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Bonobo Sex and Society

The behavior of a close relative challenges assumptions

about male supremacy in human evolution

Frans B. M. de Waal

(Originally published in the March 1995 issue of SCIENTIFIC AMERICAN, pp. 82-88)

FRANS B. M. de WAAL was trained as an

ethologist in the European tradition, receiving his

Ph.D. from the University of Utrecht in 1977. After a

six-year study of the chimpanzee colony at the

Arnhem Zoo, he moved to the U.S. in 1981 to work

on other primate species, including bonobos. He is

now a research professor at the Yerkes Regional

Primate Research Center in Atlanta and professor of

psychology at Emory University.

At a juncture in history during which women are seeking equality with men,

science arrives with a belated gift to the feminist movement. Male-biased

evolutionary scenarios – Man the Hunter, Man the Toolmaker and so on –

are being challenged by the discovery that females play a central, perhaps

even dominant, role in the social life of one of our nearest relatives. In the

past few years many strands of knowledge have come together concerning a

relatively unknown ape with an unorthodox repertoire of behavior: the

bonobo.

The bonobo is one of the last large mammals

to be found by science. The creature was

discovered in 1929 in a Belgian colonial

museum, far from its lush African habitat. A

German anatomist, Ernst Schwarz, was

scrutinizing a skull that had been ascribed to

a juvenile chimpanzee because of its small

size, when he realized that it belonged to an

adult. Schwarz declared that he had stumbled

on a new subspecies of chimpanzee. But soon the animal was assigned the

status of an entirely distinct species within the same genus as the

chimpanzee, Pan.

http://www.emory.edu/YERKES/

http://www.emory.edu/PSYCH/Faculty/dewaal.html

The bonobo was officially classified as Pan paniscus, or the diminutive Pan.

But I believe a different label might have been selected had the discoverers

known then what we know now. The old taxonomic name of the

chimpanzee, P. satyrus – which refers to the myth of apes as lustful satyrs –

would have been perfect for the bonobo.

The species is best characterized as female-centered and egalitarian and as

one that substitutes sex for aggression. Whereas in most other species sexual

behavior is a fairly distinct category, in the bonobo it is part and parcel of

social relations – and not just between males and females. Bonobos engage

in sex in virtually every partner combination (although such contact among

close family members may be suppressed). And sexual interactions occur

more often among bonobos than among other primates. Despite the

frequency of sex, the bonobo's rate of reproduction in the wild is about the

same as that of the chimpanzee. A female gives birth to a single infant at

intervals of between five and six years. So bonobos share at least one very

important characteristic with our own species, namely, a partial separation

between sex and reproduction.

A Near Relative

This finding commands attention because the

bonobo shares more than 98 percent of our

genetic profile, making it as close to a human

as, say, a fox is to a dog. The split between

the human line of ancestry and the line of the

chimpanzee and the bonobo is believed to

have occurred a mere eight million years ago.

The subsequent divergence of the

chimpanzee and the bonobo lines came much

later, perhaps prompted by the chimpanzee's need to adapt to relatively open,

dry habitats [see "East Side Story: The Origin of Humankind," by Yves

Coppens; SCIENTIFIC AMERICAN, May 1994].

In contrast, bonobos probably never left the protection of the trees. Their

present range lies in humid forests south of the Zaire River, where perhaps

fewer than 10,000 bonobos survive. (Given the species' slow rate of

reproduction, the rapid destruction of its tropical habitat and the political

instability of central Africa, there is reason for much concern about its

future.)

If this evolutionary scenario of ecological continuity is true, the bonobo may

have undergone less transformation than either humans or chimpanzees. It

could most closely resemble the common ancestor of all three modern

species. Indeed, in the 1930s Harold J. Coolidge – the American anatomist

who gave the bonobo its eventual taxonomic status – suggested that the

animal might be most similar to the primogenitor, since its anatomy is less

specialized than is the chimpanzee's. Bonobo body proportions have been

compared with those of the australopithecines, a form of prehuman. When

the apes stand or walk upright, they look as if they stepped straight out of an

artist's impression of early hominids.

Not too long ago the savanna baboon was regarded as the best living model

of the human ancestor. That primate is adapted to the kinds of ecological

conditions that prehumans may have faced after descending from the trees.

But in the late 1970s, chimpanzees, which are much more closely related to

humans, became the model of choice. Traits that are observed in

chimpanzees – including cooperative hunting, food sharing, tool use, power

politics and primitive warfare – were absent or not as developed in baboons.

In the laboratory the apes have been able to learn sign language and to

recognize themselves in a mirror, a sign of self-awareness not yet

demonstrated in monkeys.

Although selecting the chimpanzee as the touchstone of hominid evolution

represented a great improvement, at least one aspect of the former model did

not need to be revised: male superiority remained the natural state of affairs.

In both baboons and chimpanzees, males are conspicuously dominant over

females; they reign supremely and often brutally. It is highly unusual for a

fully grown male chimpanzee to be dominated by any female.

Enter the bonobo. Despite their common name – the pygmy chimpanzee –

bonobos cannot be distinguished from the chimpanzee by size. Adult males

of the smallest subspecies of chimpanzee weigh some 43 kilograms (95

pounds) and females 33 kilograms (73 pounds), about the same as bonobos.

Although female bonobos are much smaller than the males, they seem to

rule.

Graceful Apes

In physique, a bonobo is as different from a chimpanzee as a Concorde is

from a Boeing 747. I do not wish to offend any chimpanzees, but bonobos

have more style. The bonobo, with its long legs and small head atop narrow

shoulders, has a more gracile build than does a chimpanzee. Bonobo lips are

reddish in a black face, the ears small and the nostrils almost as wide as a

gorilla's. These primates also have a flatter, more open face with a higher

forehead than the chimpanzee's and – to top it all off – an attractive coiffure

with long, fine, black hair neatly parted in the middle.

Like chimpanzees, female bonobos nurse and carry around their young for

up to five years. By the age of seven the offspring reach adolescence. Wild

females give birth for the first time at 13 or 14 years of age, becoming full

grown by about 15. A bonobo's longevity is unknown, but judging by the

chimpanzee it may be older than 40 in the wild and close to 60 in captivity.

Fruit is central to the diets of both wild bonobos and chimpanzees. The

former supplement with more pith from herbaceous plants, and the latter add

meat. Although bonobos do eat invertebrates and occasionally capture and

eat small vertebrates, including mammals, their diet seems to contain

relatively little animal protein. Unlike chimpanzees, they have not been

observed to hunt monkeys.

Whereas chimpanzees use a rich array of strategies to obtain foods – from

cracking nuts with stone tools to fishing for ants and termites with sticks –

tool use in wild bonobos seems undeveloped. (Captive bonobos use tools

skillfully.) Apparently as intelligent as chimpanzees, bonobos have,

however, a far more sensitive temperament. During World War II bombing

of Hellabrun, Germany, the bonobos in a nearby zoo all died of fright from

the noise; the chimpanzees were unaffected.

Bonobos are also imaginative in play. I have watched captive bonobos

engage in "blindman's buff." A bonobo covers her eyes with a banana leaf or

an arm or by sticking two fingers in her eyes. Thus handicapped, she

stumbles around on a climbing frame, bumping into others or almost falling.

She seems to be imposing a rule on herself: "I cannot look until I lose my

balance." Other apes and monkeys also indulge in this game, but I have

never seen it performed with such dedication and concentration as by

bonobos.

Juvenile bonobos are incurably playful and like to make

funny faces, sometimes in long solitary pantomimes and at other times while

tickling one another. Bonobos are, however, more controlled in expressing

their emotions – whether it be joy, sorrow, excitement or anger – than are

the extroverted chimpanzees. Male chimpanzees often engage in spectacular

charging displays in which they show off their strength: throwing rocks,

breaking branches and uprooting small trees in the process. They keep up

these noisy performances for many minutes, during which most other

members of the group wisely stay out of their way. Male bonobos, on the

other hand, usually limit displays to a brief run while dragging a few

branches behind them.

Both primates signal emotions and intentions through facial expressions and

hand gestures, many of which are also present in the nonverbal

communication of humans. For example, bonobos will beg by stretching out

an open hand (or, sometimes, a foot) to a possessor of food and will pout

their lips and make whimpering sounds if the effort is unsuccessful. But

bonobos make different sounds than chimpanzees do. The renowned low-

pitched, extended "huuu- huuu" pant-hooting of the latter contrasts with the

rather sharp, high-pitched barking sounds of the bonobo.

Love, Not War

My own interest in bonobos came not from an inherent fascination with their

charms but from research on aggressive behavior in primates. I was

particularly intrigued with the aftermath of conflict. After two chimpanzees

have fought, for instance, they may come together for a hug and mouth-to-

mouth kiss. Assuming that such reunions serve to restore peace and

harmony, I labeled them reconciliations.

Any species that combines close bonds with a potential for conflict needs

such conciliatory mechanisms. Thinking how much faster marriages would

break up if people had no way of compensating for hurting each other, I set

out to investigate such mechanisms in several primates, including bonobos.

Although I expected to see peacemaking in these apes, too, I was little

prepared for the form it would take.

For my study, which began in 1983, I chose the San Diego Zoo. At the time,

it housed the world's largest captive bonobo colony – 10 members divided

into three groups. I spent entire days in front of the enclosure with a video

camera, which was switched on at feeding time. As soon as a caretaker

approached the enclosure with food, the males would develop erections.

Even before the food was thrown into the area, the bonobos would be

inviting each other for sex: males would invite females, and females would

invite males and other females.

Sex, it turned out, is the key to the social life

of the bonobo. The first suggestion that the

sexual behavior of bonobos is different had

come from observations at European zoos.

Wrapping their findings in Latin,

primatologists Eduard Tratz and Heinz Heck

reported in 1954 that the chimpanzees at

Hellabrun mated more canum (like dogs) and

bonobos more hominum (like people). In

those days, face-to- face copulation was considered uniquely human, a

cultural innovation that needed to be taught to preliterate people (hence the

term "missionary position"). These early studies, written in German, were

ignored by the international scientific establishment. The bonobo's

humanlike sexuality needed to be rediscovered in the 1970s before it became

accepted as characteristic of the species.

http://www.sandiegozoo.org/special/zoo-featured/pygmy_chimps.html?siteloc=2

Bonobos become sexually aroused

remarkably easily, and they express this

excitement in a variety of mounting positions

and genital contacts. Although chimpanzees

virtually never adopt face-to-face positions,

bonobos do so in one out of three copulations

in the wild. Furthermore, the frontal

orientation of the bonobo vulva and clitoris

strongly suggest that the female genitalia are

adapted for this position.

Another similarity with humans is increased female sexual receptivity. The

tumescent phase of the female's genitals, resulting in a pink swelling that

signals willingness to mate, covers a much longer part of estrus in bonobos

than in chimpanzees. Instead of a few days out of her cycle, the female

bonobo is almost continuously sexually attractive and active.

Perhaps the bonobo's most typical sexual pattern, undocumented in any other

primate, is genito-genital rubbing (or GG rubbing) between adult females.

One female facing another clings with arms and legs to a partner that,

standing on both hands and feet, lifts her off the ground. The two females

then rub their genital swellings laterally together, emitting grins and squeals

that probably reflect orgasmic experiences. (Laboratory experiments on

stump- tailed macaques have demonstrated that women are not the only

female primates capable of physiological orgasm.)

Male bonobos, too, may engage in pseudocopulation but generally perform a

variation. Standing back to back, one male briefly rubs his scrotum against

the buttocks of another. They also practice so-called penis-fencing, in which

two males hang face to face from a branch while rubbing their erect penises

together.

The diversity of erotic contacts in bonobos includes sporadic oral sex,

massage of another individual's genitals and intense tongue-kissing. Lest this

leave the impression of a pathologically oversexed species, I must add,

based on hundreds of hours of watching bonobos, that their sexual activity is

rather casual and relaxed. It appears to be a completely natural part of their

group life. Like people, bonobos engage in sex only occasionally, not

continuously. Furthermore, with the average copulation lasting 13 seconds,

sexual contact in bonobos is rather quick by human standards.

That sex is connected to feeding, and even appears to make food sharing

possible, has been observed not only in zoos but also in the wild. Nancy

Thompson-Handler, then at the State University of New York at Stony

Brook, saw bonobos in Zaire's Lomako Forest engage in sex after they had

entered trees loaded with ripe figs or when one among them had captured a

prey animal, such as a small forest duiker. The flurry of sexual contacts

would last for five to 10 minutes, after which the apes would settle down to

consume the food.

One explanation for the sexual activity at feeding time could be that

excitement over food translates into sexual arousal. This idea may be partly

true. Yet another motivation is probably the real cause: competition. There

are two reasons to believe sexual activity is the bonobo's answer to avoiding

conflict.

First, anything, not just food, that arouses the interest of more than one

bonobo at a time tends to result in sexual contact. If two bonobos approach a

cardboard box thrown into their enclosure, they will briefly mount each

other before playing with the box. Such situations lead to squabbles in most

other species. But bonobos are quite tolerant, perhaps because they use sex

to divert attention and to diffuse tension.

Second, bonobo sex often occurs in aggressive contexts totally unrelated to

food. A jealous male might chase another away from a female, after which

the two males reunite and engage in scrotal rubbing. Or after a female hits a

juvenile, the latter's mother may lunge at the aggressor, an action that is

immediately followed by genital rubbing between the two adults.

I once observed a young male, Kako, inadvertently blocking an older,

female juvenile, Leslie, from moving along a branch. First, Leslie pushed

him; Kako, who was not very confident in trees, tightened his grip, grinning

nervously. Next Leslie gnawed on one of his hands, presumably to loosen

his grasp. Kako uttered a sharp peep and stayed put. Then Leslie rubbed her

vulva against his shoulder. This gesture calmed Kako, and he moved along

the branch. It seemed that Leslie had been very close to using force but

instead had reassured both herself and Kako with sexual contact.

During reconciliations, bonobos use the same sexual repertoire as they do

during feeding time. Based on an analysis of many such incidents, my study

yielded the first solid evidence for sexual behavior as a mechanism to

overcome aggression. Not that this function is absent in other animals – or in

humans, for that matter – but the art of sexual reconciliation may well have

reached its evolutionary peak in the bonobo. For these animals, sexual

behavior is indistinguishable from social behavior. Given its peacemaking

and appeasement functions, it is not surprising that sex among bonobos

occurs in so many different partner combinations, including between

juveniles and adults. The need for peaceful coexistence is obviously not

restricted to adult heterosexual pairs.

Female Alliance

Apart from maintaining harmony, sex is also involved in creating the

singular social structure of the bonobo. This use of sex becomes clear when

studying bonobos in the wild. Field research on bonobos started only in the

mid-1970s, more than a decade after the most important studies on wild

chimpanzees had been initiated. In terms of continuity and invested

(wo)manpower, the chimpanzee projects of Jane Goodall and Toshisada

Nishida, both in Tanzania, are unparalleled. But bonobo research by

Takayoshi Kano and others of Kyoto University is now two decades under

way at Wamba in Zaire and is beginning to show the same payoffs.

Both bonobos and chimpanzees live in so-called fission – fusion societies.

The apes move alone or in small parties of a few individuals at a time, the

composition of which changes constantly. Several bonobos traveling

together in the morning might meet another group in the forest, whereupon

one individual from the first group wanders off with others from the second

group, while those left behind forage together. All associations, except the

one between mother and dependent offspring, are of a temporary character.

Initially this flexibility baffled investigators, making them wonder if these

apes formed any social groups with stable membership. After years of

documenting the travels of chimpanzees in the Mahale Mountains, Nishida

first reported that they form large communities: all members of one

community mix freely in ever changing parties, but members of different

communities never gather. Later, Goodall added territoriality to this picture.

That is, not only do communities not mix, but males of different chimpanzee

communities engage in lethal battles.

In both bonobos and chimpanzees, males stay in their natal group, whereas

females tend to migrate during adolescence. As a result, the senior males of

a chimpanzee or bonobo group have known all junior males since birth, and

all junior males have grown up together. Females, on the other hand, transfer

to an unfamiliar and often hostile group where they may know no one. A

chief difference between chimpanzee and bonobo societies is the way in

which young females integrate into their new community.

On arrival in another community, young bonobo females at Wamba single

out one or two senior resident females for special attention, using frequent

GG rubbing and grooming to establish a relation. If the residents reciprocate,

close associations are set up, and the younger female gradually becomes

accepted into the group. After producing her first offspring, the young

female's position becomes more stable and central. Eventually the cycle

repeats with younger immigrants, in turn, seeking a good relation with the

now established female. Sex thus smooths the migrant's entrance into the

community of females, which is much more close-knit in the bonobo than in

the chimpanzee.

Bonobo males remain attached to their mothers all their lives, following

them through the forest and being dependent on them for protection in

aggressive encounters with other males. As a result, the highest-ranking

males of a bonobo community tend to be sons of important females.

What a contrast with chimpanzees! Male chimpanzees fight their own

battles, often relying on the support of other males. Furthermore, adult male

chimpanzees travel together in same-sex parties, grooming each other

frequently. Males form a distinct social hierarchy with high levels of both

competition and association. Given the need to stick together against males

of neighboring communities, their bonding is not surprising: failure to form

a united front might result in the loss of lives and territory. The danger of

being male is reflected in the adult sex ratio of chimpanzee populations, with

considerably fewer males than females.

Serious conflict between bonobo groups has been witnessed in the field, but

it seems quite rare. On the contrary, reports exist of peaceable mingling,

including mutual sex and grooming, between what appear to be different

communities. If intergroup combat is indeed unusual, it may explain the

lower rate of all-male associations. Rather than being male- bonded, bonobo

society gives the impression of being female- bonded, with even adult males

relying on their mothers instead of on other males. No wonder Kano calls

mothers the "core" of bonobo society.

The bonding among female bonobos violates a fairly general rule, outlined

by Harvard University anthropologist Richard W. Wrangham, that the sex

that stays in the natal group develops the strongest mutual bonds. Bonding

among male chimpanzees follows naturally because they remain in the

community of their birth. The same is true for female kinship bonding in Old

World monkeys, such as macaques and baboons, where males are the

migratory sex.

Bonobos are unique in that the migratory sex, females, strongly bond with

same-sex strangers later in life. In setting up an artificial sisterhood, bonobos

can be said to be secondarily bonded. (Kinship bonds are said to be

primary.) Although we now know HOW this happens – through the use of

sexual contact and grooming – we do not yet know WHY bonobos and

chimpanzees differ in this respect. The answer may lie in the different

ecological environments of bonobos and chimpanzees – such as the

abundance and quality of food in the forest. But it is uncertain if such

explanations will suffice.

Bonobo society is, however, not only female-

centered but also appears to be female-

dominated. Bonobo specialists, while long

suspecting such a reality, have been reluctant to

make the controversial claim. But in 1992, at

the 14th Congress of the International

Primatological Society in Strasbourg,

investigators of both captive and wild bonobos

presented data that left little doubt about the

issue.

Amy R. Parish of the University of California at

Davis reported on food competition in identical

groups (one adult male and two adult females)

of chimpanzees and bonobos at the Stuttgart Zoo. Honey was provided in a

"termite hill" from which it could be extracted by dipping sticks into a small

hole. As soon as honey was made available, the male chimpanzee would

make a charging display through the enclosure and claim everything for

himself. Only when his appetite was satisfied would he let the females fish

for honey.

In the bonobo group, it was the females that approached the honey first.

After having engaged in some GG rubbing, they would feed together, taking

turns with virtually no competition between them. The male might make as

many charging displays as he wanted; the females were not intimidated and

ignored the commotion.

Observers at the Belgian animal park of Planckendael, which currently has

the most naturalistic bonobo colony, reported similar findings. If a male

bonobo tried to harass a female, all females would band together to chase

him off. Because females appeared more successful in dominating males

when they were together than on their own, their close association and

frequent genital rubbing may represent an alliance. Females may bond so as

to outcompete members of the individually stronger sex.

The fact that they manage to do so not only in captivity is evident from

zoologist Takeshi Furuichi's summary of the relation between the sexes at

Wamba, where bonobos are enticed out of the forest with sugarcane. "Males

usually appeared at the feeding site first, but they surrendered preferred

positions when the females appeared. It seemed that males appeared first not

because they were dominant, but because they had to feed before the arrival

of females," Furuichi reported at Strasbourg.

Occasionally, the role of sex in relation to food is taken one step further,

bringing bonobos very close to humans in their behavior. It has been

speculated by anthropologists – including C. Owen Lovejoy of Kent State

University and Helen Fisher of Rutgers University – that sex is partially

separated from reproduction in our species because it serves to cement

mutually profitable relationships between men and women. The human

female's capacity to mate throughout her cycle and her strong sex drive

allow her to exchange sex for male commitment and paternal care, thus

giving rise to the nuclear family.

This arrangement is thought to be favored by natural selection because it

allows women to raise more offspring than they could if they were on their

own. Although bonobos clearly do not establish the exclusive heterosexual

bonds characteristic of our species, their behavior does fit important

elements of this model. A female bonobo shows extended receptivity and

uses sex to obtain a male's favors when – usually because of youth – she is

too low in social status to dominate him.

At the San Diego Zoo, I observed that if

Loretta was in a sexually attractive state,

she would not hesitate to approach the adult

male, Vernon, if he had food. Presenting

herself to Vernon, she would mate with him

and make high- pitched food calls while

taking over his entire bundle of branches

and leaves. When Loretta had no genital

swelling, she would wait until Vernon was

ready to share. Primatologist Suehisa

Kuroda reports similar exchanges at

Wamba: "A young female approached a

male, who was eating sugarcane. They

copulated in short order, whereupon she took one of the two canes held by

him and left."

Despite such quid pro quo between the sexes, there are no indications that

bonobos form humanlike nuclear families. The burden of raising offspring

appears to rest entirely on the female's shoulders. In fact, nuclear families

are probably incompatible with the diverse use of sex found in bonobos. If

our ancestors started out with a sex life similar to that of bonobos, the

evolution of the family would have required dramatic change.

Human family life implies paternal investment, which is unlikely to develop

unless males can be reasonably certain that they are caring for their own, not

someone else's, offspring. Bonobo society lacks any such guarantee, but

humans protect the integrity of their family units through all kinds of moral

restrictions and taboos. Thus, although our species is characterized by an

extraordinary interest in sex, there are no societies in which people engage in

it at the drop of a hat (or a cardboard box, as the case may be). A sense of

shame and a desire for domestic privacy are typical human concepts related

to the evolution and cultural bolstering of the family.

Yet no degree of moralizing can make sex disappear from every realm of

human life that does not relate to the nuclear family. The bonobo's

behavioral peculiarities may help us understand the role of sex and may have

serious implications for models of human society.

Just imagine that we had never heard of chimpanzees or baboons and had

known bonobos first. We would at present most likely believe that early

hominids lived in female- centered societies, in which sex served important

social functions and in which warfare was rare or absent. In the end, perhaps

the most successful reconstruction of our past will be based not on

chimpanzees or even on bonobos but on a three-way comparison of

chimpanzees, bonobos and humans.

Social Organization among Various Primates

BONOBO Bonobo communities are peace-loving and generally egalitarian. The

strongest social bonds are those among females, although females

also bond with males. The status of a male depends on the position of

his mother, to whom he remains closely bonded for her entire life.

CHIMPANZEE

In chimpanzee groups the strongest bonds are established between

the males in order to hunt and to protect their shared territory. The

females live in overlapping home ranges within this territory but are

not strongly bonded to other females or to any one male.

GIBBON

Gibbons establish monogamous, egalitarian relations, and one couple

will maintain a territory to the exclusion of other pairs.

HUMAN Human society is the most diverse among the primates. Males unite

for cooperative ventures, whereas females also bond with those of

their own sex. Monogamy, polygamy and polyandry are all in

evidence.

GORILLA

The social organization of gorillas provides a clear example of

polygamy. Usually a single male maintains a range for his family

unit, which contains several females. The strongest bonds are those

between the male and his females.

ORANGUTAN Orangutans live solitary lives with little bonding in evidence. Male

orangutans are intolerant of one another. In his prime, a single male

establishes a large territory, within which live several females. Each

female has her own, separate home range.

To find out more about bonobos, check out

Bonobo Protection Fund

Bonobo Links

Sex-crazed Bonobos May be More Like Humans

Human DNA is 98.4 percent identical to the DNA of chimps and

bonobos, a lesser-known chimp-like ape.