Domesticated animals such as dogs, pigs, and horses often sport floppy ears, patches of white hair, and other features that are unknown in their wild ancestors. These traits—collectively referred to by scientists as a “domestication syndrome”—are the result of breeding less aggressive individuals. Drawing from his new book, The Goodness Paradox (2019, Pantheon Books), Richard Wrangham shows that our cousin apes, the bonobos, also exhibit a domestication syndrome, making them the first clear example of a “wild domesticate.” Self-domestication in the wild now seems likely to be a widespread phenomenon, responsible even for the evolution of our own species, Homo sapiens.
About the Speaker
Richard Wrangham, Ruth Moore Professor of Biological Anthropology, Department of Human Evolutionary Biology, Harvard University
Richard Wrangham has conducted extensive research on primate ecology, nutrition, and social behavior. He is best known for his work on the evolution of human warfare, described in the book Demonic Males, and on the role of cooking in human evolution, described in the book Catching Fire: How Cooking Made Us Human. He founded the Kibale Chimpanzee Project in 1987, and together with Elizabeth Ross, he co-founded the Kasiisi Project in 1997. He serves as a patron of the Great Apes Survival Partnership (GRASP). Wrangham holds a Ph.D. from the University of Cambridge.
Presented by the Peabody Museum of Archaeology & Ethnology and the Harvard Museum of Natural History
Recorded April 9, 2019
Transcript
[00:00:06.20] It's my pleasure now to actually introduce Richard. And he is known to many of you, I'm sure, and as with many of our professors has so many things. I could stand here for sort of 10 minutes and tell you all the wonderful things about Richard. But really, you're here to hear him and learn about bonobos. So I'm going to keep this fairly short. He is the Ruth Moore professor of biological anthropology at Harvard, where he has taught since 1989.
[00:00:33.50] His major interests are chimpanzee and human evolutionary ecology, the evolutionary dynamics of violence and non-violence, and ape conservation. He received his PhD in zoology from Cambridge University, was a research fellow at King's College Cambridge, and taught at the Department of Anthropology at the University of Michigan before arriving here at Harvard. Since 1987, he has studied wild chimpanzee behavior in Kibali National Park in Uganda. He has been president of the International Primatological Society and an ambassador for UNESCO's great ape survival project.
[00:01:13.17] He has been awarded the MacArthur Foundation Fellowship and is a fellow of the American Academy of Arts and Sciences and the British Academy. His most recent books are Catching Fire: How Cooking Made Us Human in 2009 and then, of course, what we're going to hear about this evening, The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution, although it won't be quite the book talk, as I'm sure he's going to mention in a minute. But please join me now in welcoming Professor Wrangham. Thank you.
[00:01:53.45] Thank you, Jane. So no, it's not going to be a book talk exactly, because when she asked me to do this talk, she said, but you can't give your book talk, because you already have given a book talk in January, which I did, or maybe early February. So this is a chapter talk. One of the chapters in the book is about bonobos. And it's a great excuse to be able to share my enthusiasm for thinking about bonobos as a really special case in human evolution, which may turn out to have larger lessons than just this particular species.
[00:02:29.19] So if you came here thinking the book talk, here's the three minute-- actually, the three slide version. Well, maybe it's the two slide version. The goodness paradox is the paradox that, on the one hand, humans are one of the nicest animals. We're all so cooperative and tolerant and aggressive to each other. And on the other hand, we kill more members of our own species than almost any other animal does, accepting a few things like wolves that do even more.
[00:02:54.98] So where does all that come from? Well, read the book. But briefly, about 300,000 years ago, we get the first glimmerings of the gracilization of our skulls and skeletons which indicate that we were on a path towards self-domestication. And I think that just as people have often said that humans are like domesticated animals, actually humans really were domesticated animals.
[00:03:20.45] And in the book, I tell the story of how we can argue that the way in which we became self-domesticated is that we got sufficiently good language to be able to start developing conspiracies, having safe executions of poor victims, who were the bullies, the aggressive ones, and then this led to reduction in reactive aggression.
[00:03:42.66] Well, what on earth has that got to do with bonobos? Here, we have the National Geographic doing, I think, a rather wonderful portrayal-- they really tried to work hard to get the science right-- of our two closest relatives, who are equally closely related to each other, chimpanzees and bonobos. And you may know that it was some years after bonobos actually came to the United States and were live here that anybody was able to figure out that there were two different species at stake, because they do look kind of similar to each other. But one of them clearly looks a little bit different from the others.
[00:04:22.83] So we're going to have to talk about both bonobos and chimpanzees, because if we're going think about bonobos, we have to think about them relative to their very close cousin, their chimpanzee. So we've got the males in the middle and the females on the outside. And what we're going to do is, first of all, think about chimpanzees, just to bring you up to date or just rehearse you on what we know about chimpanzees in relationship to aggression, and then think about bonobos in relationship to aggression. I just want to draw that contrast out. And then we'll think about why the differences occur.
[00:05:04.07] So very often, when people hear about bonobos, they think it's all going to be about sex. I'm so sorry. You've come to the wrong-- that's in the room just up-- so we're going to talk more about aggression, and we're going to think about what it is that learning about the strange difference between these two species can help tell us about human evolution.
[00:05:33.45] We're not going to go far with that, but that's sort of the big aim. OK, so Jane Goodall was 85 last week. In 1960, she started her studies of chimpanzees. That was in Gombe in the eastern part of the chimpanzee range. You see there, on the edge of Lake Tanganyika, it is really just beyond the great West African and Central African forests that creep up to that place in the Western Rift Valley.
[00:06:05.86] And since Jane Goodall started, a whole bunch of studies of chimpanzees, one is the one I work in Kibali. And what I'm going to say is some generalizations that have emerged from these and actually some other studies that are further west, where you can't quite see them in this wonderful view of the Earth.
[00:06:28.73] So now, the first thing that is important is to recognize that, just like bonobos, chimpanzees live in social communities. There are up to 200 individuals. They start at around 20 or 30. And within those communities, they break up into traveling parties that forage as a unit for a few hours or a few days, or even a few minutes. And generally, you find there are more males than females in those parties. But in the community, there are more females than males. So what's going on is that the females are more often on their own. They're more often traveling alone and away from the big parties, where the males outnumber the females.
[00:07:15.23] Those males are consistently rather aggressive. On the average day, you're likely to see males charging around, maybe just chasing, maybe beating others up. They use their aggression to dominate others. And when they dominate others, they're doing it partly with the help of alliances that they curry favor with all the time through grooming and other ways of being nice to each other. They are pretty brutal.
[00:07:46.73] Those male allies can recruit each other to attack members of their own group. So here are two males who are attacking an alpha male actually, the one on the left, who is called Pimu. This is in Mahali And on this occasion, he was killed. He was killed by a coalition that ended up being about four males. Some tried to defend him. Some tried to attack him. And those were the ones who won.
[00:08:16.78] Females, of course, are terrified of males. They have a very unhappy relationship with males for much of the time. They are extremely submissive. The males can force them to pretty much do anything that they want. And sometimes, the attacks that they make are quite unprovoked. Here, Carole Hooven and I are examining some sticks that a couple of males used to use to beat on the females.
[00:08:49.06] But ordinarily, you see these black and white photographs here where a male has simply launched an attack and is pulling a female around and kicking her and biting her and pounding her with his fists. And the males who do this are the males-- well, the male who does this most to a particular female turns out to be the one who is most likely to mate with her and is most likely to have her next offspring.
[00:09:17.93] So males are totally domineering. If they get some really precious food, then they are the ones that control access to the food. The female has to come and beg from a male for a monkey like this. The sexual coercion is routine in the sense that it is, as I said, the male who is most aggressive to a particular female who ends up being the one who mates her most often and, in one study, in Gombe, is the one who also has the highest chance of being the father, and in fact, has a more than 50% chance of being the father, which is amazing considering there are maybe 10 or 12 males, all of whom are meeting her.
[00:10:00.21] The females often get hurt by males. So do their young. They may get hurt by males within the community. They may get hurt by males of a different community, because adult males of communities patrol the edges when they have the time and they have the energy, they have enough food to be able to justify wasting it, as it were, in the possibility of aggressive encounters with the neighbors. If they find infants from neighboring groups, then they may kill them. You see the puncture wounds from a canine of a male and the skull of Andromeda here, who was killed in intergroup interaction.
[00:10:42.43] And more often, they get into a shouting match with the males from the neighboring group. But if the shouting match turns out to be uneven and there's only maybe one on the other side, then that's when you can get individuals who are really badly beaten up, lucky to escape with their lives, and not everybody does.
[00:11:06.16] So I said this was going to be about aggression more than sex. That is chimpanzees. Domineering males, sexual coercion, infanticide, intense aggression, and proactive killing, where individuals actually take the opportunity to go and look for opportunities to kill members of neighboring groups who are vulnerable by being alone. And that's a generalization that we can make about chimpanzees in East Africa, and to some extent in Western Africa.
[00:11:38.94] OK, so then, now, we can move to the contrast with bonobos. When bonobos were discovered as being a species that was separate from chimpanzees, people did not know that they were going to turn out to have very different behavior from chimpanzees. Indeed, at the time the discovery, basically 1929, nothing was known about the behavior of chimpanzees. So you've got these two species living in Central Africa separated by the Congo River.
[00:12:13.28] In order to find out about bonobos, people had to go into one of the most difficult countries on Earth, where the infrastructure is extremely poor. So it's not surprising that it's taken time for bonobo studies to get to the point where people have battled up the rivers and have walked sometimes 20 kilometers in order to be able to get from the nearest village into an area where you can watch bonobos. And they have started getting glimpses of bonobos.
[00:12:45.14] And that was enough for people to found long term studies. And now, in the area, on the left bank of the Congo River, there are four really significant long term studies and a few other shorter term ones. But these are the ones that are producing data equivalent to what we have on chimpanzees, the earliest going back to 1974, but interrupted by occasional periods of war and insurrection.
[00:13:13.84] So it's very exciting that we're now at the stage where we're really learning about bonobos. And just like chimpanzees, they live in social communities. And within those communities, they live in parties. And you might say, OK, well, is it going to be like chimps with the number of males in the average party being greater than the number of females? And I think you'll get the answer when you look at this picture.
[00:13:41.68] So here, we have some something like 20 individual bonobos and at least eight adult females. And they're with their babies. They are dominating the number of males numerically. There are more females than males in this party. And it turns out that this is the regular feature that you see in bonobos. So all of a sudden, you've got a sex difference that is the reversal of what we see in chimpanzees.
[00:14:12.54] And the really key feature that seems to be very important, underlying much of what goes on with the social behavior of bonobos, is that females form these alliances against males. And they form the alliances through, very often, having sex together. One of the rather charming ways in which this all begins is that, in adolescence, just like chimpanzees, a female leaves the group in which she is born and moves to a new community, where she probably doesn't know anybody very much. She might know them a little bit in bonobos.
[00:14:47.07] And when she does that, she is not greeted particularly warmly by the females in the new group, but she kind of hangs around, often apparently targeting a particular adult female in the new group. So when I say targeting, what she's doing is spending her time, very often, within a few yards of that particular female. It tends to be the female that she is closest to.
[00:15:12.12] And eventually, after a few weeks, it has been described that the target female kind of welcomes the new female, and they have a quick sexual interaction. And then basically, what happens is that she introduces her to all her friends. And then from then on, she is part of the network of females. And that network is important because they are able to support each other if a male loses his temper-- very foolishly-- with her.
[00:15:40.53] So the net result is that instead of males dominating the females, the females are co-dominant with the males. In chimpanzees, every single adult male is absolutely dominant to every single adult female. In fact, the way he becomes part of the male hierarchy is by forcing every adult female to scream, the females that used to dandle him on their knee.
[00:16:03.78] Well, in bonobos, the co-dominance is probably really in favor of females, but it's difficult because of the rarity of aggressive interactions to see exactly who is dominant at the top, as I understand it from the real bonobo experts. So you've got here an alpha female and an alpha male. And sometimes, the alpha females are clearly dominant and sometimes it's more that they're kind of in parallel. And what is so interesting is that when a male is dominant, when he is the alpha male above the other males, he is only there by virtue of the support of females.
[00:16:40.89] So here is an example of a male who I actually happened to see in the wild at Wamba in 1996. And he is now 48 years old. He used to be the alpha male. That was when his mother was alive. When his mother died, he sank. And that is the story that happens repeatedly, that as long as you've got a living mother, then you can rise in the hierarchy, but without it, you don't. So the females are very important. One example is that, whereas the males are controlling food sharing in chimpanzees, the females are much more likely to be in control of a delicious food, like a big fruit or a piece of meat.
[00:17:25.08] Compared to the regular sexual coercion of the chimpanzees, there is essentially no sexual aggression in bonobos. I am sure that experts have occasionally seen some sort of tendency for a male to get so frustrated that he tries a little something, but there's a huge difference. There is such a lack of aggressiveness in the bonobos in terms of sexual interactions.
[00:17:53.89] Now, one of the fascinating areas of development recently has been the ability to tell who are the fathers in chimpanzees and bonobos through DNA analysis of the infants. And the expectation, when people knew all that about sexual coercion not being important in bonobos, was that the paternity would be rather widely distributed among the bonobo males, because every male's got a good shot. None of them are able to use their violence to impose themselves on the females.
[00:18:30.29] So it was a fascinating surprise when Martin Surbeck and colleagues discovered that actually the way in which the paternity is distributed is even more in favor of the top males in the bonobos than it is in chimpanzees. The reproductive skew is intense. Why? It's not because the males are dominating them. It's because the females are choosing them. So if you really want to be a successful male, you get to be a favorite of the females, the kind of male that is being supported by the females, and then the males will be chosen by the females.
[00:19:12.20] Now, I don't want to portray bonobos as totally unaggressive. There was a terrible situation about-- I can't remember, five, six, maybe more years ago-- when some bonobos were released in the wild and they ended up attacking the very people that were caring for them and looking after them and imposing terrible wounds on them. They seem to have just been surprised and not understanding who it was that was there.
[00:19:36.89] And within bonobo society, there is enough aggression that you will sometimes see missing knuckles or cuts on them. Nevertheless, the overall impact does seem to be less, because the fighting is relatively mild in its intensity compared to the pounding that male chimpanzees give to each other and to females.
[00:20:00.23] And nowhere is this more striking than in the relationships between communities, between these larger social groups, where you can get, between members of neighboring communities, grooming, sexual interactions, play, and even occasional food sharing, which was reported just a few weeks ago from one of those sites. Now, to someone who studies chimpanzees over many years, this is just simply unbelievable, because chimpanzees are so far from having the capacity to be tolerant towards members of neighboring groups. Toleration, like an individual, Ana here, reported by Takeshi Furuichi, as after spending a year as a temporary immigrant in one group, the E1 group, left.
[00:20:54.71] And then five years later, the E1 group meets the group to which Ana had moved. And what happens? Ana comes over and meets her old friends. She apparently recognized them as individuals, and they spent some time together, having intense social interactions-- GG rubbing is the sex that happens among females-- before the groups quietly separated. Entirely different from anything you'd see in chimpanzees.
[00:21:24.69] Here's a particularly astonishing one. A form of play that's pretty astonishing just within a group, because what happens is that a male or a female will be playing with a juvenile that does not belong to them, in the most interesting cases, and takes them by a foot, as you see here, and swings them about. So this is the Michael Jackson moment in the forest.
[00:21:52.17] And you see, up to 40 meters above the ground. The juvenile is totally in the power of the adult. So we'd all be sort of quivering with alarm, but the juvenile looks happy. They allow this to happen, because they trust the adult to take care of them. So that happens within groups, but it also happens between groups, where a male of one group can play with a juvenile of another group.
[00:22:18.89] If a chimpanzee was doing that, we know what kind of play that would be. That would involve bites and flailing against a trunk. And here, you've got these bonobos just taking extremely relaxed attitude. It's not totally relaxed, because when the sex goes on, the males from one group sort of look at their females walking across and having sex with the males of the other group, and the males don't come across and sort of get together and be friendly about it. They kind of look rather resentful is my understanding.
[00:22:53.81] But nevertheless, this is all very, very different from what we see in chimpanzees. And there have now been sufficient decades of study to be able to say that the fact that there have been no violent deaths seen in bonobos from other bonobos attacking and killing them is statistically significant. That is to say that bonobos are really genuinely different, in terms of the amount of observation that's gone on, from chimpanzees in that particular measure of aggression.
[00:23:28.49] So there we got, chimpanzees and bonobos, two relatives so similar in body size and, by the way, in the degree of sex difference in body size that it took some time for scientists to recognize that they were different species. And on the one hand, you've got all these aggressive aspects of chimpanzee behavior. And on the other hand, with bonobos, you've got all these much more peaceful, supportive aspects-- female alliances, females co-dominant with the males, female choice much more important in sexual interactions, friendly intergroup interactions with no killing. So that's the background.
[00:24:12.82] So obviously, what we want to know is, what's going on? Why is there this remarkable difference in aggressiveness between the two species? And what I have done here is to bring up a map that shows in red the four different subspecies of chimpanzees in the different areas they live. And in blue, the subspecies, as it used to be, and now we call it the species, of bonobo. And the critical thing here is that the bonobos are separated from the chimpanzees by the Congo River.
[00:24:48.89] So what I want to do is to sort of start thinking about the evolutionary aspect between the relationship between bonobos and the chimpanzees. And a critical feature is the age of the Congo River. People used to think it was relatively recent and that maybe some populations had been divided, so that bonobos and chimps could equally well be each other's ancestor. But now, we know that the Congo River has created a deep sea canyon opposite its exit into the Atlantic Ocean.
[00:25:23.41] And people have been able to work out that that is more than 30 million years old. And the difference between bonobos and chimpanzees is thought to be between 1 and 2 million years. So the Congo River long predated the existence of the chimpanzees and bonobos. And people have been able to work out that only occasionally was there a drought sufficient to enable a crossing of the river, and I've shown here roughly where that crossing would have been, somewhere in the northeast of the range now of the bonobos.
[00:25:57.77] And the genetic evidence is not bonobos evolved from a relatively small area of the chimpanzee populations just once. It's a monophyletic evolution, so that we can reconstruct bonobos as evolving from the East African subspecies of chimpanzees, sometime either a million years ago or 1.8 million years ago to judge from when droughts happened. And the genetic data, with the uncertainty about rates of mutation, are not quite sufficient to resolve that confidently as to whether or not it was the more recent or the more distant date.
[00:26:40.67] So it looks as though we've got bonobos evolving from a chimpanzee-like ancestor. And what they did was to evolve by reducing aggression from a much more aggressive version, the different subspecies of chimpanzees all showing that contrast to bonobos. And I say here, does this mean that we're talking about self-domestication? Because I want to define domestication as the evolution of reduced aggressiveness. And the evolution of reduced aggressiveness by a species in the wild, without humans being involved at all, would be self-domestication.
[00:27:23.83] And in order to think about this, what we have to think about, of course, is the domesticated animals as a model for understanding the reduction of aggression. Now, the domesticated animals are really fascinating, because we've got an experiment that's been repeated 20, 30 times in different species, the evolution of reduced aggressiveness and domestication. In wild animals, nobody has really thought, until recently, about the possibility that when you have aggression being reduced over evolutionary time, there may be some sort of analogy to what is happening in captivity.
[00:28:03.70] But there seems to be a reason for thinking that, because in captivity, according to the great experiments done by Dimitri Belyayev and Lyudmila Trut in Novosibirsk in Siberia since 1958, if you select animals, whether it's foxes or rats or mink, for reduced fear and aggression.
[00:28:28.16] And so if you select for breeding those individuals who are least emotionally reactive, then it turns out that you get the features of domesticated animals, in addition to a reduction in aggressiveness, features like white patches and floppy ears and curly tails. I wish that we could say that bonobos had ears long enough to be floppy or tails at all or some indication of white patches. They sort of has some indication of white patches. We can talk about that later maybe.
[00:29:01.71] But much more interesting is the view from archeology, because the view from archeology is, according to Helen Leach and colleagues, that there are certain consistent ways in which it is possible to recognize when domestication has happened from a wild ancestor. The domesticates end up with a lighter body than the wild ancestor, a shorter face and smaller teeth, with, if there are sex differences, of increasing feminization of the skull and the skeleton, a smaller brain in the domesticated species, and some evidence of pedomorphism, juvenile characteristics being retained into adulthood, something that is very characteristic of dogs compared to wolves.
[00:29:51.88] Well, let's think now about bonobos, because the prediction is that if bonobos have been through a biological process of reducing their aggressiveness that is paralleling what happens in domesticated animals, then we should see these features. And the short story is that we do. The bonobos have got a lighter body, particularly in the males than the females.
[00:30:20.22] So for a given body weight, they have literally thinner limb bones and lighter muscle mass. They have shorter face and smaller teeth. Here, what you see is a view from above of a couple of skulls and jaws laid out on a bench, as it were. And you see the humerus, the arm bone, and that is scaled to the same length. So you see that for a given length of the arm bone, the chimpanzee has got the longer cranium with a longer face, and also a very slightly thicker arm bone. The skull and skeleton are also feminized.
[00:31:02.84] So one way you can see this is that the canines, the fighting weapons of the bonobo, are reduced in length compared to those of chimpanzees. And their skulls are narrower. And that is an association both with maleness and with aggressiveness in humans and other primates. The brain-- well, you can kind of tell there's a reduction in the size of the brain just by looking at the size of the head. Because here, we've got a bonobo and a chimpanzee. And obviously, the chimpanzee looks as though it's got a bigger head.
[00:31:38.81] People sometimes have called bonobos the pygmy-headed chimpanzee. But now, if you look at the most recent publication on this, where people have been able to work out the relative change in size of the brain of bonobos in blue and chimpanzees in red, then males in particular are getting this smaller brain, females a little bit compared to the last common ancestor as they infer it and certainly compared to the chimpanzees.
[00:32:10.19] And then the final topic was pedomorphism. And absolutely, you get pedomorphic anatomy. Harold Coolidge, who some of you may remember, who lived in Cambridge and was long associated with Harvard, he was the man who discovered bonobos in 1928 in the Royal Central African Museum in Tervuren in Belgium.
[00:32:32.61] And he did so by pulling out a drawer of skulls and looking at a particular skull of what he thought was a juvenile chimpanzee and he said, "What clearly looked like a juvenile chimp skull from the south of the Congo, to my amazement, the sutures were totally fused." The sutures were fused. So what that meant was that it was an adult. The juvenile characteristic-- the shape of the skull-- had been retained into adulthood. That is, pedomorphism. And so that was the basis for his rushing into print to try and publish that species.
[00:33:11.99] And nowadays, people like Dan Lieberman in our department have done careful analysis with geometric, morphometric measurements and been able to show that the crania are, indeed, underdeveloped relative to those chimpanzees at any particular age. And that's pedomorphic anatomy. And there's a bunch of ways in which the behavior is pedomorphic as well. This is one that's rather nice, because there's actually a little bit of data that really crystallizes it. These data are from captivity, but what they show is for chimpanzees and bonobos, how much time the offspring is spending away from the mother.
[00:33:49.76] In this case, more than five meters away from the mother. And what you see is that at the age, say, of about two and 1/2 years, the bonobo is spending about 20% of its time more than five meters away from the mother and the chimpanzee is spending more like 40% or more percent five meters away from the mother. Much more independent. The bonobo is behaving more like a juvenile of the chimpanzee. Again, it is pedomorphic.
[00:34:21.17] So isn't this astonishing? We got the reduced aggressiveness of bonobos compared to chimpanzees. And in all of these ways that archeologists can recognize the difference between a domesticated animal and a wild animal, bonobos show the differences. The lighter body, the shorter face, smaller teeth, the feminized skull, the smaller brain, and the pedomorphic anatomy and behavior.
[00:34:45.95] Domestication is a really frustrating area, because not every species shows every feature of the domestication syndrome. I said that bonobos don't have curly tails. They didn't have floppy ears. That's probably because the ears of them, like other primates, are so short that the difference between the cartilage inside on the external part of the ear is so little that there's very little opportunity for them to be floppy.
[00:35:12.77] But nevertheless, this looks as though there's a very strong case to say that bonobos are echoing the process that happens in domestication of animals at the hand of humans. Only they're doing it in the wild. They're losing their aggressiveness. They've come self-domesticated.
[00:35:36.79] So now, I want to go into a little speculative set of questions about why this might have happened. What kind of selection pressures would have made bonobos less aggressive? And of course, what we're talking about is those darn males. And the principle is, that I want to say, is that the present offers us a clue to the past. And what I mean by that is the way in which bonobo males are controlled in terms of their aggression nowadays can give us a clue to the way it has happened in the past.
[00:36:10.12] Well, nowadays, what happens is that if a male is aggressive, then he has a good chance of being dominated by the female alliances. He is actually apparently a little bit afraid of the females. He doesn't like to be aggressive towards the sexually active females. And maybe that is because he has learned that he will then be disfavored. And that's all because, ultimately, he cannot impose his will through simply bullying and aggressiveness.
[00:36:43.99] So I like to think that what we're talking about is the critical advantage, the critical feature of female power, of females being able to support each other against the males. Now, then the question is, well, why is it that bonobos do this? And why don't chimps do it? You'd think that chimps would just have all sorts of advantages, as females, in being able to spend a lot of time together and attack those males.
[00:37:14.01] So then the question is, what is it that allows bonobos to have relatively stable parties, parties in which you can have lots of females supporting each other? And I approach this question from the point of view of their environment, because the key features enabling animals to stay together in groups is when feeding competition allows them the luxury of each other's company.
[00:37:41.21] And if we take the forests of the bonobos and the chimpanzees, then there's not that much difference apparently between the two species. They tend to occupy areas with really rich rainforest, where they're eating a lot of fruits. These particular fruits are eaten by bonobos. But they very much like the type that are eaten by the chimps.
[00:38:03.26] However, I think a clue comes from comparing two different parts of the diet. One is the fruit diet. And that is very much the same between bonobos and chimpanzees. But then there is the herb diet. And it has been shown that the quality of the herbs that are eaten by bonobos is better than the quality eaten by chimps. They have more options apparently. In both species, this is quite a common thing to do to-- if you can't find enough fruit, often by middle of the day, then you resort to eating leaves and stems, like these ones here.
[00:38:39.87] I want to explore the idea that it's because bonobos are able to eat those sorts of things, that they're able to eat and live in stable parties, and the essential underlying notion here is that, whereas fruits are clustered into isolated fruit trees, where there's going to be competition for those trees, the herbs are a continuous layer under the trees. And it's possible for the bonobos to stay together, munching on those herbs without being pressured to get away from each other in order to be able to feed better.
[00:39:16.00] Now, with that idea in mind, then let me put this conundrum here. You've got bonobos and chimps living basically on the equator, bonobos on the left bank, chimpanzees on the right bank. And you would think that the growing conditions are the same for both of those species, because they have similar humidity and temperature. So why is it that there should be some difference in ecology for the bonobos from the chimpanzees?
[00:39:50.69] Well, here, I think, is the answer. Gorillas live only on the right bank, and gorillas eat those herbs and compete for them. So gorillas are widely spread on the west side. They are more narrowly spread on the east side, as the humidity is declining and the herbs are becoming less numerous. But everywhere they live, whether in mountains or in the lowlands, they are able to survive without eating fruits at all if there are no fruits available, by munching on these herbs.
[00:40:31.41] And that seems to me enormously important. And it seems to me explicable that gorillas only occur on the right bank, because during periods of relative climatic drought, the gorillas retreat up into the mountains, it seems, where there's still a little bit of humidity sufficient to keep those herbs going. But on the left bank of the Congo River, there are no mountains. It's all totally flat, so that during periods when the area got just a little bit drier, there wouldn't be any herbs. And if the gorillas had ever got across, they would have gone extinct.
[00:41:12.85] So the speculation I have is that you've got in the area of the bonobos abundant gorilla food, because there is all of these herbs in this nice lowland area. That allows the bonobos to live in relatively stable groups, just like the gorillas living on those herbs live in relatively stable groups. That enables the females to form alliances, and therefore the males ultimately to be controlled.
[00:41:44.46] So the ultimate sort of notion here is that the way in which bonobos became self-domesticated is all a result of the fact that there weren't any mountains on the left bank of the Congo River. Well, that, of course, is speculative. We want some kind of answer. I think it's the best that I've come across. Maybe there'll be other answers that can come up.
[00:42:12.40] But nevertheless, it seems to me the right kind of answer in the sense that we can expect species to undergo sometimes selection in favor of increased aggressiveness and sometimes in favor of reduced aggressiveness. And the way in which we're going to understand selection for reduced aggressiveness is best going to be seen by seeing what happens nowadays. And nowadays, the groups of females are able to control the males. And so we have to think about how those groups of females were made possible. And there's my answer.
[00:42:44.46] Regardless of that sort of fun speculation-- hope it's a fun speculation-- it seems to me quite remarkable that the differences between bonobos and chimpanzees that we have reviewed have boiled down to such similarity between domesticated animals and their wild ancestors. The domestication syndrome is not easy to understand, except as an incidental consequence of selection against reduced aggression. Nobody has separate explanations for all of these different features.
[00:43:20.47] What is truly remarkable is that, if this happened a million years ago, which is one of the possible genetic dates, or could be earlier, then how is it that the species has not reversed? How is it that you get these incidental consequences coming along for the ride when selection favors reduced aggressiveness? You would think that over time, the brains would have got bigger, their teeth would have got bigger. But no. They are showing these very persistent signs of the domestication syndrome, apparently going back a million years or more.
[00:44:02.39] Well, I like to think that bonobos offer us a window into a type of evolution that we don't normally think much about. And that is a non-adaptive evolution. The fact that it is associated in the bonobo case with incidental consequences of reduced aggressiveness is very interesting when you think about the fact that, at an evolutionarily stable state, where you've got thousands of different species of, say, mammals, some of them will have had ancestors that were less aggressive than themselves.
[00:44:38.90] Others will have had ancestors that were more aggressive than themselves. And every time you got ancestors that were more aggressive than themselves, then you've got reduced aggressiveness. I think bonobos are suggesting, you would expect to see a whole suite of incidental characteristics produced, like we've seen in the bonobos.
[00:44:58.70] So what about islands? Islands are totally fascinating, because it's a predictable phenomenon that you get reduced aggressiveness on islands. Back in '85, island lizards, birds, and mammals in high density populations often exhibit reduced situation-specific aggression towards conspecifics. I think islands are going to be a wonderful place to look for evidence of these incidental non-adaptive effects.
[00:45:27.77] And I had really fun going to Zanzibar off the east coast of Tanzania and looking at the red colobus there compared to the red color that I knew from Kibale. So here we are on the island. It's been isolated for more than half a million years. So that's probably how old the red colobus are on Zanzibar island. And it turns out they do indeed have lighter bodies than those on the mainland.
[00:45:55.37] The sexual dimorphism is reduced. Males are becoming more female-like. Their canines are shorter. And it's already been published that their skull is pedomorphic. And so is their coloring. If you look at these pictures, what you see is on the left an adult from Kibale, on the continent. On the right, you see an adult from the island. And look at the facial coloration. The pink around the nose looks just like the picture in the middle, which is the infant from the continent. It's retained that facial coloration right into adulthood.
[00:46:33.29] And no one has really studied the behavior of the Zanzibar red colobus enough to know anything in detail. But Thomas Struhsaker has taken movies of suckling by males of adult females, who are presumed, but not known, to be the mothers. And that suckling happens later than known in any other primate. It's been seen in adolescent males and actually even occasionally by adult males. How much more pedomorphic can you get?
[00:47:04.18] So there are a bunch of cases, I think, where we can start thinking that this kind of domestication-like process has happened in the wild. And one of the areas that I'm kind of fascinated by is the possibility that there's been a hint of this in western chimpanzees compared to the other three subspecies of chimpanzees, because in the western chimpanzees, people have long been pointing out that, although they show the characteristics that I recounted in the first part of this talk, nevertheless, their females are more gregarious, there seems to be less sexual coercion, and overall, there is less violence.
[00:47:42.37] And they also apparently have shorter crania. And we've just done some measurements of the brains of our Kibale chimps and the brains, by comparison, of the western chimps. And the brains of the Kibale males are bigger than those of the ones in the west.
[00:48:00.40] So it's just a hint that maybe, again, we're going to see some version of the domestication syndrome even among the chimps. So I think we see some kind of self-domestication in humans. We see it in our two closest relatives. And in one of those relatives, maybe we see it in the subspecies. This could be quite a common phenomenon.
[00:48:25.76] The second reason that I think that it's fun to think about this self-domestication in the case of bonobos is to emphasize the fact that, ever since Darwin, much of the fight has been, can we really explain features as being adaptive? And what we're seeing here is that, with that fight largely won, that most features of animal species, including humans, can be thought of as adaptive, nevertheless, there may be systematic ways in which incidental, non-adaptive effects are produced.
[00:48:59.91] Now, explaining why that happens, I haven't touched that. You may have noticed. And for those of you who have looked at this area at all, you may know that one idea is that when there is selection for a reduction in aggressiveness, then what that initiates is a series of very early changes in the developing embryo affecting a tissue called the neural crest cells, and that that has a whole series of incidental consequences that are very difficult to separate from just the effects on aggression.
[00:49:37.27] And those consequences seem to be very well lined up with what we see in the domestication syndrome. People are starting to test the idea quite well in domesticated animals. It's going to be fun to look at wild animals that have got the domestication syndrome and see if we can elaborate that same idea.
[00:50:00.35] And then the third aspect is, can we get a bonobo perspective on human evolution? From time to time, some people have suggested that bonobos might serve as a nice model for an ancestor of human evolution. Well, I think that the evidence increasingly suggests that's wrong. It's a nice idea, because in many ways, bonobos have more attractive features than chimpanzees do. But nevertheless, even if it turns out that we are descended from an ancestor that is more chimpanzee-like than bonobo-like, what bonobos offer is another case of the self-domestication.
[00:50:48.59] Because that same syndrome that we saw exhibited by bonobos compared to chimpanzees, it's a syndrome that we see in modern humans, in terms of their skeletal material, compared to our ancestors some 300,000 years ago. So it looks to me as though bonobos and humans converged on pacifism, converged on our, in the human case, the one angle of our pacifism, namely the reduction in reactive aggression associated with ordinary day to day encounters with our friends and neighbors.
[00:51:29.18] And again, I would think that the present is a clue to the past. And if we look at small scale societies, what we see is that if a man, as sometimes happens, is sufficiently aggressive and sufficiently willing to bully others and sufficiently willing to go against all the societal norms, just to punch his way out of any kind of social conflict and take people's meat and take people's wives, then what happens ultimately is that he is liable to be executed. And that's very much why that is a clue to how it happened in the distant past.
[00:52:05.40] So what I suggested is that self-domestication in bonobos is a kind of model for selection against aggression in animals in general, with its source of non-adaptive effects. I think that it's going to be really exciting to use bonobos and other species to help explain the mechanisms underlying domestication, because it's such a different kind of system. And then, ultimately, to help think about the processes that led to evolution of ourselves.
[00:52:38.05] So the talk I've given is based on all sorts of work that many different people have helped me with and given me all sorts of ideas and supported me. They are the sort of female allies, as it were, if I was a bonobo. I want to draw attention to two particular people here, Isaac Schanberg, who is in the audience, and Martin Surbeck, because they are people who have done the long years in the field studying bonobos.
[00:53:09.37] Isaac is here as a college fellow in the Department of Human Evolutionary Biology and Martin is arriving in the fall. He is, I think many people would say, the person who is pushing the scientific study of bonobos in the wild faster and further than anybody else at the moment. So he's coming as an assistant professor, and he's going to be a great person to have around. So many thanks to all, and thanks to bonobos for offering a wonderful model. Thank you.