Warfare is a nearly universal trait of human societies. It has influenced the evolution of human societies at least since the dawn of history, and may have influenced the evolution of human psychology. By some definitions, warfare is uniquely human; no other species engages in armed combat using manufactured weapons. But in other respects, human warfare bears much in common with intergroup aggression in a range of species, from ants to chimpanzees. Michael Wilson will discuss how an evolutionary perspective on warfare can help shed light on why people fight and what they can do to make war less likely to occur.
Related exhibition: Arts of War: Artistry in Weapons across Cultures
Michael L. Wilson, Associate Professor, Department of Anthropology and Department of Ecology, Evolution and Behavior, University of Minnesota
Presented by the Peabody Museum of Archaeology & Ethnology and the Harvard Museum of Natural History
Recorded April 16, 2015
Transcript
[00:00:05.44] I am pleased to welcome Michael Wilson, Associate Professor in the Departments of Anthropology and Ecology, Evolution of Behavior at the University of Minnesota. As an evolutionary anthropologist, he studies the behavior and biology of chimpanzees. His research on our closest living relatives reveals fascinating insights into our evolutionary past. His main focus is on intergroup relations as you will hear, and his specialties include warfare, human evolution, ecology, and evolution of social behavior, and communication. He received his BA in Biological Sciences from the University of Chicago and obtained his PhD here at Harvard.
[00:00:45.21] He has held numerous other prestigious academic appointments, including Associate Director of the Jane Goodall Institute's Center for Primate Studies at the University of Minnesota, and the Director of Field Research at the Gombe Stream Research Center in Tanzania. He has also published widely in his work and has served as an editor for a host of academic journals, including the American Journal of Physical Anthropology, American Journal of Primatology, Behavioral Ecology and Proceedings of the National Academy of Sciences. You were an editor of that. [INAUDIBLE] articles.
[00:01:17.27] His work has been featured broadly in the public sphere as well through interviews with a wide range of outlets such as National Public Radio, Scientific News, Scientific American, Nature and so forth. He has also got a blog called "Monkey Business," offering thoughts on people, primates, and evolution, and covering a range of topics and conversation-- conservation, conservation could work, economics and language, and communication. So I'm looking forward to this talk. I hope you are too. Please welcome.
[00:01:46.33] [APPLAUSE]
[00:02:03.83] Thank you for that introduction and thank you for the invitation to speak here. And thank all of you for coming out tonight on this beautiful spring evening to hear me talk about primates killing each other. So this painting from the end of the Trojan War illustrates the efforts that people have made over the millennia to attempt to understand why people go to war. And the ancient Greeks had their hypotheses for why people went to war. And specifically the Trojan War was caused by this event illustrated here, where Paris is the fellow there with the red robe, is judging a beauty contest among the goddesses.
[00:03:01.31] And because of a wedding invitation that was not issued to the goddess of discord, she threw an apple into the wedding of the people who would become the parents of the Greek warrior, Achilles. And the apple was addressed to the most beautiful, to the fairest. And these three goddesses argued about which of them should get the apple. And so Paris was chosen as an objective person to choose which of the goddesses was most beautiful. They each chose a bribe to influence his decision and the winning bride, or the winning bribe, was the love of the most beautiful woman in the world, which had the complication that she was already married to someone else. And this led to a 10-year-long siege of the city of Troy, according to these stories.
[00:04:00.83] So the Greeks believed that war was something that happens to humans because of the gods. The gods have their disputes and we get caught up in them and die horribly as a result of these divine actions. Around the same time that the siege of Troy is thought to have taken place, is the conventional dating for the fall of Jericho, which is described in the Book of Joshua. And this illustrates another hypothesis for the origin of war or the cause of war. And in this case, God told his chosen people to invade this land and take over the cities within it and kill the people in each of the cities. So that's another hypothesis for the origins of those specific wars.
[00:04:58.41] And for most of human history, this has been the understanding, that warfare as something that we do because the gods are somehow causing us to go out and kill each other. In Western political philosophy, really the first person to develop a secular hypothesis for the origins of warfare was Thomas Hobbes. And Hobbes was inspired by Euclid, the Greek mathematician, and he was inspired by Galileo. And he sought to explain humans through natural philosophy without recourse to supernatural explanations. And he attempted to explain society from first principles.
[00:05:45.55] And so he thought very hard about what would life be like in a world in which there was no government, no laws, no source of order. And he describes this really attempt to explain the society from first principles in his book, The Leviathan. And his conclusion was that in such a situation, you end up with the war of all against all. And Hobbes is usually thought of as being this very pessimistic person who saw life as nasty, poor, brutish, and short.
[00:06:26.58] Focusing on warfare is something that people were engaged in constantly, but by war of all against all, he's not really imagining battle lines. He's imagining families defending what little they have against other families who are trying to grab what they can as well. And Hobbes has ever since then seen as someone to argue against, and especially famously by Jean- Jacques Rousseau, who was a French political philosopher originally from Switzerland, and in his discourse on the origin of inequality, he takes issue with Hobbes and Hobbes imagined view of the origin of human society.
[00:07:12.17] And he presents his own imagined view of human society, in which he argues that in the state of nature, people didn't live in families. They lived independently. They hardly recognized each other as individuals, men and women just met to have sex with each other, and life was free and peaceful and happy. And he argued that this was the way things were, and that warfare was something that developed more recently as the result of property and other cultural inventions that people came up with and ruined things.
[00:07:47.31] And these two philosophers to this day, form the competing or opposing poles of debate in thinking about warfare. But they were working at a time when they didn't have much empirical evidence to really dispute one another. How do you test these claims? Did we live as solitary individuals or did we live in small family groups? And also really the basic underpinnings of why people do what they do were not at all clear.
[00:08:28.14] And the person who we owe a lot to do in terms of shedding light on why people do the things they do as well as why other animals do the things that they do is of course, Darwin. And Darwin introduced, really for the first time, a scientific foundation for understanding human behavior. Because Darwin was the first person to really provide a compelling reason for why humans behave the way they do. What is the meaning of life, as it were? And of course, the answer is to make copies of our genes. That's what we're here for.
[00:09:06.69] Now of course, Darwin didn't state it that way. Genes hadn't yet been discovered. And of course, that's an oversimplification. But from an evolutionary perspective, the introduction of these ideas from Darwin finally provided a basis for really thinking in a common currency about what human behaviors-- what effect they have on fitness, and how likely they are to evolve.
[00:09:35.40] And Darwin argued quite simply that traits that help people to make more copies of their genes-- again, I'm speaking a little anachronistically because Darwin didn't know about genes. But they became, those traits that help people to survive and reproduce become more common in the population. And Darwin introduced a number of tools that have been indispensable for thinking about the evolution of traits, including warfare.
[00:10:01.67] And this includes the comparative method, in which Darwin was the first to recognize that we are all part of a family tree. We're all members of the tree of life, and in order to understand any particular branch of the tree of life, you really need to look at the other branches and how they interrelate with one another. And looking at this tree, once you understand that we are all part of a big family, you can see that closely related species often have similar traits.
[00:10:31.12] You can understand the traits of a given species better understanding the context in the family tree, and also sometimes there are differences. Closely related species differ in interesting ways and distantly related species are similar in interesting ways. And looking at these, using the comparative method helps show that similar problems can be solved in similar ways through evolution, even among quite distantly related lineages. And so this is really the foundation for what I'll be talking about today, using the comparative method, especially focusing on one of the two living species that is most closely related to us, the chimpanzee.
[00:11:16.50] In talking about warfare, I realize that I'm getting into a very contentious subject. People have been fighting about warfare for a long time. And these debates are very heated. People argue with a lot of emotional investment about the answers to these questions such as, is warfare evolutionarily ancient like Hobbes would argue or recent like Rousseau would argue?
[00:11:42.89] Why do people kill their enemies rather than merely chasing them away? Seems inefficient, a waste of effort. How can risking one's life in war increase fitness, which depends on surviving and reproducing? How can risking your life increase your fitness? And warfare something that will always be with us or is it something that might one day go away? People argue a lot about these.
[00:12:13.46] And this raises questions about why this debate is so heated. And I think at least part of the answer is that when people make an argument, people listening to them often hear some other argument being made. People have so much invested in this issue emotionally, that they may misinterpret or over-interpret what's being said.
[00:12:41.01] And I think of this often in terms of my own experience with when my son was born. I was a graduate student here at Harvard and I told some of my friends that I was so impressed at the rate at which my son was growing that my wife must be producing elephant seal milk. And by that I meant, he's growing so fast he must be getting really good nutrients. This is really good stuff that she's producing.
[00:13:19.90] And this is what I pictured in my mind, a cute little baby elephant seal next to its mom, who's taking such good care of it. I only found out recently that all these years, what my wife thought I was saying was, my wife is like an elephant seal, which is not at all what I meant. And so tonight I want to make clear, first some things that I don't mean to say and what I don't think the chimpanzee data that I look at and analyze can say.
[00:14:02.28] I don't mean to say that war is good. I don't like war. I don't mean to say that war is inevitable, that we have some instinct for always producing warfare, kind of invariably, regardless of what the context is. I also don't mean to say that war is natural and therefore excusable, that if warfare has an evolutionary history, that somehow means that it's OK. Go ahead and make war.
[00:14:34.41] I don't mean to say that war is the product of one or a few simple genetic changes. And I also don't mean to say that war is the result of genes that are only present in certain individuals or ethnicities. I think of warfare as the product of our common evolutionary heritage, something that we all have within us. So this isn't what I mean to say, any of this.
[00:15:02.72] I also think of that as fascinating as studies of chimpanzees are, chimpanzee studies by themselves can't tell us a lot of things about human warfare. The chimpanzee data alone can't tell us important things about the past. For example, how long warfare occurred in the human lineage? Just because chimpanzees have war or something like it, doesn't necessarily mean that it's always been present in the lineage leading to humans as well.
[00:15:35.43] And looking at the chimpanzee data alone I don't think and tell us whether warfare was frequent or rare for the human ancestors. We need to look at human data as well as that. Looking at the present, chimpanzee data don't tell us why war occurs in specific instances. We need to look at those specific instances that have historical contexts. And looking at the future, just looking at chimpanzees doesn't tell us whether warfare can be reduced or eliminated. I think that definitive answers to all of these questions about human warfare require studies of human warfare. And I'll talk a little bit about some of those studies, but I'm mainly going to be talking about chimpanzees.
[00:16:21.30] So, after listening to all those things that I don't think the chimpanzee data alone can tell us, I do think nonetheless that the chimpanzee data can tell us, or at least point to some very interesting possible answers to these questions. And I don't plan to have all the answers, but I think that the data that we have been able to put together tells us some interesting things.
[00:16:44.50] And the first of these that I'm going to look at is this question of whether warfare is evolutionarily ancient or recent. This is a photo that David Bygott took in September of 1971. And it's the first really clearly documented case of an intergroup killing in chimpanzees. This was taken at Gombe National Park in Tanzania where Jane Goodall had been studying chimpanzees since 1960. And when Jane first arrived in Gombe, she had a very strong impression that chimpanzees were basically peaceful.
[00:17:21.02] She didn't even believe at first that they had clear boundaries to their groups. They seemed to come and go and intermingle with many different subgroups and get along well with one another. And it was only 11 years into the study that clear evidence to the contrary really started to emerge. And in this case males from the main study community attacked, killed, and partially ate this infant from an unfamiliar female. And since then, other intergroup killings were seen at Gombe and this raised many provocative questions about the evolutionary history of warfare.
[00:18:01.07] And so to put this in context, this is our family tree as we currently understand it. And the dates in millions of years ago are on the left there. Those are approximate dates and we have pretty wide confidence intervals on all the dates. But we do know that of the living apes, the living African apes, include humans. We are an ape, and our closest cousins are chimpanzees and bonobos, those two species.
[00:18:31.79] Our more distant cousin is gorillas. We're more closely related to chimpanzees then chimpanzees are to gorillas. We have fossil evidence from a number of earlier species of human. And so we know that at least going back four million years, a bit more, there were bipedal apes the roaming the woodlands of Eastern and Southern Africa. And we can infer a little bit about their behavior.
[00:18:59.97] There are some things that using the comparative method we can get some clues about what might have been the common behavior of our last common ancestor. They're called Pan prior, the name that Richard Wrangham has proposed for this species that no one has discovered yet. There are some things that the living apes in this group, humans, bonobos, and chimpanzees share in common. We all have pretty high quality diets. Chimpanzees rely on ripe fruit. Humans rely on fruit and meat and other things that are pretty easy to digest.
[00:19:34.28] We have fission fusion dynamics, which means that unlike a baboon troop, for example, we don't all spend all of our time together with just the same social group. Instead we fission apart into little subgroups, we fuse back together. And in chimpanzees and bonobos, and many though by no means all human groups, males tend to stay in the group that they're born in and females tend to move to a different group when they reach maturity.
[00:20:05.98] There are some things that chimpanzees and humans have in common that they don't have in common with bonobos, like tool use. Chimpanzees use lots of tools. So do humans. Bonobos hardly ever use tools in the wild. And hunting is something that chimpanzees and humans do. And so perhaps this is something our common ancestor shared and was lost in bonobos for some reason. We don't really know for sure.
[00:20:32.27] And another trait that's like this is coalition area killing, where we know that humans do this. We call it warfare. We know the chimpanzees do this and we know that bonobos don't do this. So is this something that we share from a common ancestor from Pan prior that was then lost in bonobos, or has this evolve separately in humans and chimpanzees. We don't know. And that's a hard question to answer.
[00:20:58.12] But what we can do is look at comparative data to see whether the things that cause coalitionary killing in chimpanzees and humans are similar. And we could also look at whether the rates of killing are similar in these two species. And people have argued either way. Some people have argued that chimpanzees are not as nasty to each other as humans. And that humans have a long history of much fiercer intergroup relations and more killing and other people have argued that humans actually only recently became so warlike and that the very warlike behavior of chimpanzees' violence something that disappeared at some stage with these woodland apes.
[00:21:47.10] And it's really only in the past 10 years or so that enough data have come together from different sources to start answering some of these questions, do chimpanzees and humans in non-state societies kill each other similar rates? And it turns out that the estimates that we've been able to come up with are really strikingly similar.
[00:22:11.71] So these are chimpanzee data from several long term study sites on the left end and some data from foragers, hunter gatherers and how forager horticulturalists in the Amazon from a recent paper. And in blue are killings that have occurred within a group and in red are killings that have occurred between groups. And when you add these together, humans and chimpanzees both face about a 12% risk of dying from violence in these societies. And in both cases, there's a bit more within group violence, within group killing than between group killing.
[00:23:00.51] If we compare those rates to our own societies, or state level societies in general, we see a striking contrast. And those of you who have read Steven Pinker's book, The Better Angels of our Nature, will be very familiar with these data and this contrast. But in state societies, even ancient Mexico, which has a reputation as being a very warlike society, the percent of deaths from violence in the archaeological record are closer to 5%, so less than half of what they are in these non-state societies and in chimpanzee societies. And in another more recent states the rates of death from violence are very low.
[00:23:42.41] Even in the 20th century, which is often thought of as the most warlike century in history because people killed each other on such a horrific massive scale during the two World Wars and various genocides that have occurred, even this figure shows the estimates of deaths from war worldwide during the four quarters of the 20th century. And in the quarter 1925 to 1950, with World War II right in the middle of that, the percent of deaths worldwide from warfare is still a lot less than what we would see in a chimpanzee society or in many non-state human societies.
[00:24:34.94] So going back to this question of whether warfare is evolutionarily ancient or recent, we see that hunter gatherers, the people living in the sorts of societies that we spent most of our evolutionary history living in, and chimpanzees have similar rates of killing, both between and within groups. And people have argued in a very common argument in archaeology and anthropology, the argument the Rousseau makes, is that we need these special human things like property or agriculture or settled sedentary societies, projectile weapons, these are all special things that humans have the cause was but chimpanzees don't have any of those. So this at least supports the idea that coalitionary killing is something that could date as far back as the common ancestor with chimpanzees and humans.
[00:25:54.69] And so when people talk about warfare as an invention, which is something that Margaret Mead did in 1940, she argued that warfare is something that people have invented. It's a cultural construct and archaeologists often point to things like the fortifications that we see around ancient walled cities like Babylon. They see this as the beginning of warfare. When we see people erecting walls around their cities, that's what we know that warfare has really gotten serious. Before then, people were just fighting and it didn't make such a big deal.
[00:26:31.31] But what I think is going on there is that actually what we see what those fortifications is the invention of peace. And that before people were able to build walls around their cities, they were living in a much more vulnerable world, a world much more like what chimpanzees live in and what hunter gatherers live in, where you don't have a safe place. And when people finally developed the technology and material, really luxury, to be able to build these fortifications, that's when we see a place where at least some people, once you're inside the city walls are safe. And it's outside those walls the people are vulnerable as chimpanzees always are and as most people have been through the course of human evolution.
[00:27:24.06] So the next question I want to address is why do people kill their enemies rather than merely chasing them away? And it's a real puzzle. And it's something that's unusual about humans and chimpanzees. There are lots of species that have aggression between groups. They defend territories. They fight with each other, but in most of those species it's enough to chase them away.
[00:27:46.55] If a troop of baboons meet and fight over a water hole or a fruit tree, one troop will run away. The other troop will stay put and get to use the resource. They don't kill each other. And that's just much more common in animals.
[00:28:02.23] And so why go to all the trouble of actually killing your rivals instead of just chasing them off? And people have proposed various hypotheses for this. And one hypothesis that I think has become very popular among anthropologists, is that it's because of human impacts that these killings occur. Jane Goodall, when she first started at Gombe, early on in her studies, she started feeding the chimpanzees bananas. And some people have argued that feeding the bananas somehow caused them to become more violent and to do things they wouldn't otherwise.
[00:28:42.57] And more recently, this is an aerial photo of Gombe from about 10 years ago. And you can see where the park is, it's very green and lush and the backside of the park, outside of the park boundary is quite barren. People have cleared the forest for cassava fields and their farms. And so the habitat that's available to chimpanzees is shrinking.
[00:29:10.09] And so people have argued also, well, maybe it's a result of habitat loss that the chimpanzees are facing more stress. They're competing for fewer available resources. Maybe it's something to do with that.
[00:29:25.68] But it is the case that we've seen killings at sites that don't have either of these impacts. This is a male chimpanzee who was killed in Kibale National Park where chimpanzees have never been fed bananas. And it's a nice big park without much habitat loss, within this really huge area. Here's another male who was killed in Uganda in a different community on Kalinzu where they've never fed them bananas. This is a male chimpanzee who was killed in Fongoli in West Africa, a site where the chimpanzees have not been artificially fed and they range over a very wide area.
[00:30:11.44] So at least anecdotally, many of us who have been studying chimpanzees have thought, well, it's probably not human impacts that's causing this killing. But we want to know for sure. And so I worked with the leaders of all the long term chimpanzee study sites and also the bonobo study sites to analyze the data, all the data that exist on killings at these sites. And we ran some statistical models that found that the measures of human impacts that we had, such as whether banana feeding occurred had no impact on rates of killing.
[00:30:51.19] And instead, the rates of killing depended very much on the number of adult males in the group and the population density. So just to explain this figure little, there's a bar for every one of the communities in the study. The ones that are labeled with B are bonobos and the ones that are labeled with E are eastern chimpanzees. The W's are western chimpanzees and the P stands for provisioning. So these are the groups that have a history of being fed by researchers.
[00:31:22.04] And we see that the groups with the highest rate of killings per year are not the ones with P's but here we have Ngogo, the largest chimpanzee community that's ever been documented, something like 200 individuals. And they have the highest rate of killing. They have lots of adult males and when they meet the neighbors, they can kill them pretty easily.
[00:31:44.96] So it's not human impacts that's causing these killings. Instead what is a better supported hypothesis is the balance of power hypothesis that Richard Wrangham developed. And I've really spent much of the past couple of decades testing this hypothesis, testing various predictions of it. This is a hypothesis that focuses on the costs of killing as well as the benefits. And from the perspective of a chimpanzee, there could be any number of benefits to killing. You can get more stuff basically by killing your neighbors. You can get their stuff.
[00:32:18.88] But the costs of doing so really vary, depending on how many chimpanzees are on each side of the conflict. And the cost of killing is going to be lower when your side has more guys than the other side. And just as a cartoon to illustrate this, here we have two territories, a red territory and a blue territory. They each have 10 males illustrated here as squares. And for ecological reasons, these chimpanzees travel in parties of varying size.
[00:32:52.56] So the blue community doesn't have very much food right now, so they're traveling in small groups of twos or even off by themselves. Whereas, the red community has enough food that they're able to stay together all day. And so they go on a border patrol to the edge of their territory, where they find this blue male, surround him and kill him. And now there are fewer blue males. And as a result, they are less able to defend their territory and red is able to expand the territory. So that's kind of a cartoon of the hypothesis.
[00:33:24.14] And there are various predictions from this that we can test. And I've been trying to test these predictions. One is that because these imbalances of power are so important, you should avoid going to the periphery of your range unless you've got a lot of buddies with you, unless you're in a party with many males, you shouldn't go to the edge of your range.
[00:33:45.82] But if you do have more males, then you should be more likely to fight, more likely to win, and more likely to kill. And as a result winners should gain more territory. In testing these predictions in Uganda studying the Kanyawara community, we found that when they go to the periphery of their range, they have about twice as many males as when parties stay in the core of their territory all day long.
[00:34:14.69] I did a series of playback experiments with these chimpanzees where we hid a single speaker about 300 meters away from parties of varying size in the listening community. And we had two teams for these experiments. In one team, there'd be people actually watching the chimpanzees and recording what they were doing. The other team would be at the site of the speaker. As soon as we did the playback, a single stranger's call, we'd pack up the speaker and take it away. And one of us, often me, would stay at the speaker site and see what happened.
[00:34:53.35] And what happened was that if we played the call to parties with just females, then the females stayed very quiet. They looked nervous. And if they climbed down from the trees, they would move away from the speaker. And when we played this call to parties with one or two males, they also stayed quiet. But in about half the cases, they would climb down from the trees and cautiously approach the speaker.
[00:35:17.53] And in cases where there were three males or more in the listening party, they gave a very loud vocal response right away and they moved very rapidly towards the speaker. And when I would be there waiting at the speaker site, I remember hearing footsteps coming towards me and looking and seeing a line of males with their hair out, looking very excited, looking for an enemy. And when they saw me, they knew who I was. I wasn't very interesting to them and so they moved on, looking for this enemy chimpanzee that we had invented.
[00:35:55.25] And these experimental results, we've since been able to confirm with analysis of 15 years of the actual intergroup encounters, over time, and we found a very similar pattern, in that the probability of giving a vocal response and the probability of actually approaching the calls of strangers increases as the number of males in the party increases. So there's this context dependent response.
[00:36:31.22] And looking at the patterns of killing from these data from all the long term chimpanzee study sites, we found a total of 99 killings that were quite well-documented. And most of these killings were done by groups of males. And most of the attackers were males, most of the victims were male. And the majority of the killings were intergroup. They're mainly killing males from other groups. This includes babies as well as adults. In fact, the majority of the killings were of infants.
[00:37:05.02] But these killings between groups happened during cases of overwhelming numerical superiority. So the average ratio of attackers to defenders was eight to one in these intergroup killings. And as a result of these successful killings, chimpanzees are able to enlarge their range. We see in Gombe National Park, the main study community, has a larger territory when they have more males.
[00:37:35.92] So to summarize, why do people kill their enemies rather than merely chase them away? Well insofar as people are similar to chimpanzees, in that we also have fission fusion societies and coalitions of males who defend group territories, the particular grouping patterns that we have make it possible to kill at low cost to the attackers.
[00:38:00.96] This next question, how can risking one's life in war increase fitness, is a very big one and I'm just going to touch on it with a few pieces of data. But in general, warfare presents a puzzle for evolutionary thinking. And this was something that Darwin worried about back in 1871. It's something that people still worry about. Because if the whole point of evolution is maximizing your fitness, in other words, surviving long enough to reproduce and making as many babies as you can, then why on earth would you put your life on the line? Why would you go into battle and risk your life? That seems like a crazy thing to do.
[00:38:41.08] And part of the reason why this seems crazy is because these sorts of battles that we're most familiar with really are crazy. They are the battles of huge state level conflict, the battles of World War I and World War II. And it's really crazy to be involved in those. People don't do it willingly. They do it with a gun at their back from their officer and fear of court martial and imprisonment. It's really, really terrifying to be involved in these battles, at least that's what all the people who write home about them say and the various novels that we have from these battles.
[00:39:19.27] And those battles are probably quite different from the battles that we evolved to fight. If we read the data on-- read descriptions of what warfare is like in hunter gatherer societies, it's much more like what we see in chimpanzees, where there's this preference for low cost attacks. When chimpanzees meet in a big group, chimpanzees from another group, they chase each other around and yell at other and shout at each other, but that's not where the killing occurs. The killing occurs in these ambushes and gang attacks.
[00:39:51.14] And in non-state human societies, it's very much the same thing. The encounters between big groups on both sides tend not to be very lethal. What is deadly are the surprise attacks, the raids and the ambushes where the attackers have the advantages of numbers and surprise.
[00:40:13.27] One of the ways in which attackers gain fitness benefits is through resource competition and by more effectively obtaining resources. This is a chimpanzee in a food tree. He's very excited. If you look at his nether regions, you can see a typical male chimpanzee response to a feeding excitement. And chimpanzees spend most of the day looking for food. That's mainly what they do. That's the main business of chimpanzees.
[00:40:42.44] And it turns out that intergroup encounters occur most often when chimpanzees are looking for food in border regions. For example, here's a map of the territory of the Kanyawara community that I studied in Uganda. And these red dots are the locations of a certain kind of tree, uveriopsis, which is much more common in the south. And at Kanyawara, most of the intergroup encounters that occurred, shown by these dots, are in the same area and they occur at the same time of year that uveriopsis is fruiting.
[00:41:22.55] So these abundant resources at the range edge draw chimpanzees from both sides to compete over them. Ann Pusey and her team, working at Gombe National Park have found that when the territory size is larger, chimpanzees actually weigh more. So it looks like they're getting more food. And as a result of having more food, females have shorter inter-birth intervals. They're able to produce babies more quickly.
[00:41:52.21] So in chimpanzees, this whole business of territory defense and intergroup killing seems to be a male reproductive strategy to acquire a maximum territory size, so that you have more food for yourself, for your mates, and your offspring. It's kind of a male parental care strategy sort of.
[00:42:14.03] We see similar things in human societies, although the data for human societies relating fitness to warfare are very scarce. These are very hard data to come by. And one of the very few data sets that anyone has accumulated has recently been collected by Luke Glowacki, who's a graduate student just finishing up here at Harvard. And Luke has been working with the Nyangatom cattle herders in Ethiopia.
[00:42:41.54] And what he's found is that among the Nyangatom, like many other cattle herders, cattle raiding is common. Cattle are an easily stealable resource because they come with their own transport system. And cattle are essential, cattle are the wealth in the society. And young men need to acquire a certain number of cattle in order to pay the bride wealth that they need to get married. So there's a direct link between the number of cattle that you can acquire and whether you're going to have any reproductive success whatsoever.
[00:43:16.08] What Luke has found is that men who raid frequently, which we see, we have the non-prolific and the more prolific raiders, they have more wives. So there is an indication for fitness payoffs in the society where you can acquire more wives by raiding more cattle.
[00:43:40.28] So, to summarize this, how could risking one's life in war increase your fitness? Something I didn't really discuss is that you can help your inclusive fitness, but most importantly, most fighters avoid risking their lives if possible. They avoid risky conflict. And the risks that they do take, can pay off, in chimpanzees by increasing territory size, and in chimpanzees and humans, by increasing access to other resources, whether it's a fruit or cattle.
[00:44:14.99] So finally, I will look briefly at this question of will warfare always be with Us and those of you who've been following Steven Pinker's work will know the answer is, maybe not. And this is a graph that shows the number of deaths from warfare for different regions for the time since World War II. And when we read the newspapers, we see all kinds of warfare. There's warfare in Iraq and Syria and Ukraine and we get this impression that the world is very dangerous place. But if you actually count up the number of deaths, they go up and down a lot over time, but there's a general decreasing trend.
[00:45:02.94] Even in the past 10 years, the number of deaths from war, even though our country has been directly involved in wars throughout this whole time, the number of deaths is quite small. So in answer to this question, will warfare always be with us? Maybe not. I think it's something we can be hopeful about because of this decline in rates of death that's happening worldwide. Of course, we can't be sure.
[00:45:34.41] A thermonuclear war could break out any minute. That's possible. And that would increase the rates of death from warfare, since we still have lots of nuclear weapons. But there are multiple factors that are working together that all together seem to be decreasing the likelihood of war. And this includes various things that increase the costs of war. Nuclear weapons may be an important one of those, but also things like missiles and airplanes that make the sort of massed armies fighting in trenches of World War I and II not really feasible anymore, when you can use your satellites to find out exactly where those armies are and eliminate them.
[00:46:20.44] But there's also another way the costs have increased is that there are international organizations like the United Nations that will punish perpetrators of war. And this is really led by the American Leviathan, which is playing a role somewhat like what Thomas Hobbes envisioned for the state, the government of a particular country, the United States in certain ways does this for the globe. And at the same time, the benefits of war are relatively in decline compared to other, less risky ways of getting resources. So generally speaking the resources that people need for their reproductive success are a lot easier to obtain by getting a good job and going to work on a regular basis and not going off and killing people. That's a really risky strategy to acquire resources.
[00:47:23.42] So the take-home messages that I have for all of us are, first of all, several lines of evidence suggest that warfare does have deep roots in human evolution. It's something we share in common-- well, it's very similar in humans and chimpanzees in many ways that suggests it's an inheritance from our common ancestor. And if rates of death from warfare and intergroup aggression more generally have been as high throughout human evolution as they are in chimpanzees and in hunter gatherer societies living today, then warfare is likely an important source of selection pressure, shaping our biology and our cultural evolution.
[00:48:10.65] And warfare continues to be a very important source of evolutionary pressure. Just look at how rapidly airplanes went from being something that barely existed just 100 years ago to very rapidly becoming weapons of war and destruction. But at the same time, warfare is not inevitable. And we see this in chimpanzees. Chimpanzees are sensitive to the costs and benefits of warfare. They don't go out killing each other all the time.
[00:48:38.94] They pay attention to the context. And so do people. People are sensitive to the costs and benefits of warfare. And as those costs and benefits change, the benefits have decreased relative to other strategies like trade and generally being peaceful. And as a consequence warfare has declined.
[00:49:02.92] So I would like to acknowledge the many people who've been essential for the sort of work that I've been doing over the years, including the many collaborators. These are all of the people who are involved in the long term chimpanzee and bonobo comparison that I've described. But most importantly, I'd like to thank the field assistants. These are just some of the field assistants at Gombe. And there are people like these men and women at other sites across Africa who have been working very hard for many years to follow chimpanzees and bonobos around, recording everything that they do. And I wouldn't be able to tell you anything about chimpanzee behavior without them. So thank you very much.
[00:49:50.29] [APPLAUSE]