bonobos and chimpanzees

Our next relatives in the animal kingdom are bonobos and chimpanzees. Genetically these two species of apes are almost identical. For a long time bonobos were simply seen as one of various chimp subspecies. Only in the 1930s the differences in anatomy led to a distinction as a different species. Since then some major differences in social behavior of bonobos and chimps have been found..

"The gorilla must have branched off from our family tree slightly before we separated from the common and pygmy chimpanzees. The chimpanzees and bonobos, not the gorilla, are our closest relatives. Put another way, the chimpanzees' closest relative is not the gorilla but the human. Traditional taxonomy has reinforced our anthropocentric tendencies by claiming to see a fundamental dichotomy between mighty man, standing alone on high, and the lowly apes all together in the abyss of bestiality. Now future taxonomists may see things from the chimpanzees' perspective: a weak dichotomy between slightly higher apes (the three chimpanzees, including the 'human chimpanzee') and slightly lower apes (gorilla, orang-utan, gibbons). The traditional distinction between 'apes' (defined as chimps, gorillas, etc.) and humans misrepresents the facts. The genetic distance (1.6%) separating us from pygmy or common chimps is barely double that separating pygmy from common chimps (0.7%). It is less than that between two species of gibbons (2.2%), or between such closely related North American bird species as red-eyed vireos and white-eyed vireos (2.9%), or between such closely related and hard-to-distinguish European bird species as willow warblers and chiffchaffs (2.6%). The remaining 98.4% of our genes are just normal chimp genes." (Jared Diamond, The Third Chimpanzee)

Bonobos
Bonobos from www.bonobos.org

Here is a concise description of the differences, further down you will find a more detailed article:

Bonobo: Often described as more playful and joyful. Primatologist Frans de Waal states bonobos are capable of altruism, compassion, empathy, kindness, patience, and sensitivity and described "bonobo society" as a "gynecocracy". Bonobos show more sexual behavior in a greater variety of relationships. An analysis of female bonding among wild bonobos by Takeshi Furuichi stresses female sexuality and shows how female bonobos spend much more time in estrus than female chimpanzees. (Wikipedia) Some researchers say, the life of the bonobo males is far more relaxed, healthier and longer.

Chimpanzee: larger and more robust than the bonobo. They live in groups which range in size from 15 to 150 members, although individuals travel and forage in much smaller groups during the day. The species lives in a strict male-dominated hierarchy, where disputes are generally settled without the need for violence. Nearly all chimpanzee populations have been recorded using tools, modifying sticks, rocks, grass and leaves and using them for hunting and acquiring honey, termites, ants, nuts and water. Their behavior is far more violent and aggressive.

One should think that bonobos have a far more relaxed and joyful life and therefore should have an evolutionary advantage over chimps. The distribution of the two species, however, tells a different story. Bonobos are restricted to a small area south of the Congo River and north of the Kasai River in the humid forests of the Democratic Republic of Congo. According to research estimates far less bonobos exist than chimps. Chimps are spread allover Central and West Africa. They started out from north of the Congo River. At a time when the Congo River was so big that it couldn't be crossed by apes, the populations south and north of the river were separated and took a different development. The northerners became chimps, the southerners bonobos, separated only through the big stream. In the northern area the chimp population had a tight competitor for food with the orangutans, which were not present in the southern part. The chimps had to adopt a more aggressive behavior in order not to starve. Later, when the chimp population became too big to survive in their ancestral area, they started spreading to other areas. They succeeded, because they could adjust to different environments and enemies. The bonobos on the other hand never managed to survive outside their ancestral habitat.

Chimps
Chimpanzees from www.audleytravel.com

The curious case of Bonobos and Chimpanzees

By English anthropologist and primatologist Richard Wrangham in "The Goodness Paradox" (2019).

These two sister species of great ape are our closest living relatives; and the ancestor of bonobos appears to have been very similar to a chimpanzee. Bonobos and chimpanzees look very similar. Both are black-haired apes that walk on their knuckles, weigh around 30 to 60 kilograms (65 to 130 pounds), with bigger males than females, and live in the moist rain forests of equatorial Africa. The easiest way to distinguish between the two is to note the bonobos’ smaller head topped by hair with a central parting of hair. Bonobos are also unique for their pink lips, even though the rest of their face is uniformly dark. The two species live separated by the Congo River that winds around Africa’s equator in the Democratic Republic of the Congo, chimpanzees on the right bank, to the north of the river, and bonobos on the left bank.

Bonobos and chimpanzees share much of their social behavior. Both live in communities of a few dozen individuals, including more females than males. Community members inhabit a common territory, which they defend against encroachers from neighboring communities. Within the territory, they form changing subgroups (also known as parties) of a few individuals up to twenty or thirty or more. They travel alone at times. Sons never leave their native community but daughters mostly do. Around the time when a daughter reaches puberty, she tends to leave her mother and move to a different community, where she will spend the rest of her life. Females typically mate several hundred times before giving birth to each infant, which means that they normally mate often with every adult male in their community. Mothers look after their young essentially without any help from others.

So the two species are in many ways alike, which makes their few differences in behavior all the more striking. The feature that points most directly to bonobos’ being self-domesticated is their relatively muted tendency for aggression. Bonobos are much less aggressive toward one another, and much less fearful of one another, than chimpanzees are. Zookeepers find bonobos easy to accommodate, because groups readily accept new individuals without any serious tension. Introducing chimpanzees to one another, by contrast, tends to be a painfully slow process, weeks or months of gradually familiarizing strangers with one another through wire mesh in order to minimize the risk of violence. Even after such cautious preparation, when chimpanzees who have not spent time together finally meet, they might easily fight. In the wild, too, long-term research has shown strong differences between chimpanzees and bonobos in every kind of competitive or aggressive behavior.

Chimpanzee males fight often with other members of their community. Sometimes they fight over valuable foods, such as hunks of meat. Sometimes they fight over mating privileges. Mostly, however, they fight over nothing more than status. They regularly charge at one another in displays intended to demand clear expressions of subordinacy. If the target does not give any signal of submissiveness, fighting tends to erupt. Usually, the aggressor wins, the reluctant subordinate screams passionately, and everyone else in the party runs for cover.

Males also commonly beat up on females, often in surprise attacks launched for no obvious reason. These bullying tactics can be sustained for many minutes. One attack at Kanyawara, in western Uganda, observed by the researcher Carole Hooven, lasted for a full eight minutes, during which the male grabbed sticks and beat the female intermittently with them, when he was not slapping, punching, and kicking her. The male’s aim in such attacks is to intimidate a chosen female into readily acceding to his future demands for sex.9 For each female, one male distinguishes himself from other males by being the one who most frequently attacks her. The tactic is often successful. Over subsequent weeks, a female’s most frequent aggressor tends to be her most frequent sex partner, and eventually, even though she is likely to mate several times with every male in her community, he will be the most likely father of her next baby. This stomach-churning practice is part of the reason why, as males become adult, they go through a ritual of beating up on every female. A male’s ability to intimidate females is a vital component of his strategy for having as many offspring as possible.

Chimpanzee aggression within communities can be even more extreme. Infants less than a few months old have occasionally been killed. Adults of either sex can be responsible for killing an infant, though the killer is never the mother. When a coalition of adults unites in aggression, fights can also lead to the death of an adult male. The coalition collaborates in a frenzy of grabbing, hitting, and biting until the victim is overwhelmed and immobile, sometimes dead on the spot, and at other times crawling away to die within hours or days from the injuries inflicted during the attack.

Interactions between chimpanzee communities are never relaxed or friendly. Most encounters involve wary avoidance, sometimes accompanied by “shouting matches” when the parties from each community are separated by a large enough distance for individuals to call bravely at each other. Danger looms when parties get close (which can happen either by accident or design) and when one party has many more males than the other. The males in the larger party seek to press their advantage against the other community. Sometimes they catch and kill a helpless victim, whether an infant or an adult. Targets of between-community aggression like this are lucky to escape alive.

The most extreme forms of chimpanzee violence are not frequent, and even the milder types need not occur every day. Nevertheless, long periods without emotional outbursts are rare. Particularly when fruits are abundant and subgroups are large, charges, fear screams, and beatings are an almost daily part of chimpanzee lives. Good feeding conditions can lead females to be sexually receptive and males to have abundant energy, a potent combination inviting the likelihood of raw male aggression.

Bonobos could hardly be more different from chimpanzees in terms of competition and aggression. Even though bonobo males can be ranked by dominance, competition among them rarely involves charging displays and includes no explicit signaling of status. Success in becoming high-ranking is rarely even dependent on interactions among the males themselves, because more important than their own fighting ability is the support given by mothers to their adult sons. The top-ranked males are mostly those with high-ranked mothers; when their mothers die, they are liable to fall in rank. Fighting over meat is infrequent, low-level, and more common among females than among males. Far less male intimidation of females occurs among bonobos than among chimpanzees. In one study bonobo females were more often aggressive toward males than vice versa. Male bonobos do not beat up on females, and in competition over food, females are more likely to win than males. No one has recorded any violent infanticides, nor any killing of adults, either within or between bonobo groups. Bonobos are certainly not free from disputes, and when groups from different communities meet, they sometimes fight to the point of inflicting bite wounds and scratches. But overall, the intensity of aggressive bonobo behavior is immensely less than among chimpanzees.

The primatologist Isabel Behncke documented a telling example of just how different the behavior of bonobos can be from chimpanzees. Unlike neighboring communities of chimpanzees with their predictable hostility, subgroups from neighboring bonobo communities often enjoy one another’s company to the point where they groom, have sex, and play together. Bonobo play includes a scary test of risk and trust between an adult and a juvenile that human observers call the “hang game.” Sitting on a branch up to thirty meters (one hundred feet) high in a tree, the adult holds one of the juvenile’s limbs and swings the willing plaything back and forth. The juvenile does not cling to the adult, which means that the adult holds the juvenile’s fate in his or her hand: a drop would risk a severe wound or death. Yet the juvenile shows “visible joy,” in Behncke’s words, smiling from intense fun, apparently unaware of the very possibility that the adult might drop him or her. Astonishingly, the “hang game” happens even between bonobos of different communities. The trust exhibited among bonobos is clearly fostered by a degree of nonaggression and fear reduction that would be remarkable in almost any group-living species, and even more so in this close relative of the explosive chimpanzee.

Similarly surprising, adult male bonobos from neighboring communities can play a “ball game” in which they chase one another slowly around a sapling, each trying to grab the testes of the one in front. Adult male chimpanzees occasionally play the same game with other males within their own community, the activity accompanied by excited faces and slow guttural chuckles. But the notion that any of them would play the game with a member of a different community would strike anyone who studies chimpanzees as ridiculous, given that the only relationship that has ever been seen between males from neighboring territories is instant hostility, leading to flight, shouting, or fight.

Why should two species that look so much alike be so different in their intensity of aggression? Anatomy, ecology, and psychology all play a role. Male chimpanzees have one anatomical difference from bonobos that is clearly associated with higher aggressiveness: their dagger-like canine teeth are substantially bigger. Compared to the canines of bonobos, those of male chimpanzees are longer (or “taller” as the anatomists would say); their upper canines 35 percent taller and lower canines 50 percent taller. Females show a similar but slightly smaller difference (upper canine 25 percent taller, lower canine 30 percent taller in chimpanzees). Long canines are unnecessary for fruit-eating species like chimpanzees and bonobos, and are probably even a nuisance. But canines make strong fighting weapons, so the possession of longer canines in chimpanzees reveals an important aspect of this species’ evolutionary history. Being a good fighter paid higher dividends in chimpanzees than bonobos.

We can readily imagine that male chimpanzees learn the power their canines give them. Once their canines have erupted, which happens at about the age of ten years, the males are much more dangerous as opponents. Youthful acquisition of tall canines could therefore increase the willingness to fight. Something similar happens in humans. Boys who are big for their age learn as early as three years old that they can win in fights with smaller peers. Being rewarded for successful aggression, they end up being more aggressive throughout childhood. Large body size is even a risk factor for antisocial personality disorder. Just as being big affects the psychology of young boys, the longer and more dangerous canines of chimpanzees could in theory promote more aggressive behavior.

We might expect that sex differences in size would also play a part in aggressive tendencies. In species in which males are bigger than females, such as gorillas or baboons, males tend to be more aggressive. Surprisingly, however, the little data available from the wild indicate that the difference in body weight between the sexes might be slightly less in chimpanzees (26–30 percent) than it is in bonobos (35 percent). Sheer body mass does not explain why male chimpanzees are more aggressive than male bonobos.

A second plausible explanation for differences in aggression between bonobos and chimpanzees comes from their environments. Bonobos might have less to fight about, or might live in conditions that are less conducive to aggressive behavior. We will see shortly that differences in the food supply indeed appear to contribute to differences in aggression, although the effect is likely due to the effects of food type on the grouping patterns of chimpanzees and bonobos rather than to the benefits of winning access to contested resources.

Even more than anatomy and ecology, temperament is the key to understanding why bonobos are so peaceful compared to chimpanzees. In captivity, the two species have been studied in similar conditions, with the exigencies of the wild food supply entirely irrelevant. Much of this work has been conducted in sanctuaries where people dedicate their lives to looking after orphan apes that have been rescued from the bushmeat trade. Only one such sanctuary houses bonobos, the aptly named Lola ya Bonobo, meaning Bonobo Paradise, located in the Democratic Republic of the Congo near Kinshasa. Lola has been a haven for bonobos ever since Claudine André founded it in 1994. North of the Congo River, in the neighboring Republic of the Congo, orphan chimpanzees live in a similar home, the Tchimpounga Sanctuary, maintained by the United States’ Jane Goodall Institute. Research at these two locations on dozens of bonobos and chimpanzees of all ages has done much to deepen our understanding of the two species.

The biological anthropologists Victoria Wobber and Brian Hare led the effort. Both were my graduate students. In 2005, I was delighted to accompany Hare and his wife, Vanessa Woods, on their first visit to Lola ya Bonobo. While designing experiments to study the behavior of a thoroughly non-murderous species, we occupied a guesthouse built by the former president, the murderous Mobutu Sese Seko.

Through systematic testing of dozens of Lola bonobos, as well as chimpanzees living in similar conditions in Uganda and the Republic of the Congo, Hare and Wobber confirmed and extended what we know about the two species’ psychological differences. Outright reactive aggression is not easily studied in apes, because experimenters do not want individual apes to be stressed or wounded. But reactive aggression is closely related to emotional reactivity, which can be assessed by measuring social tolerance. In the first experiment designed to find out whether sanctuary bonobos were more tolerant than sanctuary chimpanzees, Hare simply put some pieces of banana in one or two small piles in an empty room, and then allowed two individuals to enter simultaneously through the same door. The species difference was beautifully clear. When two chimpanzees entered the room, normally only one individual fed: he or she dominated access to the food while the other went off on its own, despite showing obvious interest in the banana. But in the equivalent situation, bonobos showed none of the grabbing or monopolizing of food, nor sad withdrawals: two individuals would eat side-by-side without tension. Differences between the species were the same regardless of whether the test subjects were juveniles or adults, or males or females.

A series of such studies all found the same kind of results. Bonobos shared food voluntarily, were more tolerant of others joining them at food, and were more skilled at tasks that demanded mutual tolerance, such as cooperating to get food that is out of reach. A particularly surprising finding came from studying whether bonobos preferred to eat alone or in the presence of another. Even when the potential companion belonged to a different social group, bonobos were so unaggressive that they voluntarily opened a door to allow the companion to join them and share their food pile. This left less food for the bonobo who opened the door, but that was not a concern. Company appeared to matter more than food.

The contrasts in tolerance were supported by other differences. Bonobos were more playful and affiliative. When two bonobos were introduced into a room with food, a typical response was rushing toward each other for a sexual interaction before approaching the food. The interaction could vary from mild rubbing of each other’s genitals all the way to full copulation, but, either way, the effect was the same as often seen in the wild: bonobos enjoy giving and receiving sexual pleasure, and they often use sex to relieve or deflect social tensions. After having sex together, whether in captivity or the field, bonobos fed next to each other easily. Chimpanzees never turned to play or sex in such tests.

In short, experiments with captive chimpanzees and bonobos have contributed a major part of the explanation for the pronounced species differences in frequency of reactive aggression. The differences are rooted in contrasting psychological tendencies. The greater tolerance shown by bonobos reflects a reduced emotional reactivity. Bonobos’ placid nature indicates a lower propensity for reactive aggression than found in the boisterous, hot-blooded, charming, but dangerous chimpanzee.

Neurobiologists are beginning to investigate how brain mechanisms contribute to these differences. In keeping with the behavioral results, relevant differences have been found in brain regions of the amygdala and cortex. Recall that higher levels of serotonin in the brain are associated with reduced reactive aggression. Strikingly, in bonobos, the amygdala contains twice as many serotonergic axons (nerves responding to serotonin) as it does in chimpanzees, suggesting one way in which bonobos have evolved a greater ability to regulate aggressive and fearful impulses. The biology of the brain appears, as expected, to be adapted to efficiently producing the kinds of emotional responses and social interactions that are characteristic of each species.20 Genes must underlie the differences.

So how did this difference in psychology evolve?

Bonobos and chimpanzees have been separate species for at least 875,000 years, and possibly as long as 2.1 million years, based on our current understanding of genetic differences between species. That is, ancestors of chimpanzees and ancestors of bonobos diverged from their common ancestor species between roughly 0.9 and 2.1 million years ago. From that divergence until now, chimpanzees and bonobos evolved into the two species we observe today. An average chimpanzee generation is about 25 years, which indicates that the psychological and anatomical differences between bonobos and chimpanzees evolved during at least thirty-five thousand generations of separate evolution. If bonobos are self-domesticated, their ancestor must have been more aggressive than bonobos are. So a critical question for the self-domestication hypothesis is whether, thirty-five thousand generations ago or more, the common ancestor of chimpanzees and bonobos was more aggressive than bonobos are today.

Behavior does not fossilize, and anyway no relevant fossils from that period have been found. However, bonobos have a series of anatomical distinctions from chimpanzees that are unique to their own lineage and that tell a story about their origins. Most notable is the feature that prompted the discovery of bonobos as a separate species from chimpanzees: the bonobo’s juvenilized skull.

It took sharp eyes to realize that the skull of the bonobo was special. As early as 1881, a bonobo skull had arrived in the British Museum of Natural History, but no one noticed that it was different from that of a chimpanzee. From 1910 onward, more bonobo bones went to Belgium. Later on, Western scientists even had the chance to see a living individual. Prince Chim, a juvenile, was brought to the United States in 1923 before dying of pneumonia while in the care of the American primatologist Robert Yerkes. Yerkes assumed that Prince Chim was a chimpanzee, albeit one with a quite exceptionally delightful personality. Everyone else who met Chim thought the same, including one Harold Coolidge, a twenty-year-old student on his way to join an expedition in West Africa searching for primates. No one realized that Prince Chim belonged to an undescribed species. Dead or alive, bonobos stayed under the radar for almost fifty years after their 1881 introduction to scientists.

The eventual breakthrough came out of nowhere. After his return from Africa, Harold Coolidge went to Tervuren, Belgium, in 1928 to measure gorilla skulls. Here, in his own words, is what happened when he visited the Royal Museum for Central Africa:

I shall never forget, late one afternoon in Tervuren, casually picking up from a storage tray what clearly looked like a juvenile chimp’s skull from south of the Congo and finding, to my amazement, that the epiphyses were totally fused. It was clearly adult. I picked up four similar skulls in adjoining trays and found the same condition.

The skull bones were fused! So, even though it looked like a juvenile’s skull, it had stopped growing. It had to be the skull of an adult. In juveniles of all mammals, flexible tissues called sutures link the growing bony plates of the skull. The sutures provide sufficient flexibility to allow the skull bones to move independently, accommodating the growing brain. Only when the brain has reached its maximum size do the sutures fuse, creating a stable structure. Coolidge mislabeled the sutures “epiphyses,” but he understood perfectly the significance of what he was seeing.

Coolidge’s observation meant that he was looking at a new variety of ape, similar in anatomy to a chimpanzee but differing by its adult skull being relatively small, rounded, and seemingly not fully developed, like the skull of a juvenile chimpanzee. Within days a German anatomist heard of Coolidge’s finding and scooped him by quickly publishing an account of bonobos as a new taxon, but he made a mistake by calling them merely a subspecies. Coolidge had the last laugh by calling bonobos a full species. In 1933 the newly identified species was named Pan paniscus Schwarz 1929.

As scientific scrutiny caught up with bonobos, the strange phenomenon of a juvenile-like skull in an adult body became more than than merely a distinction from chimpanzees. It also presented a way to reconstruct the evolutionary history of the newly named species. Chimpanzee and bonobo skulls are different now, but what was the skull of their common ancestor like? That is, since the divergence of the two species, in which species has the skull become more changed to a different form? The obvious possibility was that the bonobos’ juvenile-like skull was an example of paedomorphism or adult retention of a characteristic found in the juvenile of its ancestor. If this skull was a case of paedomorphism, we could conclude that bonobos had evolved from a chimpanzee-like ancestor. An alternative idea, however, also deserved consideration. The species difference could have come by peramorphism in chimpanzees rather than by paedomorphism in bonobos. Peramorphism refers to an adult characteristic being extended beyond its form in the ancestral species, and is thus the opposite of paedomorphism. If peramorphism explained the difference between bonobos and chimpanzees skulls, we could conclude that the bonobo skull morphology must have been the type found in the ancestor.

There is a way to tell whether the chimpanzee skull is peramorphic or, alternatively, the bonobo skull is paedomorphic: check the other apes. If the close relatives of bonobos and chimpanzees have a skull anatomy akin to that of bonobos, chimpanzees are the odd ones out; so the chimpanzee’s skull would be peramorphic, whereas bonobos would be relatively unchanged from the ancestral state. However, if the skulls of other apes are more like those of chimpanzees, the bonobo skull must be paedomorphic, a novel anatomy derived from an ancestor with a chimpanzee-like skull. The answer is easy. The skulls of other apes, including gorillas, orang-utans, and extinct species such as Australopithecus, are much more similar in these respects to skulls of chimpanzees than to those of bonobos. As the closest relative of bonobos and chimpanzees, gorillas provide the most relevant and revealing comparison. They follow the chimpanzee growth pattern so closely that they have been said to “resemble an ‘overgrown’ chimpanzee.” In short, bonobo skulls are paedomorphic; chimpanzee skulls are not peramorphic. The common ancestor from which both bonobos and chimpanzees descended most likely had a skull that looks like a modern chimpanzee skull. Bonobo skulls are the ones that changed; bonobos are the oddity.

Bonobos are oddities in other ways as well. Chimpanzees, gorillas, and orangutans all have limited estrus periods, and male dominance over females. Bonobos differ by having wildly extended estrus periods, and extensive female dominance over males. They have gone their separate way.

Why is it so important that we know that bonobo skulls changed so much? The skull houses the brain, and the brain directs behavior. Chimpanzee skulls retain the same ancestral style of cranial growth as gorillas and other great apes, suggesting that chimpanzee behavior has been relatively stable since they evolved from the last common ancestor of chimpanzees and bonobos. In contrast, bonobo skulls changed greatly, implying changes in bonobo brains and behavior. Identification of bonobos as a species separate from chimpanzees, and then establishing that bonobos are the species that changed more radically, indicates that the peaceability of bonobos is a new phenomenon, breaking away from the ancestral mold.

Thanks to the fact that bonobos and chimpanzees are closely related yet differ greatly in their propensities for aggression and their skull morphologies, and they are blessed with a number of informative relatives, we can have considerable confidence that bonobo skulls, brains, and behaviors have changed more from their common ancestor than have those of chimpanzees. In other words, the low reactive aggression of bonobos is a newly evolved phenomenon. So we can entertain a strong prediction: Bonobos should show the domestication syndrome.

Brian Hare, Victoria Wobber, and I tested that prediction in 2012, and found the first evidence for the domestication syndrome in a wild species. The cranial anatomy of bonobos turns out to fit the domestication syndrome extraordinarily closely. To start with, bonobo brains (or cranial capacities) are smaller than those of chimpanzees. The reduction is especially marked in males, where it can reach as much as 20 percent. This echoes the decrease in brain size of almost every species of domesticated vertebrate compared with their wild ancestors. All of the other major cranial features of the domestication syndrome are present as well. The faces of bonobos are relatively short, projecting less than faces of chimpanzees. Bonobos have smaller jaws, and smaller chewing teeth. The skulls also show reduced exaggeration of maleness, with males more feminized than in chimpanzees, and sex differences smaller.

These unusual features of bonobos have long been known, but they have not previously been discussed in relation to the theory of domestication. The physical anthropologist Brian Shea did as much as anyone to document the unique features of bonobo crania. He suspected that the key to understanding bonobos was the fact that the skulls of males and females were much more similar than they are in chimpanzees. “It seems likely,” Shea wrote, “that the reduced sexual dimorphism in the facial region of [bonobos] is related to social factors such as lowered male-male and male-female aggression, increased female bonding, increased food-sharing, and perhaps aspects of sexual behavior.” But how the morphological skull features are related to the behavioral tendencies remained unclear. And why should bonobos have smaller brains or smaller chewing teeth? They live in forests that are very similar to those occupied by chimpanzees a few miles away across the River Congo. The adaptive problems faced on either side of the river seem too similar to explain these important species differences in any simple way.

In the light of self-domestication theory, the differences make sense. As in captivity, we can reconstruct that, when there was selection against aggression in bonobos, the domestication syndrome emerged. Small brains, short faces, small teeth, reduced sex differences, and a paedomorphic skull —all are features both of domesticated animals found in bonobos. Admittedly, bonobos do not have floppy ears or white spots in their coats. Perhaps thirty-five thousand generations have eradicated those common components of the self-domestication syndrome, or perhaps the bonobos never acquired them. The frequency of these characteristics varies among domesticated animals. Few cats are floppy-eared, and water buffalo rarely have white spots. Still, depigmentation does occur in bonobos. Most individuals sport around their lips a striking pinkness, a loss of pigmentation that could well be associated with a delayed migration of neural-crest cells similar to the process known to occur in domesticated species. And at their rear end bonobos feature a tuft of white hair like that of chimpanzees in infancy. Bonobos differ from chimpanzees in this regard by retaining the white tail tuft (paedomorphically) until adulthood. Beyond anatomy, bonobo social behavior also remarkably fits the behavioral pattern Belyaev identified in domesticated silver foxes. In addition to reduced aggressiveness, there are two features of social behavior that are especially characteristic of domesticated animals: sex and play.

Domesticated animals such as dogs and guinea pigs show a greater variety of sexual behavior than their wild counterparts. So do bonobos compared with chimpanzees. Homosexual behavior is a striking example. In young primates, males readily mount both males and females in a premature version of sexual copulation that includes no actual intercourse. As they mature, the males shift their mounting to females, so that during adulthood homosexual mounts are very rare. Chimpanzees follow this pattern, but bonobos exhibit extensive homosexual behavior as adults. If homosexual mounting found among adulthood is a retention from the juvenile period, as these observations across species suggest, it is paedomorphic.

Adult homosexual behavior among bonobos is especially prominent among females. Among behavioral biologists it is prosaically called genitogenital rubbing, whereas in the Congo it is known as hoka-hoka. Hoka-hoka typically involves two females face-to-face, excitedly swinging their genitals side-to-side. The interaction sometimes ends in an orgasm-like pause including a tense face and contracted limbs. Hoka-hoka frequently follows social tension, such as when a subgroup of females finds an especially exciting food, or after a conflict between two females. If hokahoka is paedomorphic in chimpanzees, therefore, one should expect to see juvenile female chimpanzees engaged in similar behavior. They hardly ever do, but there are occasional reports of it.

A lovely example happened in Uganda’s Kibale National Park in 1994, several years after I started studying chimpanzees there. Our research team had discovered a juvenile chimpanzee being kept illegally as a pet in a local village, presumably orphaned after her mother had been killed for meat. With permission from the appropriate authorities, we rescued her. We called her Bahati and tried to introduce her into our wild study community. Bahati was about five or six years old. Females normally join a new community when they are around twelve years old, so she was definitely younger than she should have been to try entering a new group. She was also in poor physical condition, since she had not been able to climb trees while she was kept in the village. For three weeks, the researchers Lisa Naughton and Adrian Treves camped with Bahati in the forest to help her build up her strength and re-adapt to forest foods.

One day, a subgroup of our chimpanzee study community was nearby, and Lisa and Adrian led Bahati to them. The males were fascinated to meet the new juvenile. After some tense charging (which persuaded Bahati to stay close to Lisa and Adrian), some of the males approached her more gently and hugged her. It was a big relief for the humans that the wild chimpanzees were so welcoming to Bahati. Bahati seemed to feel the same. At any rate, while Lisa and Adrian watched nervously, Bahati went off with the wild chimpanzees to spend her first night away from humans since her capture, months earlier. She stayed with her new friends, day after day. A few weeks later, I was filming Bahati as she traveled with some of our chimpanzees. By then, the males no longer showed much interest in her, but she had developed friendly relationships with others of her age. At one point, a female of her own age, Rosa, waited for Bahati. As Bahati approached, Rosa rolled onto her back and opened her arms to encourage the still-timid orphaned stranger. Bahati embraced her, the two young ones hugged, and they swung their pelvic regions against each other. I had never seen this behavior before in chimpanzees, but it was instantly familiar. It looked like hoka-hoka between female bonobos. The meaning seemed clear. The behavior of Bahati and Rosa was a rare chimpanzee juvenile act. Bonobos had extended and elaborated it paedomorphically into a characteristic feature of their adult social lives.

Like homosexual behavior, social play is seen more in domesticated species such as dogs than among wild ancestors such as wolves; and it is also found among young primates more than among adults. And again, as Isabel Behncke showed, like a domesticated species, adult bonobos play more than do adult chimpanzees. The primatologist Elisabetta Palagi made careful comparisons between bonobos and chimpanzees in captivity where the two species lived in similar conditions. Not only did adult bonobos initiate play and use play faces more often than adult chimpanzees, but, interestingly, bonobos also played more roughly. One might have expected rough play to be the choice of the more aggressive chimpanzees, but since roughness demands more tolerance from the partner, rougher play is explicable by the overall nonaggressiveness of bonobos. In bonobos, sex and play are often linked. In Behncke’s words, “erect penises, playful intromission, and exploration of mature females’ [sexual] swellings” are some of the many elements that can make up a bonobo play session, unlike anything seen in chimpanzees.

The case for bonobos’ being self-domesticated could hardly be stronger. Bonobos are unequivocally less aggressive than chimpanzees. Their common ancestor with chimpanzees is best reconstructed as having a chimpanzee-like skull, brain, and behavior. And the bonobos’ differences from chimpanzees are characteristic of the domestication syndrome, whether anatomical (in the cranium) or psychological (in their sex and play). Those features of bonobos have not been explained by conventional adaptive logic. Thus no convincing reasons have been advanced using the parallel-adaptation hypothesis, which would state that bonobos’ reduced aggressiveness, paedomorphic skull, short face, and small teeth have all evolved in parallel as a result of responding to a series of separate selective pressures. However, Belyaev found a pattern in domesticated animals that appears applicable to bonobos. Belyaev’s rule, as we can call it, is that selection against reactive aggression leads to the domestication syndrome. That idea fits bonobos, because selection appears to have occurred against reactive aggression, generating features of the domestication syndrome. Seen in this light, many of the characteristics that so richly distinguish bonobos from chimpanzees did not evolve as adaptations. Instead, they evolved as incidental side effects of selection against reactive aggression. Genetic tests of this hypothesis will be rewarding, especially comparison of neural-crest genes in chimpanzees and bonobos.

The evidence offered by bonobos has exciting implications for vertebrate evolution. It suggests that in other species that have experienced a downward regulation of aggression, a self-domestication syndrome will likewise be found. If so, the male nipple of mammals might prove to be less unusual than it has seemed. Much as the male nipple results from a developmental constraint rather than being an evolved adaptation, so might short faces, small teeth, white patches, and other traits of the domestication syndrome also prove to have been produced by developmental constraints —even in wild animals. The bonobo evidence implies that reduced aggression has routinely led to incidental side effects.

It has a further implication of direct relevance to human evolution. If bonobos could be self-domesticated, the bonobo case supports the idea that humans could be self-domesticated too. But what the evidence of self-domestication does not do is explain why bonobo aggression was reduced.

You might think that more aggressive individuals would always fare better in the competition for evolutionary success. In fact, of course, too much of anything is a bad thing. An animal that fights too often, or too intensely, wastes energy and takes unnecessary risks. The trick is to get the balance right, to fight in the right contexts and at the right level of intensity, and only when the payoff is worthwhile.

So what is it about bonobos that makes aggression less profitable for them than it is for chimpanzees? Among chimpanzees, males practice the most frequent and dangerous forms of violence, and yet male bonobos are relatively unimposing, so the question is really about males. Ultimately, bonobo psychology has evolved to the point where males show less interest in dominating others, whether female or male, than chimpanzees do. The deeper question is why, over evolutionary time, males with gentler, less aggressive proclivities tended to have higher reproductive success. Female power is clearly an important part of the answer. A male bonobo who confronts an adult female might well win if she is the only female in earshot. But female bonobos are rarely far from other females. The challenging male must expect that, if he makes a female scream, within seconds he may be confronted by a coalition of females ready to attack him, and so effective in doing so that his best response will be to run away. Female support for one another explains why males give up easily when competing with females over food, or why males rarely try to bully females, or why males do not, on average, outrank females. Coalitionary attacks need not be common. The primatologists Martin Surbeck and Gottfried Hohmann found that, although females in the wild can use coalitions well, they do so rarely, mostly when males threatened their young. Despite their size disadvantage, females very effectively suppress bullying by males. Males seem to have learned where the ultimate power lies: numbers beat physical strength.

The reason that female bonobos can predictably present a united front appears mundane: they stay close to each other. Bonobo parties consistently contain a core of female associates and tend to have more females than males. Mostly the females are unrelated to each other by any ties of close kinship, because they are immigrants who joined the community as strangers when they were late juveniles or early adolescents. In the absence of kin ties, immigrants take several weeks or more of patiently following a group to be accepted. Eventually they engage in hoka-hoka, play, and grooming to become well integrated into the network of resident females. From then on they can count on support.

Chimpanzee parties, by contrast, are numerically dominated by males. Females tend to travel alone or in smaller subgroups. It seems likely that because of this relatively dispersed way of life, female chimpanzees fail to gain confidence in each other’s support against males. The only circumstances in which female chimpanzees have been seen to successfully mob aggressive males has been in those zoos where groups of females were alone together for months before males were introduced. Without males, chimpanzee females developed mutual trust. In the wild, most adult females apparently spend too little time together to learn to depend on each other. As we peel back the layers of bonobo evolution, we note that females can form their defensive coalitions because they form stable associations, and males are less aggressive because females render their aggression ineffective. But why are female bonobos more capable of forming stable associations than female chimpanzees? Animals are ultimately adapted to their environments, so the obvious place to start looking for an answer is in the special characteristics of the habitats of bonobos. In most respects, the habitats of chimpanzees and bonobos are very similar. The critical need for both species is access to rain forests or riverine gullies that harbor trees producing abundant fruit. Their habitudes differ in latitude, since chimpanzees live on average farther north and bonobos farther south. The Congo River, which separates the two species, curves so wildly, however, that in places chimpanzees live north, west, east, and even south of bonobos. Thus, in the equatorial region, the climate, soils, and forest types occupied by the two apes cannot be strictly distinguished. The forests vary on both sides of the river, and there is no evidence of any systematic difference in botanical structure or fruit production for the two ape species. Nevertheless, a major zoological difference between their habitats affects the availability of the bonobos’ food. Gorillas are found throughout the equatorial areas occupied by chimpanzees but are absent from bonobo habitats. The presence or absence of gorillas seems to account for a cascade of effects linking bonobo diet choices to their grouping patterns and social alliances and, ultimately, to their reduced aggressiveness. The cascade starts with gorilla competition for the foods eaten by chimpanzees. By contrast, bonobos, having no gorillas living in the same area, are relieved from that competition. So bonobos have more food choices than chimpanzees do. Gorillas are the only other great ape in Africa. They resemble bonobos and chimpanzees in being largely confined to rain forests, and their diets are broadly similar, too: they eat fruits when they can find them easily, and leaves and stems when fruits are scarce. But gorillas are much larger than the other apes. Female gorillas are two to three times the weight of female bonobos and chimpanzees, and males are three to four times heavier, averaging around 170 kilograms (375 pounds). Gorillas’ larger bodies mean that individuals find it harder to eat enough fruits during times when there are few productive trees. As a result, gorillas readily resort to a diet dominated by leaves and stems. Populations of gorillas high in the mountains even eat leaves and stems full-time, because at altitudes above 1,800 to 2,400 meters (6,000 to 8,000 feet) the climate is too cold to support more than the occasional edible fruit.

The plant types preferred as food are mostly the same for all three African apes. They are the young leaves, basal stems, or growth tips of large fast-growing plants such as Zingiberaceae (gingers), Marantaceae (arrowroots), and Acanthaceae (acanthuses). These plants tend to occur in “meadows,” often occupying gaps in the forest created by a treefall. The ability of gorillas to specialize in eating these herbaceous foods, and their willingness to do so once fruits are scarce, appear to create a problem for chimpanzees. Every morning chimpanzees eat newly ripened fruits as their first main meal within a few minutes of leaving their night beds. They continue eating fruit until ripe fruits become too scarce to be found easily, which might be around midday. Then they resort to finding a patch of leaves or stems to eat. But if gorillas have been there first, the herbaceous food patch will be inadequate for the chimpanzees. So the chimpanzees have to search for other food patches. Mothers travel slowly because of the presence of their young, and cannot keep up with fast-walking males. Needing to eat for many hours each day, they disperse, often alone, to find the small patches of food that will give them the calories they need. Bonobos, in contrast, without feeding competition from gorillas, have free rein to eat all of the ape foods that flourish in the environment. No other animal in the habitat of bonobos offers serious competition for eating these choice herbs, so bonobos can take the best of them. And that makes all the difference. An ape that can rely on cropping the herbaceous meadows can travel in relatively stable subgroups, slowly working their way from plant to plant. That is what gorillas do. The bonobos’ daily access to “gorilla foods” appears responsible for subgroups’ relative stability (as is also the case with gorillas) when compared with the shifting and smaller subgroups of chimpanzees.

The chain of logic takes us to a final question. Why are there no gorillas in the bonobo habitats? While the ancient distribution of gorillas is unknown, we do know that there are no mountains on the south side of the Congo River, where bonobos live. On the north side of the river, where chimpanzees live, there are mountains to the west and the east. The western mountains of Nigeria, Cameroon, and Gabon are the centers of diversity of the western gorilla. The eastern mountains of the Democratic Republic of the Congo, Rwanda, and Uganda are the core areas of the eastern gorillas. Mountains are the sites where gorillas can survive when a hot, dry climate means that the flat lowlands lose their lush herbs. If the lack of mountains on the south side is as significant as I think it is, we can reconstruct bonobo history as follows: The beginning of this history is undisputed, thanks to recent geological data. For longer than apes have been in existence the Congo River has been a barrier to animals moving south: maritime sediments show that the Congo has been pouring into the Atlantic Ocean for 34 million years. So the ancestors of chimpanzees, bonobos, and gorillas would always have lived north of the Congo River. Opportunities to cross to the south of the Congo River came during the Pleistocene era, which began about 2.6 million years ago and included cold, dry, Ice Age periods. Signs of low rainfall are found in the outflow area of the Congo River, in the form of marine sediments recording deposits of African dust. The deposits of dust are thought to coincide with reductions in the extent of forest, when the climate was dry. One such dry period occurred about 1 million years ago. The reduced rainfall probably caused the upper reaches of the Congo River to become sufficiently shallow that in a few places, even a non-swimming species like a great ape could make its way across. The ancestors of chimpanzees and bonobos duly crossed the river. They found an area much like the drier regions occupied by chimpanzees today. As long as there were riverine gullies where fruit trees could survive, the ancestral species would have thrived.

Gorilla ancestors might have crossed, too. But even if they did, the south side of the Congo River was unsuitable for them. The lack of mountains meant that there were no areas sufficiently damp to house the moist herbs and stems that gorillas needed for food. So, even if gorillas did cross the Congo River a million years ago, they would have died out in that area not much later.

Then, perhaps after a few thousand generations, the rains came back, the river became a barrier once more, and the lowland habitats grew lush again. South of the river there was plenty of food for two species of ape, but there were no gorillas there to crop the abundant forest herbs. The only ape was the ancestor of chimpanzees, which now evolved to become the ancestor of bonobos. The proto-bonobos thrived, eating both tree fruits and the newly abundant high-quality herbs. Mothers shifted from a frequently solitary foraging style to begin travelling, gorilla-like, in stable, larger subgroups of individuals of both sexes sharing the herb meadows. Males who tried to bully the females could now be repulsed.

With females’ power came a greater ability to choose the less aggressive males as mates. Females greatly extended their periods of sexual receptivity, and in so doing evolved a concealment of estrus. They could afford to have long periods of sexual attractiveness, since the presence of interested males was not a big problem in the big patches of gorilla food: there was little competition for the lush herbs. The males became much less sure of when to compete with each other, so intimidation of females no longer paid off as it does for male chimpanzees. As selection increasingly favored the less aggressive males as mates, the self-domestication syndrome emerged. Homosexual behavior emerged spontaneously and was then woven into the bonobo social system as a means to strengthen bonds and reduce tensions.

Genetic evidence indicates that the timing of this picture is a little more complicated than I have presented it so far. The first crossing of the Congo River by chimpanzee ancestors appears to have been followed by at least two other dry periods when ancestors of chimpanzees and ancestors of bonobos coexisted again and briefly bred together. This interbreeding did not have much genetic impact, however; less than 1 percent of genes in central African chimpanzees can be traced to bonobos. So a series of Pleistocene droughts allowed chimpanzee ancestors to cross the great river barrier and evolve into bonobos. Bonobos were left occupying a relatively small area compared with other great apes, and their numbers have been reduced by habitat loss and hunting so that now there are only between ten and fifty thousand in the wild. We are extraordinarily fortunate that they exist. They provide an illuminating contrast to chimpanzees, and they also attest to the power of Belyaev’s rule. They give us the best indication yet found that the process of becoming peaceful can have effects in the wild similar to those in captivity.