Category Archives: 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.







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.
The first Congo war.









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.







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.






 There they find a single blue male off by himself.






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







Now Blue only has 9 males.






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.

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.

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.








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.









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.








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:







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.







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?








War is natural, but it is not inevitable.

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

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

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

A place to stay.

Enough to eat.

Somewhere old heroes shuffle safely down the street.

You can relax on both sides of the tracks

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

And everyone has recourse to the law.

And no one kills the children anymore.


Evolution and Warfare Workshop at NIMBioS

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

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

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

Descriptive flyer (PDF)

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


Planet of the Apes

28 July 2014

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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







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.



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.





The Declining Effectiveness of Violence

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

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

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

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

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

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

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

Causes of Violence

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

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

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

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

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

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

Decline of Violence

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

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

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

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

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

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

Ineffectiveness of Violence

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

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

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

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

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

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

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


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

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

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

Hunting and Fighting

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

Red colobus leaping to escape chimpanzees.

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

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

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

Frodo being groomed by Apollo.

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

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

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

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

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

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

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

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

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

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

Fighting over Food

Why do chimpanzees get into fights with their neighbors? As we report in this month’s Animal Behaviour, one of the major reasons seems to be food.

Since the 1970s, researchers have known that male chimpanzees defend group territories, and that fights between groups can be deadly. But what triggers these fights? Do chimpanzees go looking for trouble? Do they get into fights over mates? Or do they fight over some other resource, such as food? And when they do meet the neighbors, what determines whether they fight or flee? Are they more likely to respond aggressively if they are defending mates, or young infants, or food? Or does strength in numbers matter more? We sought to answer these questions, using 15 years of data on social behavior and ecology.

These data come from the Kanyawara community of chimpanzees in Kibale National Park, Uganda, where I did my PhD dissertation research as a student of Richard Wrangham and Marc Hauser. For the past decade, I’ve been doing fieldwork in Tanzania instead of Uganda, but I’ve continued to collaborate with Richard and his team. Such collaboration is made easier now that the long-term data have been entered into an Access database, and the original datasheets are all scanned, making it a simple matter to consult the original records from anywhere with good internet access.

Working with these data brought back many vivid memories, such as the day I first saw chimpanzees in the wild, back in June 1996. I had come to Kibale to spend a summer doing pilot research. We hiked for hours in the forest to find the chimpanzees, following narrow trails up and down steep hills, stepping carefully along narrow logs lain across boggy patches in the valley bottoms. Everything looked green and wet. Towering rainforest trees hung with vines. Dense stands of wild ginger and other leafy tropical plants crowded the understory.

As we crept along a steep trail, an opening in the trees showed the misty green expanse of the valley below. At last, I caught my first fleeting glimpse of a chimpanzee – I think it was Stout, an adult male – as he crossed the trail. His long black hair glistened with rain as he knuckle-walked calmly but quickly through the forest, looking more like a gorilla than I had expected, having only seen chimpanzees in movies and zoos.

During my first weeks at Kanyawara, the chimps traveled in large parties with many males. I saw most members of the community, including old Lamy, with his crippled foot, new mothers Outamba and Tongo carrying their first infants, matriarch Lope and her family, including the gangly teenager Makoku and young Rosa, grey-bearded Stocky, tough young males Imoso and Johnny, and Big Brown, who was still the alpha male.

During my first few days in the field, the chimps hunted and killed a red colobus monkey, and had a brief, noisy, and (for me) completely confusing encounter with their neighbors, during which the stranger chimps came within sight of us, crashing through the underbrush, displaying at and running towards our chimps before running they turned and ran away. As it turns out, I had come just in time for the last weeks of Uvariopsis season.

Uvariopsis congensis is a small tree that grows in large groves in the understory of the forest. It tends to fruit synchronously, usually in May and June. The fruits are small and red, and look rather like fleshy red peanuts, with two or three seeds.

At Kanyawara, the Uvariopsis groves occur mainly in the southeast of the range, along the border with another chimp community, which has not been habituated. And it turns out that that these Uvariopsis groves are a key focus of intergroup competition.

Back when I was doing my dissertation work, the number of intergroup encounters that had been observed at Kanyawara was still too small for meaningful statistical analysis. But now, in the 15-year dataset, we have records of 120 intergroup encounters. As is typical for chimpanzees, most of these encounters were shouting matches rather than full-fledged fights – chimpanzees gave loud pant-hoots and waa-barks and other calls, challenging their neighbors, and then ran either towards or away from them. But when chimpanzees came close enough to see each other, they were always hostile. Two chimpanzees were killed, and several others badly injured.  We found that these encounters happened mainly when chimpanzees visited border areas, especially the southern border.

So why did males visit border areas in the first place? To find mates, when few females in their own community were receptive? To find neighbors to attack, whenever they could gather in big enough parties to have a good chance of beating the neighbors? Or to look for food?

To answer these questions, we needed to use multivariate statistics to consider all these factors at once: mates, party composition, and food. We knew that male chimpanzees were highly motivated to search for mates, and we had previously found that parties that visited the range periphery had more males, presumably because that provided safety in numbers. But we also suspected that food would be an important part of the answer.

Chimpanzees spend most of their time searching for food, eating food, or digesting food. Though chimpanzees eat a variety of things, including monkeys, honey, ants, termites, leaves and stems, they spend most of their time eating ripe fruit from trees. In a tropical forest, the availability of tree fruits varies in a complex way over space and time. Some trees occur in groves in particular areas, whereas others are scattered about the forest. Some tree species, like figs, may produce food throughout the year, but other species fruit synchronously, with a big crop occurring every one or two years. To test how food influences chimpanzee movements, we needed data on both where and when key foods were located.

We had lots of data on feeding behavior. But it could be that chimpanzees travel to different parts of their range for social reasons (such as looking for mates or checking on the neighbors), and just happen to eat whatever they find along the way. So we needed to test whether the feeding data reflected where food was actually located. We therefore examined data from vegetation plots, and confirmed that both feeding data and vegetation plot data agreed that certain tree species were more common in the north, while others were more common in the south. For key species, we also found a good correlation between feeding behavior and independent measures of whether a sample of those trees had fruit in a given month. Additionally, the strong correlation between feeding records and counts of seeds from dung samples gave us confidence that our data on feeding behavior provided a good measure of what chimps were actually eating.

So what did we find?

Males tended to stay closer to the range center, and were less likely to travel to the south, when they were with more sexually receptive females. Party composition also mattered: as we had found previously, chimps were more likely to visit dangerous border areas when in parties with many males. And as we suspected, food had a big impact on chimpanzee movements. They visited the southern border mainly when southern fruits were in season. One species in particular had a strong effect on encounter rate: Uvariopsis. Most of the intergroup encounters took place in the southeast, in or near the Uvariopsis groves, and encounters were more likely to occur on days when chimps spent more time eating Uvariopsis.

So chimpanzees at Kanyawara visited borders for various reasons, but food seemed especially important. In particular, a good crop of Uvariopsis attracts chimps from both sides of the border, resulting in a spike in the rate of intergroup encounters during Uvariopsis season.

When encounters did occur, though, we found that the response depended mainly on the number of adult males that were present. Whether they were with mates, infants, or food didn’t matter as much as whether they had enough males to put up a good fight. These results were very much in line with what we had previously found with the playback experiments.

So is food generally the main driver of intergroup competition in chimpanzees? I’m currently working on data from Gombe to test whether similar factors apply there. It seems likely that in addition to food, the relative power of communities is a key factor. An interesting case in point is just 12 km from Kanyawara: the Ngogo community, studied by John Mitani and David Watts. This is the biggest chimpanzee community ever studied, with about 150 members. These chimps live in a range about the same size as Kanyawara’s, but in higher quality forest, with a lot more food, and three times the chimpanzees. And a lot more violence. The Ngogo chimps frequently patrol their boundaries, and in a ten-year period, Ngogo males killed 21 of their neighbors.

While the Ngogo chimps were killing their neighbors and expanding their range, the Kanyawara chimps were losing territory to their powerful southern neighbors (a group of unhabituated chimpanzees whose range appears to be sandwiched between Ngogo and Kanyawara). By 2006, the Kanyawara range was less than half as big as it had been in 1998, and the southern chimps had pushed the boundary a full kilometer north. It may be that chimps in a powerful community, like Ngogo, are more likely to go looking for trouble, whereas chimps in a weak community, like Kanyawara, visit dangerous border areas only when the abundance of food there makes the risk worthwhile.

Evolution is not just for Democrats

Reading the news these days, one might get the impression that evolution is just for Democrats, not Republicans. For example, in a recent (23 August 2011) post for the Washington Post, Richard Dawkins calls Republican Governor of Texas a “fool” and an “ignoramus” for expressing some doubts about evolution, and implies that these labels apply to anyone voting Republican.  The next day (24 August 2011), Ann Coulter published her own post, calling Richard Dawkins “retarded” and trotting out a series of tired old arguments against evolution.

Coulter is just plain wrong on evolution. At the same time, though, Dawkins is wrong to disparage the political opinions of people who happen to vote Republican.

Coulter is wrong about evolution in many ways. For example, she claims that if “Darwin were able to come back today and peer through a modern microscope to see the inner workings of a cell, he would instantly abandon his own theory.” Darwin, however, spent many years peering through microscopes at the inner workings of barnacles, and it was the fascinating complexity of their anatomy that deepened his understanding of evolution. Modern understanding of the inner workings of cells has further deepened scientific appreciation for how evolution works. The patterns of DNA encoded within each living cell have confirmed Darwin’s insight that every living thing on earth is part of one big family, the Tree of Life.

The Tree of Life

Comparing the similarity of DNA sequences has revealed some interesting surprises – for example, humans and chimpanzees are closer kin than chimpanzees and gorillas, even though gorillas look basically like giant chimpanzees – but has also generally confirmed the big picture of the tree that would be expected from shared descent with modification: humans and chimps are twigs on the primate branch, sprouting from the mammalian limb, emerging from the great trunk of vertebrate life, which near the roots of the tree joins with other great trunks and side branches: fungi, plants, and a variety of different bacteria.

Similarly, Coulter brings out the old argument from probability theory, that “it is a mathematical impossibility, for example, that all 30 to 40 parts of the cell’s flagellum – forget the 200 parts of the cilum! – could all arise at once by random mutation.” But as Dawkins clearly explains in The Blind Watchmaker – which Coulter doesn’t seem to have read, or at least not understood – evolution doesn’t throw together everything at once. Evolution works with small gradual changes, building on what exists already. It would indeed be statistically highly improbable to throw even a small number of things together at random and have them work together at all, much less well. But evolution doesn’t do that. Instead, evolution blindly, mindlessly tinkers with what already exists, changing a little bit here and there at random, and then lets these different variants fight it out in the arena of natural selection.

Coulter has made a career of strident political writing, and while the tone of her piece may be more appealing to those who already agree with her than persuasive to her opponents, it is at least consistent with her genre. Richard Dawkins, however, is a scientist. Indeed, he is one of our clearest thinkers and writers on evolution. It is disappointing that he here conflates views on evolution – which he rightly calls a scientific fact – with views on politics – which are, after all, opinions.

We have abundant evidence from fossils, geology, genetics, molecular biology, physiology, development, biogeography, and numerous other fields of study that evolution has occurred and continues to occur all around us. In contrast, while political opinions bear some relation to facts, they nevertheless pertain mainly to matters about which people with intelligence, experience and expertise continue to debate vigorously. Political questions are generally much more complicated and harder to be sure of than questions in the natural sciences, and they often relate to values – what people care about – rather than things that can be objectively determined to be true or false. Many political questions concern economics – which is routinely insulted as “the dismal science” because economists have such a great diversity of opinions. And why is economic opinion diverse? Is it because economists are more stupid or quarrelsome than other academics? Or is it because economies are just incredibly complex and difficult to understand?

Contrary to what readers of Coulter and Dawkins might be led to believe, evolution is not just for Democrats. The scientific truth of evolution doesn’t depend on a person’s political affiliation. People with all sorts of different political views have made important contributions to evolutionary theory. The great population geneticist J.B.S. Haldane was an idealistic Marxist and from 1937 to1950 a member of the Communist Party. Haldane’s colleague Ronald Fisher, described by Dawkins as “the greatest biologist since Darwin,” was politically conservative. Nobody today much cares about Haldane’s or Fisher’s political views. It’s their scientific ideas that have survived the test of time.

Darwin’s own political views might be hard to categorize today, because political views change greatly over time. Darwin shared Lincoln’s exact birthday (February 12, 1809) and Lincoln’s abhorrence of slavery. Maybe, if Darwin had been American, he would have voted Republican. I don’t know. But we don’t remember Darwin for the stances he took on the pressing political issues of his day. We remember him for his idea of evolution by natural selection, which remains just as powerful today as it was 150 years ago, and will continue to be so 150 years from now, or 150 thousand years from how, when the political issues we care so much about today will long be forgotten.

Indeed, evolution by natural selection will necessarily occur wherever life exists in the universe. If intelligent beings exist on a planet orbiting, say, Alpha Centauri, we can be sure of two things: (i) life on their planet undergoes evolution by natural selection and (ii) the question of whether to vote Republican or Democrat will be entirely, er, alien to them.

Hyenas can count

A new study in press at Animal Behaviour shows that hyenas can count. This study builds on work done in other animals, including work that I did on chimpanzees, and finds similar results: animals that fight in groups don’t like to pick fights if they seem to be outnumbered.

Game theory predicts that animals should use numerical assessment when deciding whether to get into a fight with a rival group. You generally don’t want to fight you can’t win. I found this to be the case with chimpanzees, and other studies have found this to be the case for lions and howler monkeys too. Game theory predicts that hyenas should do this too.

To test this prediction, Sarah Benson-Amram and others on Kay Holekamp‘s research team at Michigan State University did playback experiments with hyenas at Masai Mara in Kenya. A write-up of this study in Nature discusses how a particularly nice feature of this study is the way they set up the experimental stimuli. Previous studies have simulated larger groups by playing back recordings of multiple individuals calling at once. This is, in fact, what often happens in the wild – members of groups of lions, chimpanzees, howler monkeys, wolves, hyenas and such often call at the same time, probably to announce to the world that they are in a big group, so watch out. But experimentally, this raises the question of whether the animals are responding to the number of individuals, or just the total amount of noise in the vocalization. This study very elegantly disentangles these variables by always playing three non-overlapping whoops in each playback. In the single-intruder playback, the three whoops are by one individual. In the three-intruder playback, the three whoops are by three different individuals. And on hearing these playbacks, the hyenas responded differently to whoops by different individuals. When they heard the same individual calling repeatedly, their vigilance response decreased. But when they heard a new individual calling, they looked longer at the speaker.

So for hyenas to respond differently to the different numbers of callers, they have to be keeping track of the different individuals that call. This is trickier than simply distinguishing between a single caller and a chorus of callers – but it makes sense that hyenas should be able to do this, since their livelihoods depend on defending group territories and keeping track of how many rivals they face, even if they’re not all calling at once.

This method has the happy byproduct of also demonstrating that hyenas can tell individuals apart from their calls. It makes sense that they should be able to do this – but it’s nice to have experimental support for this prediction.