For example, suppose by coincidence that both the surviving human population in San Francisco, and the chimpanzee population in Muir Woods, started out at about 1,000 individuals. (Gorillas reproduce more quickly than chimpanzees, and orangutans reproduce more slowly, but since in the movie most of the apes are chimps, I’ll focus on them.) In a best case scenario, chimpanzee populations could potentially grow at about 2% per year. (Most wild chimpanzee populations “grow” at about 0% per year, though, because mortality is high and food supplies are limited — which in turn limits fertility and growth.)

Projected populations of humans and chimpanzees post-apocalypse, starting from 1000 individuals in each population.
Projected populations of humans and chimpanzees post-apocalypse, starting from 1000 individuals in each population.

So starting out with 1,000 chimps, in ten years there would be only about 1,221 chimps (if they somehow found food and didn’t suffer high mortality from predation, warfare etc.). Human hunter-gatherers, though, can grow at much faster rates, such as around 4%, even without medical care and with all of the hardships that hunter-gatherers face. At this rate, starting with 1,000 humans, we’d have around 1,492 people by the end of ten years — so about 270 more humans than chimps. And realistically, survivors in California would be farmers, not hunter-gatherers, with potentially even faster population growth. So if Ape Village apes are reproducing like normal chimpanzees, and if the starting population was in the hundreds, a population of 2,000 ten years later is not realistic.

Why can human populations grow so much faster than chimpanzees?

In some ways it is surprising that this can even be possible. After all, humans take longer to reach maturity than chimpanzees. Female chimpanzees have their first birth around age 14 (males reach full size around age 16, but for population growth, it’s females that matter more). Humans hunter-gatherers take longer to mature, with an average age of first birth at 18-20 (Hill & Kaplan 1999). Moreover, even though humans live longer than chimpanzees, human females stop reproducing in their forties — so their reproductive careers are, on average, shorter than those of chimpanzees.

However, once humans do grow up, they can reproduce quickly.  Chimpanzees have an average interval of around 5 years (Jones et al., 2010), whereas hunter-gatherers have an interbirth interval of only 4 years (Hill & Kaplan 1999).

How can women reproduce more quickly than chimpanzees? A big part of the answer must be cooking. Thanks to fire, humans can extract more energy from the environment, by increasing the energy available from food, and by making otherwise unpalatable foods safe to eat (Wrangham et al., 1999). Cooking likely helps children grow faster, by providing soft, energy rich foods from a young age, whereas chimpanzee children continue drinking their mother’s milk for longer, as they gradually add tough, hard adult foods to their diet. This surely has a big impact on human fertility and growth rates.

In the movie, the apes in Ape Village had fires in each house, so maybe they were cooking? That would certainly help them reproduce more quickly.

Another reason human populations can grow so much faster than chimpanzee populations is that humans have much lower mortality than chimpanzees, even in hunter-gatherer populations without access to medicine. Hunter-gatherers regularly live into their 50s, whereas the median age of survival for wild chimpanzees is about 30, and few live into their 40s. What accounts for this difference?

I suspect cooking is probably important for reducing mortality as well.  Cooking and other food extraction and preparation technology likely help people obtain food even in difficult times of the year, whereas chimpanzees in seasonal environments may become weak and more likely to die from diseases. Cooking also must help people live longer by providing soft foods that they can continue to eat into old ages, as their teeth wear down.

So super-intelligent mutant apes potentially *could* take over the world, but only if most of the humans are killed off, and apes learn how to cook.

 

Works cited:

Jones, J. H., M. L. Wilson, C. M. Murray and A. E. Pusey (2010). “Phenotypic quality influences fertility in Gombe chimpanzees.” Journal of Animal Ecology 79(6): 1262-1269. get pdf

Keele, B. F., J. H. Jones, K. A. Terio, J. D. Estes, R. S. Rudicell, M. L. Wilson, Y. Li, G. H. Learn, T. M. Beasley, J. Schumacher-Stankey, E. E. Wroblewski, A. Mosser, J. Raphael, S. Kamenya, E. V. Lonsdorf, D. A. Travis, T. Mlengeya, M. J. Kinsel, J. G. Else, G. Silvestri, J. Goodall, P. M. Sharp, G. M. Shaw, A. Pusey, E. and B. H. Hahn (2009). “Increased mortality and AIDS-like immunopathology in wild chimpanzees infected with SIVcpz.” Nature 460: 515-519. get pdf

Hill, K. and H. Kaplan (1999). “Life history traits in humans: Theory and empirical studies.” Annual Review of Anthropology 28: 397-430. get pdf

Wrangham, R. W., J. H. Jones, G. Laden, D. Pilbeam and N. Conklin-Brittain (1999). “The raw and the stolen: cooking and the ecology of human origins.” Current Anthropology 40(5): 567-594. get pdf

 

 

 

 

 

Mitumba

On Thursday, 12 June, Deus, Rebecca and I visited the Mitumba community in the North of Gombe.

Gombe is one of the few sites where researchers can study neighboring chimpanzee communities. At most sites, researchers focus on a single study community, and don’t know very much about the neighbors. This was the case at Gombe for many years, but since the Mitumba chimpanzees were habitatuated in the 1990s, we have been able to follow chimpanzees from two communities simultaneously. And now, with monitoring of the Kalande chimpanzees, we can track the movements of nearly every chimpanzee in the park.

The prospect of studying intergroup interactions from both sides of the event is what led me to start working at Gombe, and has been a focus of my research ever since. Deus has played a big part in this, as he collected data on the Mitumba chimpanzees, first as a research assistant on the intergroup relations project, and then as a PhD student.

When I first started working at Gombe, we worried that males from the larger, more powerful Kasekela community would kill the remaining males from Mitumba and take over their range. And sure enough, soon after I started working at Gombe, the young male Rusambo was found dead, with severe wounds, the day after Kasekela males traveled deep into Mitumba’s range (Wilson et al., 2004).

Edgar keeping a close eye on Flirt
Edgar keeping a close eye on Flirt

However, since then, the Mitumba males have held on, and even seem to have expanded their range a bit. And while intergroup incursions from Kasekela have continued, including the killing of the young infant Andromeda (Wilson, 2013),  it is the Mitumba males who have proved a greater threat to themselves. Mitumba males Edgar and Rudi killed their former alpha male, Vincent, in 2004. Soon after, the young male Ebony was found dead. Since then, Edgar and Rudi have fought bitterly and Rudi has disappeared. Edgar seems determined to keep Mitumba for himself, as if he were a gorilla silverback.

When we arrived in Mitumba, we walked up the steep, narrow valley of Mitumba stream. Gnarled old Mgwiza trees grow along the stream banks, and Lusieno trees tower overhead.

 

Flirt climbing down from her feeding tree.
Flirt climbing down from her feeding tree.

We found chimps close to the stream. Edgar followed Flirt closely. She had a full swelling and he clearly wanted to keep her all to himself. Flirt is one of Fifi’s daughters. She was born and raised in Kasekela. She was orphaned when her mother died in 2004, but managed to survive, spending much of her time with older brother Frodo. Flirt is one of the very few chimpanzees I ever saw Frodo groom.

After Flirt reached sexual maturity, she did what female chimpanzees usually do:  transferred to a new community. With Edgar showing so much interest in her, maybe she will conceive an infant soon.

Young male Apple followed them from a distance.  Apple is one of the2014-06-12 Apple sitting in tree A rising generation of Mitumba males, along with Kocha and Ramba. If these boys survive to maturity, then Mitumba may have a chance at keeping Kasekela at bay and maintaining Mitumba as a viable community. But with Edgar’s track record of attacking other males, maybe he won’t let these young competitors survive. If Edgar continues killing all the young males, his line may end in a Pyrrhic victory: eliminating not only his competitors, but his only allies against the mighty Kasekelans.

After coming down from the trees, the Mitumba chimpanzees soon climbed up the steep slide of the hill, giving us a chance to experience some classic Mitumba chimp viewing: crawling on hands and knees through vine tangle.

 

Kalande

On June 10th, I traveled to the south of Gombe to visit the range of the little-known Kalande community of chimpanzees.

Map of Gombe National Park and chimpanzee ranges (from Rudicell et al., 2010)
Map of Gombe National Park and chimpanzee ranges (from Rudicell et al., 2010)

The Kalande community is one of three chimpanzee communities at Gombe.

The most famous, most intensively studied chimpanzees live in the Kasekela community in the center of the park. These are the chimpanzees that Jane Goodall has studied since 1960. They have the biggest range and the most members of the park’s three communities.

To the north of Kasekela live the Mitumba chimpanzees. This is a smaller community, which Deus Mjungu studied for his PhD research. The Mitumba community has fewer chimpanzees than Kasekela, but is still vigorous. They have a small range, but it includes excellent chimpanzee habitat with lots of food trees.

To the south is the Kalande community (also known as Bwavi). Most of the Kalande community’s range is grassland and woodland, with narrow strips of forest along the stream valleys. We know less about the Kalande chimpanzees than any of the others in Gombe. For the most part, these chimpanzees are still unhabituated, meaning they fear people. Researchers can follow the Kasekela and Mitumba chimpanzees around all day long, but they are lucky to get even fleeting glimpses of Kalande chimpanzees.

We aren’t even completely sure how many chimpanzees live in Kalande. Based on sightings and samples of genetically distinct individuals, there seem to be at least 9 chimpanzees in Kalande, but we don’t know for sure.

Skull from a male chimpanzee found dying in Kalande in 1994 or 1995.
Skull from a male chimpanzee found dying in Kalande in 1994 or 1995.

A small team of researchers monitor the Kalande chimpanzees. They collect fecal samples for genetic analysis, which enables us to keep track of individuals, even when we don’t know what they look like. Kalande has the highest rate of infection with the virus SIVcpz, which likely contributed to the decline of this community (Rudicell et al., 2010). Many females have left Kalande for other communities, both as part of the natural emigration process (females usually leave to join a new community when they are sexually mature), and because as Kalande declined, it eventually came to have too few males. Females seem to prefer living in communities with many males, both because many males are better able to defend the feeding territories that females need to survive and raise their offspring, and because females need unrelated males as mating partners. As the number of adult males in Kalande dropped down to one, or perhaps even zero, some Kalande females left for good, while others seem to have kept their Kalande home base, but visit Kasekela for mating.

Kat and Kazi, photographed when visiting Kasekela (20 April 2006)
Kat and Kazi, photographed when visiting Kasekela (20 April 2006)

Kati, for example, is a Kalande resident who has probably lived there since 1998.  Based on genetic data, we think she is the daughter of Patti who was known as Tita when she was younger. Since 2006, Kati has been making occasional visits to Kasekela. I saw her with her young son Kazi on one of these early visits. Of the Kalande chimps, Kati seems to fear people the least, which would make since if she grew up in Kasekela.

Deus and I took the boat to Kalande, where we met Ashaabu, one of the new Kalande research assistants. Ashaabu got his start working as a village Forest Monitor for his village’s forest reserve (part of the Greater Gombe Ecosystem project). Before going into the forest, we talked with Ashaabu for a while about which chimpanzees he has been seeing.

Kazi, who was just a little boy back in 2006, now seems to be the alpha male of Kalande, even though he is just a gawky adolescent. Based on how old Kazi looked back in 2006, I think he must be at least 12 years old now. Ashaabu says Kazi is around the size of the Kasekela male Fundi, who is about 14. The old male Renadi (or Leonard) hasn’t been seen for a number of years now, and I suppose must be dead. There might be another adolescent male, Pamera, but we don’t know for sure if he is still alive. Ashaabu has regularly seen Kati, Kazai, Katarina (Kati’s new baby), a big female without an infant, an adolescent female (who I think might be Pairott), and another young female around Kazi’s size. (Perhaps this is really Pamera? Might be hard to tell he’s a male if he’s still young and seen only briefly from a distance.) Ashaabu also mentioned Obedina, a female who had a big belly last year who might also have a new baby now.

After talking, we hiked into the forest, climbing a steep rocky path into Nyamagoma valley. Nyamagoma is the southernmost valley of the park, just north of Kazinga village. The path wound through an open woodland with a view of the lake below.

Ashaabu collecting Msongati fruits.
Ashaabu collecting Msongati fruits.

Along the way, we collected fruits and leaves for the isotope and nutrition projects. Given that Kalande has so much woodland, it will be interesting to see if the Kalande chimpanzees, or their foods, differ isotopically at all from those in Kasekela.

Ashaabu and Deus below a chimp nest.
Ashaabu and Deus below a chimp nest.

We followed the path down towards Nyamagoma Stream, where tall trees grew, shading the steep valley in green light. We didn’t see any chimpanzees, but we did see a number of nests. Chimpanzees build a new nest (or bed) in trees each night to sleep safely out of reach of any predators that might be lurking about. We found one cluster with five fresh nests, suggesting that up to seven chimpanzees might have slept there (if the group included Kati and Obedina and their new babies). It was encouraging to see so many fresh signs of chimpanzees using this valley. The Kalande community is still hanging in there, and perhaps they might recover, if the Kasekela males don’t catch Kazi and finish him off.

Ashaabu taking data on his tablet.
Ashaabu taking data on his tablet.

Ashaabu carried with him the tablet computer he had used as a Forest Monitor. He used the tablet to take pictures of the nests and enter the data, including GPS locations of the nests. It was quite stunning for me to think each of the villages around Gombe now has its own Forest Monitors, collecting data like this on their own village forest reserves, and loading it up regularly into the Cloud.

 

 

 

 

 

 

 

 

Director of Chimpanzee Research for Gombe Stream Research Centre.

 

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The Declining Effectiveness of Violence

In The Better Angels of Our Nature, Steven Pinker argues that violence has declined over the course of human history. This weekend I had the good fortune to participate in a workshop that Pinker organized at Arizona State University on the Origins of Violence.

A number of the speakers were people whose work was central to the argument of Better Angels, which made the workshop feel a bit like stepping into the world of the book. Kind of like that scene in Annie Hall where a guy in line for the movies is holding forth on Marshall McLuhan’s ideas, and Woody Allen produces Marshall McLuhan in person to give his own views.

In addition to talks that elaborated on research described in Better Angels, there were also some pleasant surprises, particularly from a session of talks on why violence seems be declining, not only in frequency, but also in its effectiveness as a strategy.

In Better Angels, Pinker does a superb job of bringing together evolutionary theory, historical data, psychology and political philosophy to summarize and explain an important and under-appreciated finding: violence has decreased at multiple scales over the course of human history and prehistory. Despite claims on the news and by political leaders that the world is a more dangerous place than ever, the data indicate that the world has actually become substantially less dangerous, across the board.

Better Angels is also a big book (832 pages), and a surprisingly common theme of online commentary is “I haven’t finished reading the book, but here is my opinion of it anyway.” For example, here:  “I still haven’t finished all of its 800 pages,” and here: “I may as well admit that I haven’t read all of Steven Pinker’s new book.” But it’s a book worth reading all the way through.

Like Pinker’s book, the workshop covered a broad range of scholarship: behavioral ecology, ethnography, neuropsychology, political science, and history. A common theme was shared interests in answering big picture questions with empirical work, strong theoretical foundations, and a willingness to follow the results given by data even when contrary to conventional wisdom. Participants also included speakers invited for the Great Debate, a public event held Saturday evening, among them: evolutionary biologist Richard Dawkins, and one of my favorite science fiction writers, Kim Stanley Robinson.

For the workshop, 14 invited speakers each gave a 15-minute talk, followed by 5 minutes of discussion. Talks covered a wide range of material, but roughly speaking, focused on three main topics: (1) the causes of violence, (2) the decline of violence, and (3) the increasing ineffectiveness of violence. My own work focuses on the causes of violence, and Better Angels exhaustively outlines the evidence for the decline of violence, so I was least familiar with the third group of speakers, whose findings I found particularly novel and interesting.

Causes of Violence

The workshop began on Jane Goodall’s 80th birthday, so I thought it was very appropriate that we began with a discussion of chimpanzees killing each other. That was my talk. Richard Wrangham gave the next talk, in which he discussed violence in chimpanzees and warfare in human hunter-gatherers, drawing attention to some key similarities and differences. In both species, imbalances of power reduce the risks to attackers, making it cost-effective to kill rather than merely chase away enemies. In contrast to intergroup aggression in chimpanzees, human warfare appears to be more dangerous for the attackers, since humans are armed with weapons. This raises questions about what motivates people to undertake those risks. According to Wrangham, the answer is cultural rewards: warriors gain net benefits because their societies reward them for their deeds.

The remaining sessions all focused on humans, starting with a talk by Polly Wiessner, who is an extraordinary ethnographer. She has carried out decades of fieldwork both among the Ju/’hoansi Bushmen in Namibia and the Enga in Papua New Guinea. She compared and contrasted patterns of violence in the two societies. The Bushmen have a strong ethic of nonviolence but nonetheless have a high homicide rate, and few social mechanisms for dealing with the aftermath of violence. The Enga have lots of violence, including frequent warfare, but also have elaborate social mechanisms for negotiation and reconciliation.

Rob Boyd argued that group violence is common in other animals, but only because kinship underlies such violence (as in social insects, and competition between clonal colonies of sea anemones). Boyd argued that humans are unusual because we have lots of group violence among large groups of people who aren’t particularly close kin. In contrast to Wrangham, who focused on the rewards gained by human participants in warfare, Boyd focused on punishment as the mechanism that convinces warriors to risk their lives to benefit other group members. (For more on this, see contrasting papers: Matthew & Boyd (2011) and Glowacki & Wrangham (2013))

Adrian Raine talked about evidence from brain imaging studies about the mechanisms underlying criminal violence. Murderers tend to have low prefrontal cortex activity, indicating that they have trouble inhibiting their impulses towards violence. In contrast, psychopaths (such as serial killers) have strong prefrontal cortex activity, but unusually weak activity in key parts of the amygdala, suggesting that they kill because they have a problem feeling moral emotions. They might know that their actions are wrong; they just don’t feel that they are wrong.

David Courtwright spoke on themes developed in his excellent book Violent Land, which describes the impact of having unbalanced sex ratios in frontier societies. The take-home message: gathering lots of young, unmarried men in one place leads to lots of violence, especially if you add alcohol. Courtwright also talked about two other frontiers: air (early aviation was heavily male-dominated) and night (when city streets become more male-dominated and thus more dangerous).

So to summarize the causes of violence talks: the behavioral strategy of violence has deep roots in biology, but in humans we see an unusual level of group-level violence conducted by people who are not necessarily close kin. This seems to be supported by unusual patterns of grouping (reducing the costs of killing), combined with an unusual system of rewarding warriors and/or punishing cowards to promote participation in warfare. Among individuals, violence is promoted by dysfunction of particular brain regions, and among groups, violence is promoted when large numbers of unmarried men are gathered together. Societies may vary greatly in how they handle violence, but people everywhere face both the risk of violence, and the question of how to prevent violence.

Decline of Violence

The central argument of Pinker’s Better Angels of Our Nature is that warfare and other forms of violence have declined over time.  Pinker based his argument on extensive work by a number of other scholars, including several of the speakers invited to the workshop.

One of the key people who has assembled data on the high rates of violence in non-state societies is Azar Gat, author of War in Human Civilization. At the workshop, Gat talked about the ongoing debate between Hobbeseans and Rousseauites over the origins of warfare. Hobbeseans follow political philosopher Thomas Hobbes in arguing that the ancestral condition of humanity was war of all against all. Rousseauites follow Jean-Jacques Rousseau in arguing that the state of nature was a peaceful world of noble savages. Gat argued that while this debate might seem unresolvable, in recent years Rousseauites have shifted their positions, effectively conceding that the evidence indicates humans have a long history and prehistory of some forms of violence, including homicide and feuding, even if Rousseauites continue to disagree about the evidence for early war.  Gat also argued that, in contrast to the Rousseauite anxiety that arguing for evidence of a warlike past promotes a warlike future, the existence of prehistoric warfare is no impediment to building a more peaceful future.

Another key person documenting the decline of violence is Manuel Eisner, who has conducted meticulous studies of European historical records, leading to the discovery of a striking decline in homicide rates in Europe over the past 600 years. He presented new data both on overall homicide rates, and on regicide (take home message: it’s dangerous to be king!).

The workshop also included two people who have documented the recent decline in warfare: John Mueller and Joshua Goldstein. Based on studies of the number of ongoing wars per year, Mueller has concluded that “war has almost ceased to exist.” Great powers no longer engage in direct warfare with each other, and even minor powers seem increasingly reluctant to fight wars.  Goldstein has argued that, despite the US being involved in wars in Iraq and Afghanistan, “the decade since 9/11 has been the most peaceful worldwide in the past century.”

Christopher Fettweis presented detailed work on mortality from warfare over the years in Africa. Back in 1994, Robert Kaplan predicted “The Coming Anarchy,” in which West Africa in particular would be overwhelmed by rapidly growing populations, chaos and warfare. In contrast, what the actual data show is that over the ensuing decades Africa has become increasingly peaceful. Fettweis argued that Africa likely now enjoys a lower risk of death from warfare than ever before. Today, the average person living in Africa is less likely to die from war than the average American is to die from homicide.

So despite claims from political leaders that the world is “more dangerous than it has ever been,”  the risk of dying from violence, including homicide and war, actually has decreased considerably across history and (as far as we can tell) prehistory.

Ineffectiveness of Violence

There are many possible reasons for the decline of violence. One important reason may be the growing recognition that violence is a decreasingly effective way to accomplish goals.

 Max Abrahms argued that based on empirical data, terrorism usually fails to accomplish the goals of terrorists. In particular, killing civilians – the most feared practice of terrorists – is usually self-defeating: groups that kill civilians are more likely to fail in attaining their goals. So why do terrorists ever do this? Abrahms argued that it is mainly the lower-ranking members of organizations with weak leadership that carry out such attacks. Attacking “hard” targets like military bases is, well, hard. It’s easier to attack “soft” targets like cafes and buses, so that’s what low-level terrorists do, especially when their more experienced bosses don’t have sufficient control to stop them. In contrast, the more experienced, higher-level terrorists gradually learn that killing civilians is counter-productive and focus instead on different goals. Based on this finding, the current US strategy of using drones to “decapitate” Al Qaeda by killing leaders is likely increasing, rather than reducing, the likelihood of attacks on civilians by this organization.

Continuing on the topic of terrorists, Adam Lankford took a critical view of suicide terrorism, in the context of self-sacrificial behavior in general. Lankford argued that intentionally self-sacrificial behavior is rare: mother mammals rarely give up their lives to save their young (their fitness benefits more if they live to breed another day), and soldiers don’t fall on grenades. Soldiers do sometimes move towards grenades to throw them back at the enemy, or attempt to smother them with a backpack, helmet or boot, but they don’t lie down on a grenade and wait for it to explode into their bodies. Lankford further argued that many suicide terrorists are really just suicidal. Moreover, Lankford argued that suicide terrorism isn’t terribly effective, that it fails in about half the cases, and even when successful rarely kills many people. Lankford’s results, combined with Abrahms’s arguments that terrorism against civilians is counter-productive, suggest that terrorism is just a really bad strategy.

Instead, based on Erika Chenoweth’s findings, people seeking political change would be better off doing so nonviolently. With a meticulous, skeptically motivated empirical study, Chenoweth found that nonviolent campaigns are about twice as effective as violent rebellions in accomplishing their aims, such as removing leaders and winning territorial independence. Since the end of World War II, nonviolent campaigns have also become more common as a strategy. And unlike violent rebellion, they don’t benefit much from foreign intervention – they need to be home-grown to work effectively. The key seems to be getting a sufficiently large proportion of the society involved: groups with at least 3.5% of the population participating always achieved their aims. Additionally, the methods used to gain power have important impacts on how groups govern if they are successful. Nonviolent groups are more democratic and respect human rights more so than groups that gained power through violent means.

None of this means that we live in a world without danger of warfare and other kinds of violence. The recent events in Ukraine make that clear. Nonetheless, those very events help illustrate some key points. First, the nonviolent protests against Ukraine’s government succeeded in removing an unpopular leader. Second, the subsequent invasion of Crimea by Russia resulted in only two casualties and an election. While the invasion clearly carried the menace of violence, and the election was certainly far from fair, both were a far cry from what would have happened in previous decades.

Given the rhetoric of pundits and political leaders that we live in a dangerous world, it is important to realize that, compared to almost all of recorded history, the world today is remarkably peaceful. Moreover, in today’s world, peaceful methods are frequently more effective than violent ones.

Interested in learning more about what these speakers have to say? Check out the Origins of Violence Reading List!

References

Glowacki, L. and R. W. Wrangham (2013). “The Role of Rewards in Motivating Participation in Simple Warfare.” Human Nature-an Interdisciplinary Biosocial Perspective 24(4): 444-460.

Mathew, S. and R. Boyd (2011). “Punishment sustains large-scale cooperation in prestate warfare.” Proceedings of the National Academy of Sciences of the United States of America 108(28): 11375-11380.

Pinker, S. (2011). The Better Angels of Our Nature: Why Violence Has Declined, Viking.

Hunting and Fighting

In chimpanzees, intergroup aggression and hunting look quite similar in several ways. Both hunts and intergroup attacks are mainly the business of males. In both cases, groups of males climb, leap, and run after a victim, which, if they catch, they will bite and pummel until it stops moving. Attacking males bristle their hair like fighting cats, making them look even bigger than they really are. They bare their teeth in fearful grimaces and scream.

Red colobus leaping to escape chimpanzees.

Since the 1970s, researchers have speculated that hunting and fighting in chimpanzees are related, resulting, perhaps, from the same psychological mechanisms. For example, chimpanzees’ close relatives, bonobos, have not been observed to kill other bonobos, and bonobos rarely hunt. Perhaps the two behaviors are linked? For example, my thesis advisor Richard Wrangham has speculated that as bonobos evolved from a chimpanzee-like ancestor, “males lost their demonism, becoming less aggressive to each other. In so doing, perhaps they lost their lust for hunting monkeys, too.” (Wrangham & Peterson 1996: 219)

Ten years ago, when I was a post-doc with Anne Pusey, in the early stages of extracting records on intergroup aggression from the long-term data at Gombe, I looked at seasonal patterns of intergroup encounters, as part of an effort to understand why such encounters occurred. From the limited sample of years for which I’d extracted data, intergroup encounters occurred most often in the late dry season (September and October) with a smaller spike in the middle of the wet season (February). When I compared notes with Anne’s graduate student, Ian Gilby, I was struck that he had found essentially the same pattern with hunts: the number of hunt attempts per follow peaked in the late dry season and mid wet season. Additionally, comparing five years for which we both had data, we found that years with more hunting success also had more patrols. Was this because hunting and intergroup fighting were caused by the same factors?

We agreed we should work on this further, and now that work has born fruit: a forthcoming paper in Animal Behaviour by Gilby, Wilson and Pusey.

Frodo being groomed by Apollo.

A key inspiration for this work was one male chimpanzee, Frodo. Frodo was the best hunter at Gombe when Ian was doing his dissertation research. Frodo was involved in all four intergroup killings that I described in my first Gombe paper (Wilson et al., 2004). I had watched video, frame by frame, of Frodo brutally attacking a young male chimpanzee from the Kalande community. Maybe Frodo was a great hunter because he was a great fighter? Frodo was also exceptionally persistent in pursuit of estrous females, and genetic studies found that Frodo had fathered more babies than any other Gombe male. I began to wonder if in chimpanzees, hunting and fighting were both byproducts of selection for skills needed to achieve mating success.

Male red colobus monkey jumping down to chase Titan, Fundi and Frodo.

Ian kept Frodo very much in mind when he went on to work as a post-doc with Richard Wrangham on data from Kanyawara. Ian proposed that a key part of group hunting in chimpanzees was the presence of particular individuals who really liked to hunt – which Ian termed “impact males.” As a group-level activity, hunting suffers from potential collective action problems. Hunting is risky and dangerous. Red colobus monkeys don’t want themselves or their babies to get eaten, and they fight back fiercely, biting with their sharp teeth. Individuals therefore might be tempted to free-ride – let others do the hunting, and then get meat afterwards, by begging, stealing, or scavenging what’s left when others are done eating. But if everyone free-rides, no one will hunt. So how does hunting get started? Ian proposed that some individuals, the impact males, are strongly motivated to hunt – and once they get started, others are encouraged to join in, because the costs of joining a hunt already in progress are less than the costs of starting the hunt.

Titan, Fundi and Frodo looking at red colobus monkeys, deciding whether to hunt.
Titan, Fundi and Frodo looking at red colobus monkeys, deciding whether to hunt.

The logic of “impact males” made sense to me, not only for hunts, but also for territorial behavior. Patrolling boundaries is energetically expensive and potentially dangerous, as patrols can meet a big group of hostile neighbors. Patrols therefore seem vulnerable to the same collective action problems as hunts. So maybe they are solved the way: some individual males are strongly motivated to go on patrols, reducing the costs for everyone else to join in. After all, if I know at least one of my buddies is going on patrol, I know I won’t be alone at the edge if I go with him. And perhaps – maybe these are all correlated for the same reason? Maybe the same males are impact hunters and impact patrollers? And maybe these same males are the ones who are really good at winning dominance interactions, gaining high rank, and getting access to fertile females and fathering lots of babies? Maybe these are all correlated traits, parts of an overall personality profile or behavioral syndrome of what it takes to be a successful male chimpanzee?

We now had lots more data to work with than we did a decade ago: 32 years of data on both hunting and boundary patrols. And we found that hunting and patrolling boundaries were indeed correlated, not just on a monthly basis, but also on a daily basis. However, it turned out that hunting and patrolling were mainly correlated because of the influence of other variables. Both hunting and patrolling were more common when males were in large parties – which we expected, because parties with more males are more likely to succeed, both in hunts and in intergroup encounters. But the main reason hunting and patrolling were correlated was because both involved long-distance travel. When chimpanzees patrol their borders, they necessarily travel a long ways. And when they travel a long ways, they are more likely to encounter monkeys.

We found that there were indeed impact males for both hunting and patrolling. But only one of the males who was an impact hunter was also an impact patroller. And surprisingly, good old Frodo was neither an impact hunter nor patroller. He had a positive impact on hunting probability, but not enough to merit status as an impact hunter. And the probability of patrolling was the same, with and without Frodo in the party.

So these results suggest that hunting and fighting, despite their many resemblances, may result from different psychological mechanisms. In some ways this is not really surprising, given that in species whose brains have been studied in detail, such as rats and cats, aggression and predation involve quite different regions of the brain. Additionally, Marissa Sobolewski has looked in detail at the hormones of male chimpanzees from the Ngogo community, going on two different kinds of patrols: hunting patrols, in which males are looking for monkeys, and border patrols, in which males are presumably looking for neighbors. Marissa found that testosterone levels were elevated for border patrols, but NOT for hunting patrols. This strongly suggests that hunting and fighting do indeed result from different psychological mechanisms in chimpanzees.

Our findings from the long-term data are thus rather different from what we thought we’d find. This illustrates the importance of looking at data, not just theory, when doing science. It’s easy to fall in love with hypotheses, but they need to stand the test of empirical work. Nonetheless, I still suspect there may be interesting links in the psychology of hunting and fighting. For example, tendencies towards pugnaciousness and risk-seeking seem likely to benefit both hunting and fighting (and acquiring mates). But from the data we’ve examined so far, being a top hunter doesn’t automatically make one an influential boundary patroller.

(For more on Frodo, see Ian’s biography of Frodo, Lisa O’Bryan’s description of Frodo in retirement and this picture of baby Frodo.)