Sexual Politics of the Chimpanzees

Emlen and other researchers have undermined many cases of so‑called altruism. In only a few species, it now seems, do animals help out strangers without any regard for blood ties. Vampire bats are one. They spend each night searching for animals whose blood they can drink. But if they fail, they can return to their cave and beg for some blood from an unrelated bat that had a more successful night. Robert Trivers, a Rutgers University anthropologist, has dubbed cases like these “reciprocal altruism.” Evolution can favor reciprocal altruism, Trivers argues, because in the long run two unrelated animals that help each other may enhance their chances of survival more than if they behave more selfishly. Vampire bats burn up their food quickly, so if they go two or three days without blood they will starve. Donating blood to an unrelated bat may be a sacrifice, but it is also an insurance policy.

Reciprocal altruism may be particularly likely to evolve in species with big brains. If you have the mental capacity to recognize individuals and keep a scorecard of who has been good to you and who has been taking advantage of your kindness, you can use reciprocal altruism to your advantage. So it shouldn’t come as a surprise that some of the best evidence for animals helping strangers comes from our closest relatives–chimpanzees and bonobos (a separate ape species sometimes known as the pygmy chimpanzee).

Chimpanzees cooperate with unrelated chimps, do favors for them, and sometimes even make sacrifices for them. They may join together on hunting expeditions, looking for duikers or colobus monkeys and sharing their kills. Reciprocal altruism may help chimpanzees gain social power–two subordinate males, for example, can make an alliance to overthrow the top male in their group. And chimpanzees don’t just hand out favors blindly. They keep track of their kindness, and if they are betrayed they will cut off their generosity or even punish a cheating chimp.

In chimpanzee society, males can take advantage of reciprocal altruism while females cannot. Whereas males spend their whole lives in the band where they were born, female chimps leave when they reach adulthood. Once a female chimp joins another group, the demands of raising children keep her from establishing long‑term relationships with the new chimps. Lugging a nursing baby makes it impossible for her to keep up with the group as it searches for fruit. Because baby chimps depend on their mothers for up to four years, a female chimpanzee may end up spending 70 percent of her adult life away from her group.

As a result, male chimps have all the power. They bond with other males, making alliances that help them climb their way up through the chimp hierarchy. Reciprocal altruism also lets male chimps cope with their unstable supply of food. Chimpanzees rely mainly on fruit, which requires them to travel continually to find ripe trees. To supplement their diet, males can hunt together for meat and share the spoils, and they can also form raiding parties to attack smaller groups of chimps, taking over their fruit trees.

Meanwhile the females, who never get a chance to form alliances and enjoy the benefits of reciprocal altruism, can’t gain the power that male chimps have. If a group of chimpanzees comes across food, the females invariably wait while the males have their fill. Male chimpanzees also inflict violence on females. They will hit them to coerce them into having sex, and when a new female shows up in a group with a baby, the male chimps may kill it. “Chimpanzee society is horridly patriarchal, and horridly brutal,” says Richard Wrangham, a primatologist at Harvard University.

As in other species, female chimpanzees do not suffer passively. They do what they can to protect their babies and find good mates. Compared to other apes, young female chimpanzees take a long time to reach sexual maturity; some primatologists have suggested that the delay is a strategy for reducing the chances that they will come into a new group bearing an infant that may get killed.

Once a female chimpanzee does reach sexual maturity, she uses sex to protect her babies. Each time she becomes sexually receptive, her genitals swell and turn pink, and she approaches all the males of her group. Dominant males tend to have the most sex with her, but they cannot keep the females from mating with others. On average, a female chimp will have sex 138 times with 13 different males for every infant she gives birth to. Yet the signal of her swollen genitals is misleading: she is actually fertile for only a short time. As a result, about 90 percent of the times a female chimp has sex she actually cannot conceive. As with lions, female chimpanzees may be having sex with many males in order to defend against the infanticidal instinct of males, by making it harder for males to determine the paternity of their babies.

Love, not War

The evolutionary conflict between the sexes sometimes leads to maleon‑female violence, as in the case of chimpanzees, but it doesn’t have to. If the conditions are right, apes may evolve a tranquil existence, in which sex becomes far more than a matter of the survival of genes. It becomes a tool for keeping the peace.

The peaceful apes in question are the bonobos. Bonobos are relatively new to science; scientists first recognized they are different from chimpanzees only 70 years ago. In 1929 a German anatomist was studying the skull of a juvenile chimpanzee in a Belgian colonial museum when he realized that it actually belonged to a small adult of a different species. Bonobos, which live south of the Zaire River in the Democratic Republic of Congo, are not only smaller than ordinary chimpanzees but more slender, with long legs and narrow shoulders. Their lips are reddish and their ears are small and black. Their faces are flatter than a chimp’s, and they have long, fine black hair neatly parted in the middle.

The differences between chimps and bonobos are more than just anatomical. During World War II the Allied forces bombed the German city of Hellabrun. One of the zoos in the city had a colony of chimpanzees, which were not affected by the terrific sound of the explosions. Another zoo nearby kept a colony of bonobos, and all of them died of fright. A few years later two German primatologists studying bonobos at Hellabrun noticed that their sex lives were quite unlike that of chimpanzees. They wrote that chimpanzees mated more canum (like dogs) while bonobos did so more hominum (like people). Unlike any other primate except humans, bonobos had sex face‑to‑face.

The German primatologists were ignored by other ape experts, and it wasn’t until the 1970s that a new generation of scientists rediscovered that bonobos were dramatically different from chimpanzees. As with chimpanzees, male bonobos stay in the community where they are born and a female must leave to find a new community when she reaches adulthood. But when she arrives she doesn’t face a gang of bullying males ready to kill her baby and force her into sex. In bonobo society, the females dominate. If you toss a bunch of bananas into their midst, the females eat first, and the males wait their turn. If a male bonobo tries to attack a female, he’s liable to be stormed by a pack of angry females. They’ve been known to pin an offending male to the ground as one of them gives his testicles a painful bite. Male bonobos live in a hierarchy of their own, but it is the sons of the dominant females who rank high; among themselves, males form hardly any bonds at all.

A female bonobo joining a new community also enters a perpetual orgy. While female chimps have swollen genitals for 5 percent of their adult life, female bonobos are sexually receptive 50 percent of the time. Their sex lives start early: young bonobos start trying to mate long before they can possibly conceive. And bonobo sex is not just heterosexual. Young males will fence with their penises or give oral sex to each other. Females, meanwhile, specialize in rubbing their genitals together until they reach orgasm (what primatologists call the “g‑g rub”).

Among bonobos, sex is not just for reproduction, or even for protecting babies against angry males. It is a social tool. A new female will work her way into a bonobo community by approaching a resident female and giving her lots of sexual satisfaction. This favor wins her an alliance, and as she makes more of them, she can make her way toward the core of the community.

Sex can also defuse the tensions that build up in bonobo society. When bonobos come across food–be it a fruit tree or a termite nest–they start screaming in excitement. But rather than fighting over the food as chimpanzees might, the bonobos proceed to have sex. Likewise, if a male has a fit of jealousy and chases another male away from a female bonobo, the two males may later reunite for some scrotal rubbing. Sex keeps the underlying competition from escalating into all‑out war. “The chimpanzee resolves sexual issues with power; the bonobo resolves power issues with sex,” writes Frans de Waal, a primatologist at Emory University, in his book Bonobo: The Forgotten Ape.

Chimps and bonobos share a common ancestor that primatologists estimate lived 2 or 3 million years ago. Richard Wrangham and his colleagues have proposed that the difference stems from where the two primates live. Bonobos live in humid jungles, where the supply of fruit is much more reliable year‑round than the open forests where chimpanzees often live. And even if bonobos should run out of fruit, they can turn to the herbs that grow in abundance in their forests.

Thanks to the abundance of food, bonobo groups don’t have to move as quickly as chimps do. Female bonobos can keep up even when they have babies in tow. With enough food for everyone, the females don’t compete with one another, and can form long‑term alliances. By cooperating, the female bonobos can keep the males in line. As a result, infanticide is unknown in bonobo society. Because males are peaceful in their own groups, they don’t wage war against other groups. When two groups of bonobos meet up, they have sex rather than fight.

“It looks as though a relatively simple change in the feeding ecology is responsible for this dramatic difference in sexual behavior,” says Wrangham.

For female bonobos the benefits of this social structure are clear: they start becoming pregnant several years earlier than chimpanzees and can have more offspring. The difference, researchers suspect, lies in the fact that a female chimp has to cope with the threat of infanticide. Bonobo females, thanks to their social power, no longer have to worry about it.

Alliances, betrayals, deception, trust, jealousy, adultery, motherly affection, suicidal love–it all sounds rather human. When biologists talk about divorce among birds or adultery among mice, these words always bear invisible quotation marks around them. Nevertheless, we humans are animals–males with abundant sperm, females with scarce eggs–and our ancestors were subject to evolution just as much as any pipefish or jacana. Could it be that inclusive fitness, reciprocal altruism, and conflicts between males and females have something to do with the way we act, or even the way we think?

Drop this question at a bar full of biologists and prepare to dodge the flying pint glasses. Why are humans such a tender subject? To understand the murkiness of the matter, we first have to understand where humanity came from.

PART FOUR