Category Archives: Chimpanzees

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, Gombe, Language and communication

Do Chimpanzees Have Dialects?

I grew up speaking English, first in Minnesota, then in central Illinois. On visits back to Minnesota, friends teased me for having a “southern” accent, like when I described a gar fish as looking “right like an alligator.” I encountered other varieties of English when we moved to Indiana — twangier than the downstate Illinois drawl — and when I went to college in Chicago. But these varieties of American English paled in comparison to what I encountered in Britain, when I spent a year as an exchange student at Cambridge University. As a fan of British television shows (Monty Python, Doctor Who, David Attenborough) and music (the Beatles, Pink Floyd), I expected to understand what people were saying. It turned out to be much trickier than I expected. Instead of the Received Pronunciation of BBC broadcasts, I encountered rapid speech with unfamiliar slang (“naff,” “grotty,” “phoar!”), new vocabulary words (“bog,” “crisps,” “snog”), expressions (“over the top,” “get off with,” “taking the mickey”), with consonants often ghosted as glottal stops. And there wasn’t just one variety to learn. People from different cities, regions, islands and social classes all spoke differently. Chris from Birmingham (“Brum”) spoke in a rapid series of unintelligible words and knowing looks that often left me completely baffled. 

Different varieties of languages – dialects – likely emerge as an inevitable consequence of cultural evolution. People learn their home language from their parents, but they don’t learn it exactly the same. They add new words and phrases and ways of speaking that they pick up from their friends, and in the modern world, other sources like books, television, and the internet. As these differences accumulate, ways of speaking diverge. Descent with modification leads to the formation of new languages, just as in biology it produces new species.

Do other animals have dialects? Peter Marler, a pioneer in the study of animal behavior, demonstrated that some animals do. Songbirds have two main categories of calls: short, simple calls for everyday use, such as predator alarm calls, and more elaborate songs, mostly used during the mating season. Birds sing to defend territory, attract mates, and once mated, to communicate with their partner. Over the course of a long career, Marler conducted an elegant series of field studies and laboratory experiments, demonstrating that in species such as white-crowned sparrows, songs must be learned. A sparrow raised in isolation produces abnormal songs, but if he hears recordings of songs while growing up, he produces a normal song. Because sparrows learn their songs, local variants emerge and gradually change over time, producing regional dialects (Marler, 1970). (Here’s a video describing recent work on dialects in white-throated sparrows.)

Subsequent studies found evidence of such ability to modify calls — vocal learning — in some other species. Humpback whales, for example, sing long, complicated, haunting songs during their mating season. Like in sparrows, males do most of the singing. Whale songs vary not just by region, but also by year. In a given year in Hawaiian waters, male humpbacks sing songs that closely resemble one another, but which differ from what the whales are singing off the coast of Australia — and which also differ from last year’s favorite styles in Hawaii (Mercado et al., 2004). 

Do species more closely related to humans, such as chimpanzees, show any evidence of vocal learning and dialects?

Marler pioneered the study of primate communication as well as birdsong. He visited Gombe National Park, Tanzania, in 1967, where he carried out the first modern field study of chimpanzee vocal communication. Based on his recordings from Gombe, Marler identified a set of 13 basic calls for chimpanzees. He also found that chimpanzee calls generally resembled those of gorillas (Marler, 1976). The similarity of calls among these apes suggests they are mainly under genetic control, rather than learned. However, John Mitani, working as a post-doctoral researcher with Marler, found some evidence for dialects in chimpanzee pant-hoot calls (Mitani et al., 1992). 

When Jane Goodall gives public talks, she often begins by giving a pant-hoot: a loud call that begins with soft hoos, followed by grunts, barks or screams, and often ends with grunts again. (Jane usually leaves out the screams, though, as she thinks those sound too aggressive.)

Spectrogram of chimpanzee pant-hoot call, from Desai et al. (2022)

Pant-hoots are loud, and enable chimpanzees to communicate over long distances through the forest. Mitani recorded pant-hoots from chimpanzees at Mahale Mountains, some 160 km south of Gombe, and compared them to Marler’s recordings of pant-hoots from Gombe. Analysis revealed some subtle differences: Mahale chimpanzees produced shorter grunts at a faster rate than Gombe chimpanzees, and produced pant-hoots with higher-pitched screams (Mitani et al., 1992). Later studies reported similar geographic variation in calls among groups of captive chimpanzees (Marshall et al., 1999), between chimpanzees in Uganda and Tanzania (Arcadi et al., 1996), and among communities of chimpanzees at Taï Forest in Côte d’Ivoire (Crockford et al., 2004). 

The idea that chimpanzees have distinct pant-hoot calls is an intriguing one. Moreover, the study led by Cathy Crockford (2004) reported that neighboring communities differed from one another more than they did from more distant communities. This suggested that chimpanzees produced calls that announced their community membership to other chimpanzees. This would make sense, given the hostile relations between chimp communities. A pant-hoot call would function like a sports team uniform or national flag, announcing the caller as a member of team Mitumba or Kasekela. Perhaps vocal learning evolved in our own ancestors in response to selection pressure from intergroup aggression?

As a graduate student, I conducted a series of playback experiments with chimpanzees in Kibale National Park, Uganda (Wilson et al., 2001). I used recordings that Mitani had made of chimpanzees from Mahale. In each experiment, I played back a single pant-hoot from a Mahale chimp from a speaker hidden hundreds of meters from Kibale chimpanzees. This single simulated intruder provoked an immediate and striking response. If the listening party consisted only of females, they silently climbed down from the tree where they had been resting or feeding and moved off in the opposite direction. If one or two males heard the call, they stayed silent, and either stayed in place, looking towards the source of the call, or slowly approached the speaker. If three or more males heard the call, though, they immediately gave a loud vocal response and rapidly approached the speaker, as if they were looking for an intruder to attack. So chimpanzees can clearly tell friend from foe by their voice. But were they responding to differences in dialect? Or just differences in familiar versus unfamiliar individuals?

As Mitani and his team continued to analyze calls from different chimpanzee sites, though, they dialed back on claims of dialects. When Mitani began recording calls at Kibale, he found that chimpanzees in Kibale do produce pant-hoots with build-up elements, despite the previous report that they don’t (Mitani et al., 1999). They also found that much of the variation in acoustic structure occurred within individuals, rather than between communities. 

So, do chimpanzees have dialects or not? In an effort to answer this question, I worked with my graduate student, Nisarg Desai, and a team of Tanzanian field assistants to record and analyze chimpanzee vocalizations. Team members followed chimpanzees through the forest, carrying a hand-held “shotgun” microphone and a digital recorder, recording as many calls from each individual as possible.

Three researchers at Gombe National Park, Tanzania
The Chimpanzee Dialects Project team at Gombe: Nasibu Madubmi, Hashimu Salala, and Nisarg Desai.

When Nisarg analyzed the data, he found that individuals varied a lot in the calls, but community membership explained little of the variation between the calls. Analyzing calls that Pawel Fedurek had recorded in Kibale, Nisarg again found lots of variation among individuals, but no clear geographical variation (Desai et al., 2022). 

Acoustic analyses from Desai et al (2022). Pant-hoots from the different communities showed lots of overlap in their acoustic features.

Based on Nisarg’s findings, and on the subtle differences reported from other studies, I’m inclined to think that chimpanzees do not have dialects after all. Chimpanzees and other apes may yet prove to have some limited capacity for vocal learning. But any such capacities in nonhuman apes pale in comparison, not just to humans, but also to many birds and whales in terms of vocal learning. This aspect of language seems to have emerged only after our ancestors diverged from the Pan-human ancestor.

Figure showing results from acoustic analysis of chimpanzee pant-hoot calls.
Most of the variation in acoustic features occurred among individuals within communities, rather than between communities. (Desai et al., 2022)

All of this raises questions about what promotes vocal learning in other species, and whether similar factors promoted vocal learning in human ancestors. To be continued!

Researcher with chimpanzees
Nisarg Desai with chimpanzees Sandi, Ferdinand, and Siri at Gombe National Park, Tanzania

References

Arcadi, A. C. (1996). Phrase structure of wild chimpanzee pant hoots: patterns of production and interpopulation variability. American Journal of Primatology, 39(3), 159-178.

Crockford, C., Herbinger, I., Vigilant, L., & Boesch, C. (2004). Wild chimpanzees produce group‐specific calls: a case for vocal learning?. Ethology, 110(3), 221-243.

Desai, N. P., Fedurek, P., Slocombe, K. E., & Wilson, M. L. (2022). Chimpanzee pant‐hoots encode individual information more reliably than group differences. American Journal of Primatology, e23430.

Marler, P. (1970). Birdsong and speech development: Could there be parallels? There may be basic rules governing vocal learning to which many species conform, including man. American scientist, 58(6), 669-673.

Marler, P. (1976). Social organization, communication and graded signals: the chimpanzee and the. Growing Points Ethology, 239.

Marshall, A. J., Wrangham, R. W., & Arcadi, A. C. (1999). Does learning affect the structure of vocalizations in chimpanzees?. Animal behaviour, 58(4), 825-830.

Mercado, E., Herman, L.M. & Pack, A.A. Song copying by humpback whales: themes and variations. Anim Cogn 8, 93–102 (2005). https://doi.org/10.1007/s10071-004-0238-7

Mitani, J. C., Hasegawa, T., Gros‐Louis, J., Marler, P., & Byrne, R. (1992). Dialects in wild chimpanzees?. American Journal of Primatology, 27(4), 233-243.

Mitani, J. C., Hunley, K. L., & Murdoch, M. E. (1999). Geographic variation in the calls of wild chimpanzees: a reassessment. American Journal of Primatology: Official Journal of the American Society of Primatologists, 47(2), 133-151.

Wilson, M. L., Hauser, M. D., Wrangham, R. W. 2001. Does participation in intergroup conflict depend on numerical assessment, range location, or rank for wild chimpanzees? Animal Behaviour 61(6): 1203-1216. https://doi.org/10.1006/anbe.2000.1706

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.