Self-Domestication in Bonobos and Other Wild Animals


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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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? 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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? 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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.

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