Category Archives: Intergroup Conflict

Chimpanzees, Humans, Intergroup Conflict

Hawks, Doves, and Golden Rules

Amy Kass, standing on one foot

In college, I took a class taught by Amy Kass called “Human Being and Citizen.” We read and discussed selections from the Bible, Homer, Aristotle, Plato, Shakespeare, Kant, Rousseau, Dostoevsky, and others. After one class, frustrated and confused by the arguments presented in these widely varying texts, I approached Mrs. Kass and asked her,  

“What is your foundation for morality?” 

She looked at me with a wry smile and a glint in her eye, and said, 

“Standing on one foot?”

I understood her point—the impossibility of responding adequately to such a question—but I didn’t get the reference she made until some years later, when I came across a story told in the Talmud (Shabbat 31a) in which a gentile seeking to be converted to Judaism approaches the famed Rabbi Hillel with the challenge, 

“Convert me on condition that you teach me the entire Torah while I am standing on one foot.” 

Rabbi Hillel responded, “What is hateful to you, do not to your neighbor: that is the whole Torah, while the rest is the commentary thereof; go and learn it.”   

Rabbi Hillel lived in Jerusalem during the reign of King Herod, and is thought to have died when Jesus was still a child. Hillel’s version of the Golden Rule would thus belong to a tradition that already existed when, according to Luke, Jesus gave his own version of the Golden Rule in the Sermon on the Mount:

“Do unto others as you would have others do unto you” (Luke 6:31).

Jesus presents the Golden Rule with a positive spin: “do unto others” rather than “do not what is hateful.” Either way, this rule provides a compass for navigating social life, for living well with others. It is so familiar that it seems obvious, even trivial. But it is a radical rule, both in its simplicity, and in its departure from other views of how we should act in the world.  

For example, in Plato’s Republic, written around 375 BCE, Thrasymachus proclaims:

“I say that the just is nothing other than the advantage of the stronger” (338c, p. 15).

In other words: “Might makes right.” 

Another definition emerges in conversation with another character, Polemarchus:          
 “Justice is doing good to friends and harm to enemies” (332d, p. 8).

While Plato doesn’t endorse either of these views, none of the characters in his Republic propose anything quite like the Golden Rule as a guide to living. Even Plato’s hero, Socrates,  comes to a rather modest conclusion: justice is “minding one’s own business” (433b, p. 111).

The Golden Rule may not be a perfect guide to every situation—after all, people’s preferences vary. That would be one reason that holy books contain commentary, not just a simple statement of one rule for living. But the rule provides a good starting point: taking the perspective of other people rather than just looking out for your own interests. 

The chimpanzees that I study in Tanzania act as if they live according to those two views from Plato’s characters: “might makes right,” and “do good to friends and harm to enemies.” Chimpanzees live in a zero-sum world of intergroup relations. A win for my group is a loss for yours. Most of the stories in history books—invasions, conquests, the rise and fall of empires—follow similar zero-sum rules.

This week marks the one-year anniversary of the Russian invasion of Ukraine. Since 24 February 2022, nearly every day has brought appalling new stories. Cruise missiles blasting civilian apartment buildings, schools, and hospitals. Massacres, torture, rape. The war seems anachronistic, a throwback to the tank battles of World War II and the trench warfare of World War I. In 1945, hoping to prevent World War III, the founders of the United Nations agreed to “live together in peace with one another as good neighbours,” vowing “that armed force shall not be used, save in the common interest.” The United Nations Charter signaled a profound shift away from the view that war is a legitimate tool of national interest, to something more like the Golden Rule.

Unlike chimpanzees, we don’t have to fight our neighbors. Instead, we can engage in mutually beneficial exchanges. We help each other out. We can trade information. Where is that migrating herd of wildebeest heading? Where is a waterhole that is not dried up? We can exchange favors. You can come hunt on my land for a while, if you let me take water from your land. We can exchange goods, such as decorative pigments and shells, and material for making tools. The archaeological record in Africa indicates the people have engaged in such trade for hundreds of thousands of years, based on the finding of materials such as obsidian many kilometers their sources—much further than people are likely have traveled on their own.  

As humans, we depend on help from others. In recent centuries, developments in economics and technology have multiplied exponentially the ways in which we depend on others around the globe. A major lesson from the modern world is that everyone gains from living as good neighbors, and everyone loses from war. My hope is that Russia’s invasion of Ukraine will remind the world of the horrible costs of war, resulting in renewed efforts to build an international community based more on the Golden Rule than the law of tooth and claw.  

 Of course, while we can aim for an international community composed entirely of good neighbors, the world we live in contains bad neighbors, who must be dealt with. In the book of Luke, immediately before pronouncing the Golden Rule, Jesus proclaims:

“Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you. If someone slaps you on one cheek, turn to them the other also. If someone takes your coat, do not withhold your shirt from them.  Give to everyone who asks you, and if anyone takes what belongs to you, do not demand it back.”

Perhaps George Price, who underwent a profound conversion to Christianity late in life, had this passage in mind when working on the Hawk-Dove game, which he published in a 1973 paper with John Maynard Smith. In this foundational work of evolutionary game theory, the authors modeled “turning the other cheek” with the strategy of Dove: when conflict arises, retreat from the aggressor. In a world composed entirely of Doves, everyone would benefit: nobody would fight, so nobody would suffer injuries. However, mathematical and computer modeling found that the world of Doves is vulnerable to invasion by Hawks: individuals that always fight to take what they want. Everyone is worse off in a world with Hawks, but Doves alone cannot prevent a Hawk invasion. Doves can, however, invade a world composed of entirely of Hawks, because Doves avoid the costs of constant fighting. Maynard Smith and Price concluded that the Evolutionarily Stable Strategy consists of a mix of Hawks and Doves—or of individuals using a mixture of both strategies. 

In the long peace that followed World War Two, the peoples of Western Europe learned to live as Doves, enjoying the benefits of being good neighbors to one another, rather than seeking conquest and empire. The mostly peaceful collapse of the Soviet Union in 1991 brought with it the hope that Russia too would become a Dove, and everyone would win. With the invasion of Ukraine, however, Russia revealed itself to be a Hawk. So long as Hawks exist, the Golden Rule alone will be an insufficient guide to international relations. To defend against Hawks, nations must instead follow the advice of Polemarchus: “Do good to friends and harm to enemies.” And if the logic of the Hawk-Dove game is sound, that there will always be Hawks.

 (If you are interested in exploring the dynamics of the Hawk-Dove game, see this NetLogo version of the game developed by Kristy Crouse here. )

References

Babylonian Talmud: Tractate Shabbat, Folio 31a https://halakhah.com/shabbath/shabbath_31.html

Bloom, A. (1968). The Republic of Plato. Basic Books.

The Holy Bible, new international version. (2011). Biblica.


Smith, J. M., & Price, G. (1973). The Logic of Animal Conflict. Nature, 246(5427), 15-18.

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Chimpanzees, Intergroup Conflict

Gombe Chimpanzees, Yellowstone Wolves, Agent-Based Models, and the Benefits of Larger Territory Size

12 August 2022

I’m very happy to announce the publication of a new paper from our lab, led by newly minted Ph.D., Dr. Kristy Crouse.

Work on this paper began back in August, 2012, when Kristy emailed me, asking for advice about graduate studies. Kristy had recently earned her bachelor’s degree from the University of Minnesota, where she had taken pretty much all of the classes I had taught over the past two years. When we met, she said, “I have a background in Anthropology and Computer Science. Is there anything I can do with that?” I had some ideas. We began meeting together to discuss them, together with Clarence Lehman, an ecologist who also has a professional background in computer science.

At the time, I had started to compile data on lethal aggression in chimpanzees. Male chimpanzees defend group territories. There are some obvious benefits to having a larger territory. Perhaps the most obvious benefit is that, all else being equal, a larger territory should have more fruiting trees, shrubs, and vines, providing more food for chimpanzees to eat. Analysis of long-term data from Gombe had found that when the territory size was larger, chimpanzees traveled in larger parties, and females reproduced more quickly (Williams et al., 2004). Controlling for age and reproductive state, individuals weighed more when the territory was larger (Pusey et al., 2005).

Another benefit of large territory size occurred to me. Chimpanzees are most likely to meet their neighbors along the periphery of their range. Geometrically, as the territory increases in size, the perimeter increases linearly, while the area of the territory increases by the square of the radius. As a result, with increasing territory size, the periphery should constitute an increasingly smaller proportion of the total range. If intergroup killings occur mainly in the periphery, and the periphery constitutes a smaller proportion of larger territories, then larger territories should provide an additional benefit: more safe area within the territory. As a result, overall risk of death from intergroup aggression should be smaller in larger territories.

An illustration of the conceptual model with small (a) and large (b) square territories.


Another consequence of this occurred to me. If mortality from intergroup aggression is lower in larger territories, then females should have higher fertility. Chimpanzees tend to kill male rivals more often than females, but they do sometimes kill females from other communities. More importantly, attackers often kill the infants of females from neighboring communities. In a larger territory, females should suffer fewer such losses, and so be able to produce more offspring. About half of these will be males, who stay in their birth community and add to the ranks of territorial males. With increased production of male defenders, chimpanzees in larger territories should be increasingly able to win intergroup battles, and thus further increase their territory size. A virtuous cycle would therefore ensue: larger territories reduce intergroup mortality, leading to higher female fertility, leading to faster production of male defenders, leading to greater odds of winning intergroup fights, resulting in larger territories. The rich would just keep getting richer, until some other factor, such as infighting within the group, led to a change, such as a group fission.

The geometry seemed simple enough. But would it hold up in reality? Real territories are not perfect circles. Intergroup killings could take place anywhere, not just along the periphery. We could test the model empirically with data, but good long-term data on such systems are scarce, and sample sizes are bound to be small. This seemed an excellent system to test using agent-based computer models.

Kristy set to work on the project, and soon developed a working model. She created artificial chimpanzees that lived in color-coded territories. They roamed their virtual landscape, and beat up on any neighbors they encountered.

Snapshot of a LethalGeometry simulation.

And sure enough, analysis of the resulting data indicated that per capita intergroup mortality was higher in smaller territories.

Results from the agent-based model


In February, 2013, I presented on this model to the Behavior Group in the Department of Ecology, Evolution and Behavior in Minnesota. In addition to describing results from Kristy’s modeling, I also analyzed data from multiple chimpanzee study sites, collated for another paper (Wilson et al., 2014). Both the empirical data and the modeling data were consistent: individuals in larger territories experienced a lower risk of intergroup mortality.

This seemed to have implications for human societies as well. In a world of hostile neighbors, killing is most likely to occur along the edges of territories, where invaders first encounter defenders. Over historical time, the maximum size of territories has increased, from the home ranges of hunter-gatherers, to the city-states of early agricultural societies in places like Mesopotamia, to the empires that gradually grew and swallowed up those city-states. Are people living within empires safer than people living in smaller states, or in hunter-gatherer societies? Does the virtuous cycle of reduced mortality and increased fertility support the growth of empires?

Kristy applied to graduate school, and I began serving as her co-advisor, along with Clarence. In 2014, while I was on sabbatical at the University of Montpellier in the south of France, Clarence visited with me. We talked about this project we had been working on with Kristy. It seemed like with just a bit more work, we could wrap the paper up and submit it for publication.

Once Kristy started grad school, though, coursework and teaching and grant proposal writing and other pressing matters demanded her time. Moreover, Kristy wasn’t satisfied with her model. She saw ways she could do things more elegantly, so she continued to tinker with it. Work continued on the paper, and graduate student Nisarg Desai joined to help with statistical analysis. Finally, in the fall of 2019, Kristy submitted the manuscript to a high-profile journal. The editor rejected it almost immediately, without sending it out for review. This was discouraging. Kristy set the paper aside for a while to focus on her dissertation research.

We eventually developed the paper in two new ways.

First, we recruited additional collaborators to provide more empirical data. Kira Cassidy had published some very nice work on intergroup aggression in wolves, which parallel chimpanzees in many ways: they defend group territories, and have high rates of intergroup killing (Cassidy et al., 2015). I knew Kira from having served on her master’s thesis committee. I contacted her and asked if she would be interested in collaborating on this comparison with chimpanzees and virtual agents. She agreed, and brought in Erin Stahler, another biologist working on the Yellowstone wolf project.

Second, Kristy was becoming increasingly interested in a fundamental problem of agent-based models. How do you know that the model is doing what it is supposed to do? Every model is a simplified view of something more complex; an abstraction from reality, an approximate match to the real system. As British statistician George Box famously said, “All models are wrong, but some models are useful.” How do you know whether a model you have created is useful?

Other researchers in agent-based modeling and suggested methods for developing models to ensure the they match the target system sufficiently well to provide useful information about that system. How exactly to follow those suggestions, however, was not always clear from the existing literature. So Kristy took our paper as an opportunity to explore these issues in more detail. We had a simple geometrical model, an agent-based model, and two sets of empirical results from different species. These different sets of information provide ways to check one another, to give us more confidence that what we are modeling applies to real-world systems.

We submitted this revamped version of the paper to Ecological Modelling, where it has now been published.

We hope this paper will be useful to others, both for those interested in further exploring the impacts of territory size on mortality and those seeking to develop agent-based models of other systems. 


References

Box, G. E. (1976). Science and statistics. Journal of the American Statistical Association, 71(356), 791-799.


Cassidy, K. A., MacNulty, D. R., Stahler, D. R., Smith, D. W., & Mech, L. D. (2015). Group composition effects on aggressive interpack interactions of gray wolves in Yellowstone National Park. Behavioral Ecology, 26(5), 1352-1360.

Crouse, K. N., Desai, N. P., Cassidy, K. A., Stahler, E. E., Lehman, C. L., & Wilson, M. L. (2022). Larger territories reduce mortality risk for chimpanzees, wolves, and agents: Multiple lines of evidence in a model validation framework. Ecological Modelling, 471, 110063. https://doi.org/10.1016/j.ecolmodel.2022.110063

Pusey, A. E., Oehlert, G. W., Williams, J. M., & Goodall, J. (2005). Influence of ecological and social factors on body mass of wild chimpanzees. International Journal of Primatology, 26(1), 3-31.

Williams, J. M., Oehlert, G. W., Carlis, J. V., & Pusey, A. E. (2004). Why do male chimpanzees defend a group range?. Animal behaviour, 68(3), 523-532.

Wilson, M. L., Boesch, C., Fruth, B., Furuichi, T., Gilby, I. C., Hashimoto, C., … & Wrangham, R. W. (2014). Lethal aggression in Pan is better explained by adaptive strategies than human impacts. Nature, 513(7518), 414-417.



Intergroup Conflict

To End War, We Need to Understand Its Origins, Evolution and Causes

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Michael L. Wilson and Richard W. Wrangham

Embed from Getty Images

In a recent opinion piece for Scientific American, John Horgan writes “Ending war won’t be easy, but it should be a moral imperative, as much so as ending slavery and the subjugation of women. The first step toward ending war is believing it is possible.” As part of his argument he declared that “far from having deep evolutionary roots, [war] is a relatively recent cultural invention.” We cherish the goal of ending war, but regard the claim of war being independent of our evolutionary past as unjustified, irresponsible and improbable. It is possibly based on a misunderstanding of what it means for a behavior to have evolutionary roots, but regardless of its derivation, we see it as dangerous. The problem is that if Horgan’s view prevails, we close our minds to potentially important contributions towards understanding the causes of wars.

Like any other sane individual aware of the power of nuclear weapons, we of course agree that is desirable to reduce major wars to zero. We consider this goal to be part of the “possibilist agenda” —  an approach that begins by ruling out what is impossible, given the laws of the universe, rather than pessimistically focusing on what results are most probable to occur. 1  Is ending war possible? Well, we don’t know of any laws of physics, or even principles of evolutionary biology, that would make it impossible to end war. The task before us remains finding the best means to achieve this goal. This includes gaining a clear, scientific understanding of what war is, what its causes are, and what factors can prevent it.

There are reasons for optimism. The examples Horgan cites shows that extended peace is a reasonable possibility, given that wars have been absent for decades in parts of the world. As Stephen Pinker described in exhaustive detail in his 2011 book, The Better Angels of Our Nature, deaths from international warfare declined steeply after 1945. Many of the richer countries that formerly engaged in long and bitter wars with one another — the United States, Japan, Germany, France, England — are currently closely allied in long-term friendships. The United States and Canada, for example, haven’t fought each other in earnest since the War of 1812, and as each other’s largest trading partners, benefit greatly from trading rather than invading. At the same time, the rise of authoritarian and nationalistic leaders worldwide, and events such as Russia’s invasion of Ukraine, provide reasons for concern. 

We also agree that much about war involves “relatively recent cultural inventions.” Certainly contemporary weapons are entirely different from those used by the hunter-gatherers of Nataruk, Kenya, 10,000 years ago to execute their enemies, or by the several hunter-gatherer societies of India’s remote Andaman Islands who lived in permanent war with each other. Equally novel are the hierarchies that give leaders power to order warriors into battle, and the military training that promotes efficient tactics, and ideologies that encourage self-sacrifice even to the point of suicide, and the assembly of huge armies. There are many such novelties. But so what? They cast no direct light on the question of the antiquity of war, or more importantly on the question of whether the possible practice of lethal intergroup aggression by our remote ancestors has left a legacy in the human psyche.

Whether war has deep evolutionary roots remains an unsettled topic, contrary to Horgan’s assertions. For example, Horgan links to his 2016 blog post, in which he describes a study led by Hisashi Nakao of skeletal trauma in hunter-gatherers living in Japan 12,000 to 2,800 years ago, which reports that only a small proportion of skeletons showed evidence of violent death. However, as noted in a 2020 review by Mark Hudson and colleagues, “[i]ndividual finds of arrowheads embedded in human bones and other similar, clear-cut traces of violence have long been known in Japan as elsewhere.”  Although Nakao and colleagues consider their findings to represent a low rate of violence,  their sample includes cases of “blunt force injuries to the cranium and embedded stone and bone projectile points in the post-crania with no signs of healing – providing a prevalence of 1.8 per cent of adults dying violently.”   

Do these data indicate a high or low rate of death from violence? The actual rate of deaths from violence is difficult to determine from archaeological evidence alone: not all deadly violence damages bones, and only a fraction of deaths end up in the archeological record. But if we take the rate of 1.8% adult deaths from violence at face value, this is about 2.5 times the rate of death from homicide in the United States in 20202, a year marked by a dramatic increase in violence. What does this tell us about the evolutionary roots of war? It provides additional evidence that people in prehistory faced a threat of death from violence, but also provides additional evidence that rates of violence vary across space and time, for reasons that are worth exploring further. 

For anyone seriously interested in understanding the causes of war many other kinds of evidence are thought-provoking. Warfare is documented among hunter-gatherers in every inhabited continent, and was especially prominent where the neighbors were also hunter-gatherers rather than dominant farmers. Chimpanzees, our evolutionary cousins, practice a form of intergroup aggression that is strikingly similar to the hit-and-run aggression characteristic of human small-scale societies. In chimpanzees and in humans the initiators and the victims of lethal intergroup aggression are overwhelmingly males. In both species, males are more aggressive than females, a difference that starts long before maturity.

Why are chimpanzees and humans two of the few species of mammals that regularly take advantage of opportunities to kill members of neighboring groups? Are the reasons wholly different in the two species, or do they share some aspects of motivation and explanation, in which case, what are they? To rule out such questions is unworthy of an essay in a scientific journal. They concern topics being actively explored in behavioral ecology, evolutionary psychology and the neurosciences, among other areas.

There is much to learn, but we believe that some current hypotheses are helpful. One is that warfare has evolved from the human ability for proactive aggression. Proactive aggression involves making plans to attack vulnerable victims using overwhelming power, a practice followed by chimpanzees and humans. It benefits the aggressors by leading to increased territory size, or greater safety. Importantly, however, proactive aggression is very predictably inhibited when potential victims turn out to be sufficiently well defended that the aggressors risk getting hurt. Thus, even though proactive aggression is reined in for most of the time, it pays dividends under some circumstances; and human brains are likely very sensitive to such assessments. Would Russia have dared to invade Ukraine in 2014, much less escalated its attack in 2022, if NATO had accepted Ukraine’s application to join in 2008?

The challenge of preventing major wars is mainly undertaken by politicians and lawyers, but we think that every contribution might help. If a deep legacy of using coalitionary proactive aggression has left its mark on the human psyche, what does that mean for differences in war motivation between men and women? Or for how war leaders perceive the costs of wars in which they are directors rather than participants? Or for the point at which leaders will perceive the benefits of peace as outweighing the costs of war? Or for how individuals categorize others as friend or foe? Is it possible that the psychological mechanisms for evolutionarily-based risk assessments are predictably affected by certain medicines that could make leaders more or less risk-prone? If so, we should all be especially alert to a leader’s ailments. To ignore such questions is to bury one’s head in the sand.

Retaining an open mind about the etiology of war is surely the wisest course in a world still threatened by major conflict, despite the many institutional efforts, from the UN down, to control it.

Additionally, whether war has deep roots in our psyches, or is a recent invention like agriculture and empires, evolutionary game theory suggests that in looking to the future, we need to consider, not just our hopes and dreams, but also the strategies available to all players in the game. Players don’t act in a vacuum: they act in a world of competing strategists. We may prefer to plan for peace rather than war, but if other players pursue war as a strategy, we must either prepare to oppose them, or be defeated by them.

Russia might one day fulfill its promise of becoming a Canada to Europe and Asia: a peaceful, prosperous neighbor to the north. In the meantime, however, Russia’s leaders remain trapped in a zero-sum view of the world, seeing neighboring states more as opportunities for conquest than commerce. 

As an example of a fatalism that will lead to more war, Horgan offers this quotation from Kaja Kallas, the prime minister of Estonia: “Sometimes the best way to achieve peace is to be willing to use military strength.” People living in Estonia, however, have bitter historical experience with being invaded and occupied. If you live next door to Putin’s Russia, rather than Trudeau’s Canada, the best way to prepare for peace may indeed be to prepare for war. International peace will not be achieved by simply giving in. Peace among nations surely requires making the costs of war so high that when appeals to law or morality fail, potential aggressors recognize that it is in their own best interests to find a constructive solution.

References

Horgan, J. (2016, April 4). Japanese study deals another blow to deep-roots theory of war. Scientific American Blog Network. Retrieved May 12, 2022, from https://blogs.scientificamerican.com/cross-check/japanese-study-deals-another-blow-to-deep-roots-theory-of-war/ 

Horgan, J. (2022, April 27). Will war ever end? Scientific American. Retrieved May 12, 2022, from https://www.scientificamerican.com/article/will-war-ever-end/ 

Hudson, M., Schulting, R., & Gilaizeau, L. (2020). The Origins of Violence and Warfare in the Japanese Islands. In G. Fagan, L. Fibiger, M. Hudson, & M. Trundle (Eds.), The Cambridge World History of Violence (The Cambridge World History of Violence, pp. 160-178). Cambridge: Cambridge University Press. doi:10.1017/9781316341247.009

Nakao, H., Tamura, K., Arimatsu, Y., Nakagawa, T., Matsumoto, N., & Matsugi, T. (2016). Violence in the prehistoric period of Japan: The spatio-temporal pattern of skeletal evidence for violence in the Jomon period. Biology Letters (2005), 12(3), 20160028.

  1.  “Following the possibilist agenda means working tirelessly to imagine both possibilities and impossibilities and then laying plans to arrange events so that the desirable can be realized and the undesirable avoided—working to avoid unintended consequences. In this way, by superposing such thoughts onto recognized physical–biological–social problems of the world . . . seemingly intractable problems may have possibilist solutions.”  PNAS [↩]
  2. Proportion of deaths in the United States from homicide = 0.73% in 2020, based on 24,576 homicide deaths and 3,358,814 total deaths [↩]
Aggression, Intergroup Conflict

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.

 

Intergroup Conflict

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.

 

Intergroup Conflict, Language and communication, Life Histories, Nutrition

Planet of the Apes

1 Comment

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

 

 

 

 

 

Gombe, Intergroup Conflict

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.

 

Conservation, Gombe, Health, Intergroup Conflict, Isotopes

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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Intergroup Conflict

The Declining Effectiveness of Violence

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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.

Gombe, Intergroup Conflict

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.)