American Scientist

For hundreds of years the apes have served as funhouse mirrors for what the human species once was, or perhaps might have been had evolution taken a different course. Among the four species of great apes, the chimpanzees have received the lion's share of attention as models of early humanity. Until the 1960s, however, when Jane Goodall first set out for Tanzania, we didn't know much about wild chimpanzees. What Goodall found shocked us: Chimpanzees were not only extremely clever, they also had complex societies and adept tool-using abilities, and they loved raw meat. In the decades that followed, field researchers observed other "human qualities" in wild chimpanzees: intercommunity warfare and lethal territorial aggression, cooperative hunting for other mammals (with the spoils of the hunt ritually shared and used as the bargaining chips of political and sexual barter), and the manufacture and use of tools made of plant products and, at some sites, of stone! These studies turned our view of chimpanzees (and of ourselves) on its head.

That chimpanzees are not vegetarian pacifists came as a surprise in anthropological circles when Goodall first reported the chimps' omnivorous appetites. Some scholars even alleged that the lethal aggression seen during encounters between neighboring social groups was aberrant behavior, occurring only in animals disturbed by human contact. But as the field data accumulated it became clear that the brutal side of chimpanzees is quite real. Males strive to ascend a rigid dominance hierarchy and on reaching high rank wield their political power in brutal ways. Sexual coercion and beating of females who do not submit to male desires are routine. Males patrol the perimeter of their territory, attacking and sometimes murdering their unwary neighbors. Chimpanzees at two study sites in Tanzania (Gombe National Park and Mahale National Park) were observed to fission into two separate communities, after which the larger community in each case systematically exterminated the smaller community. Such "warfare" has been seen in only two primate species, humans and chimpanzees.

Chimpanzees are also efficient and ruthless predators, consuming hundreds of prey animals including monkeys, antelope and wild pigs. Their attacks on their favorite prey, the red colobus monkey, are brutal and dramatic. The hunts often involve hand-to-hand combat between a chimp and a monkey, a match that is usually won by the chimp. Small-bodied juveniles are killed by a bite to the neck, whereas adult monkeys are thrashed against the ground or a tree limb. The meat is distributed in Machiavellian fashion by high-ranking males who share with allies and kin, but withhold the prize from rivals. They also use meat to entice ovulating females to mate with them—an orgy of meat eating and sex straight out of Tom Jones.

The Make-Love-Not-War Ape

Since the mid-1980s, the closely related, but only recently studied, bonobo has come to serve as an evolutionary counterpoint to the chimpanzee. They may look very much like chimpanzees—they were once called pygmy chimpanzees—but bonobos appear to be an ape of a different character. Studies of bonobos reveal a society molded by cooperation, alliance formation and recreational sex "as social communication." As primatologist Frans de Waal of Emory University puts it, "...[T]he high points of bonobo intellectual life are found not in cooperative hunting or strategies to achieve dominance but in conflict resolution and sensitivity to others."

Female bonobos band together in coalitions to dominate males, avoiding the sort of physical domination and sexual coercion that male chimpanzees routinely inflict on their females. Such female coalitions are nearly unknown among chimpanzees, where the male bonds are the cause and consequence of everything from communal hunting of small game to the fierce defense of their territorial borders.

Then there is the sex. Bonobos are often said to be, more than anything else, the sexy ape. They mate more often, in more positions and with more recreational than procreational intent than any mammal other than Homo sapiens. Copulation rates recorded by de Waal and Parish for captive bonobos in the San Diego Zoo and at the Yerkes Regional Primate Center in Atlanta are sky-high compared with such activities among wild chimpanzees. Bonobos also engage in female-female pairings, in which two females rub their genital swellings together ("GG rubbing" in the lexicon of bonobo researchers) to ease tensions between individuals. Male bonobos will also engage in same-sex genital rubbing. Such same-sex bonding is absent in chimpanzee society.

Relative to chimpanzees, bonobo society appears to be sex oriented and "less dominated" by males. As de Waal states, "The chimpanzee resolves sexual issues with power; the bonobo resolves power issues with sex."

An even more striking difference between female chimpanzees and bonobos is said to link the bonobos more closely to the human family tree. The females of nearly all mammalian species are reproductively active only during a constricted time period surrounding ovulation. This estrus period characterizes all of the higher primates, except human beings. Females of our species, although more likely to conceive around the time of ovulation, are freed from the bonds of a strictly defined period of "heat." The result is that sex serves not only for procreation, but also as a mechanism of social communication and reinforcement of long-term pair bonds.

Bonobo females are often said to be released from the bonds of estrus because they maintain their sexual swellings for a much longer portion of their menstrual cycles than chimpanzees do and therefore mate throughout much of the cycle. Since female apes of either species show little interest in mating except when they are swollen, this translates into more sex for the bonobos. Being "released from estrus," bonobos have come to use sex as much for communicating with males as for conceiving offspring, as in our own species.

In war as well as in romance, bonobos and chimpanzees appear to be strikingly different. When two bonobo communities meet at a range boundary at Wamba, a research site in the lowland rain forests of the Democratic Republic of Congo, bonobo researcher Takayoshi Kano observed that not only is there no lethal aggression as sometimes occurs in chimps, there actually may be socializing and sex between females and the enemy community's males.

When it comes to hunting and meat eating, we see a final striking contrast between bonobos and chimps. Bonobos catch monkeys in their rainforest habitat almost as well as chimpanzees do, but they don't seem to know what to do with them. Bonobos capture baby monkeys and then use them as dolls or playthings for hours, only to release the monkey unharmed (though worse for the wear) when they become bored with them. It's as if the protein and fat value of the prey hasn't dawned on their kinder, gentler nature.

Held Captive to Sex?

How real are these distinctions between chimpanzees and bonobos? Captive bonobos are indeed hypersexual, far exceeding their chimpanzee kin in both the quantity and quality of their sexual couplings, but whether this accurately reflects the behavior of wild bonobos is another question. Many of the stark behavioral contrasts are based on comparisons between wild chimpanzees and captive bonobos. Most of the available bonobo data come from captive groups in the San Diego Zoo and at Yerkes. Animals in captive settings are known for their tendency to display greater frequencies of various social behaviors compared to their wild counterparts. There is often not much else to do in captivity, where animals have no need to spend their day foraging for food. Their behavior patterns do not necessarily reflect those that evolved for living in an African forest.

So we should more appropriately turn to studies of wild bonobo populations. Although much less studied than chimps, we know about naturalistic patterns of bonobos from two long-term study sites in the Democratic Republic of the Congo: Wamba, the site directed by Kano, and Lomako, which has been occupied by two separate research teams from the United States and Germany.

The field data show that in two important respects, female bonobos are not more sexual than their chimpanzee counterparts. First, there is no difference in frequency of copulations when wild bonobos from Wamba are compared with wild chimps at either Tanzanian site, Gombe or Mahale. Second, the idea that bonobo females are released from estrus results from data on the duration of sexual swelling taken mainly from animals at Yerkes, where they maintain their sexual swelling for 23 days, nearly half of their 49-day cycle (in captivity). This dwarfs the receptive period of wild female chimpanzees from Gombe, who swell for about 13 days of their 36-day cycle. The equation changes if we consider wild bonobos rather than captive specimens, whose excellent nutrition may produce earlier menarche and ratcheted-up reproductive cycling. Bonobos from Wamba in the Congo are swollen for only 13 days of a 33-day cycle, numbers that are much closer to those of wild chimpanzees. A recent report about bonobos in the Antwerp Zoo shows that even in captivity, bonobos do not necessarily have longer swelling durations than chimpanzees. The supposed release from estrus that is said to characterize bonobos has been overstated because the data are based on captive animals.

Other aspects of bonobo behavior bear a second look as well. Female bonobos, it is true, are often dominant to males, but this domination occurs in only two settings, when either food or sex is involved. A male often gets sex by acceding to a female's desire to feed and so might be thought of as strategically submissive in select situations. Cleverness through subordination is certainly not unknown in other primate societies.

And are bonobos entirely peace-loving? About half of all the intercommunity encounters seen by Kano's team involved aggression of some sort. The difference between chimpanzee aggression and bonobo aggression is that bonobo attacks and injuries are often directed by females at males, rather than the reverse as in chimpanzees. There are even reports from zoos of female bonobos brutalizing a male so badly that his penis was severed.

Meat eating, while certainly less common among bonobos than among chimpanzees, may be under reported because bonobos are so little studied. Barbara Fruth and Gottfried Hohmann of the Max Planck Institute for Evolutionary Anthropology in Leipzig have observed extensive meat eating and meat sharing by bonobos at Lomako, but most "chimp-ologists" still refer to the bonobo as the "vegetarian" great ape.

Mirrors or Projections?

In recent years, some anthropologists have placed human beings at an evolutionary crossroads. One path leads to a chimpanzee-like world of male brutality and violence, where might makes right, and subordinates must grovel to avoid a beating. The other path leads to a kinder, gentler vision of humanity, one in which violence is not strength, and compassionate bonding is not weakness. It's not Camelot; it's bonobo society. This starkly black-and-white view of the two apes has become well entrenched in the public mind and in the mind's eye of many behavioral scientists. Sexy apes versus brutal ones represents a dichotomy that appeals to us—our possible evolutionary paths laid out in plain and simple terms.

The popular view, however, may have more to do with ideology than science. There is currently a trendy caricature of the human male and female as being so distinct from one another as to be from different planets—"men are from Mars" and "women are from Venus," the saying goes. Such notions are fine in a pop-culture setting, but do they serve us well in science? Are we projecting such simple conceptions a little bit too much on our primate cousins?

It wouldn't be the first time that idealized notions of ourselves have influenced the interpretation of data among evolutionary biologists. In the 1960s, the brotherhood of predominantly male anthropologists foisted "Man the Hunter" on students and the public alike, arguing that the male role of bringing home the bacon accounted for the rapid expansion of the human brain in hominid evolution. It wasn't until several years later that female anthropologists weighed in with the reminder that something had to account for expansion of women's brains in the course of our species' evolution. Such lessons remind us that we would do well to consider how our depictions of primate societies may become intertwined with our own political views.