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up temporarily. Consequently, for the male chimp, the polygamy strategy does not work. The only way to control access to such a large group of females is to share the job with other males. Hence the sexual favours of a troop of chimps are shared among an alliance of males. One becomes the ‘alpha’ male and takes a greater share of the matings, but he does not monopolise.

      This difference in social behaviour, stemming from a difference in diet, was wholly unsuspected until the 1960s. And it was only in the 1980s that a remarkable consequence became clear. The difference has left its mark on the anatomy of the two ape species. For gorillas the reproductive rewards of owning a harem of females are so great that males which take great risks to get them have generally proved more fecund ancestors than males of a more cautious disposition. And one risk that is worth running is growing to a very large size – even though it takes a lot of food to run a big body. Consequently, an adult male gorilla weighs about twice as much as a female.

      Among chimpanzees, males are not under such pressure to be big. For a start, being too big makes it harder to climb trees, and it means you have to spend more time eating. Better to be only a little larger than a female and to use cunning as well as strength to rise to the top of the hierarchy. Besides, there is no point in trying to suppress all sexual rivals, because you will sometimes need them as allies to defend the home range. However, because most females are mating with lots of males within the troop, the male chimps that most often became ancestors were in the past the ones that ejaculated often and voluminously. The competition between male chimps continues inside the female vagina in the form of sperm competition. Consequently, male chimpanzees have gigantic testicles and prodigious sexual stamina. As a proportion of body weight, chimpanzee testicles are 16 times greater than gorilla testicles. And a male chimp has sex approximately one hundred times as often as a male gorilla.

      There is a further consequence. Infanticide is common in gorillas as it is in many primates. A bachelor male infiltrates a harem, grabs a baby and kills it. This has two effects on the baby’s mother (apart from causing her great, though transient, distress): first, by halting her lactation it brings her back into oestrus; second, it persuades her that she needs a new harem master who is better at protecting her babies. And who better to choose than the raider? So she leaves her mate and marries her baby’s killer. Infanticide brings genetic rewards to males, who thereby become more fecund ancestors than males that do not kill babies; hence most modern gorillas are descended from killers. Infanticide is a natural instinct in male gorillas.

      But in chimps females have ‘invented’ a counter-strategy that largely averts infanticide: they share their sexual favours widely. The result is that any ambitious male, if he were to start his reign with a killing spree, might be killing some of his own babies. Males that hold back from baby-killing therefore leave more offspring behind. To confuse paternity by seducing many males into possible fatherhood, the females have evolved exaggerated sexual swellings on their pink bottoms to advertise their fertile periods.²⁰

      The testicle size of a chimp is a meaningless statistic on its own. It only makes sense by comparison with the gorilla testicles. That is the essence of the science of comparative anatomy. And having looked at two species of African ape in such a way, why not include a third? Anthropologists are fond of claiming an almost limitless diversity of behaviours in human cultures, but there is no human culture so extreme that it even begins to compare with the social system of either the chimpanzee or the gorilla. Not even the most polygamous human society is exclusively organised into harems that are passed from one male to another. Human harems are built up one by one, so that most males, even in societies that encourage polygamy, only have one wife. Likewise, despite various attempts to invent free-love communes, nobody has succeeded in achieving, let alone sustaining, a society in which every man has repeated brief affairs with every woman. The truth is, the human species has just as characteristic a mating system as any other: characterised by long pair bonds, usually monogamous, but occasionally polygamous, embedded in a large chimp-like troop or tribe. Likewise, however variable testicle size is among men, there is no man living whose testicles (as a proportion of body weight) are as small as a gorilla’s or as big as a chimpanzee’s. As a proportion of body weight, our testicles are nearly five times as large as gorillas’ and one-third the size of chimpanzees’. This is compatible with a monogamous species showing a degree of female infidelity. The difference between species is the shadow of the similarity within the species.

      An intriguing explanation of the human pair bond once again focuses on food. The primatologist Richard Wrangham puts it down to cooking. With the taming of fire and its adoption for cooking – which is a form of predigestion of food – there came a reduced need for chewing. Suggestive evidence for the controlled use of fire now goes back to 1.6 million years ago, but circumstantial evidence hints that it may have happened even earlier. At around 1.9 million years ago the teeth of human ancestors shrank at the same time as the body size of females grew. This indicates a better diet more easily digested, which in turn sounds like cooking. But cooking requires you to gather food and bring it to the hearth, which would have provided ample opportunities for bullies to steal the fruits of others’ labour. Or, since males were at that time much bigger and stronger than females, for males to steal food from females. Accordingly, any female strategy that prevented such theft would have been selected, and the obvious one was for a single female to form a relationship with a single male to help her guard the food they both gathered. These increasingly monogamous males would then not be competing with each other so fiercely for every mating opportunity, which would result in their becoming smaller relative to females – and the sex difference in size began to shrink 1.9 million years ago.²¹ Later, the pair bond developed into something even deeper when ancestral human beings invented a sexual division of labour. Among all hunter-gatherers, men are usually more interested in and better at hunting; women are more interested in and better at gathering. The result is an ecological niche that combines the best of both worlds – the protein of meat and the reliability of plant food.²²

      But, of course, there are not three species of African ape; there are four. The bonobos that live to the south of the Congo river may look rather like chimpanzees, but they have been evolving apart for two million years, ever since the river split their ancestral range in two. Like chimps they eat fruit; like chimps they live in large home ranges shared by multi-male troops. It follows that their sex lives, and their testicle size, should be like those of chimpanzees. But, as if to teach us scientific humility, they are astonishingly different. In bonobos, females are usually able to dominate and intimidate males. They do this by forming coalitions and coming to each other’s aid. A male bonobo in trouble can count on his mother’s support more than he can count on that of his male friends. An adult female bonobo, supported by her best friends, can usually outrank any male.²³

      But why? The secret of the bonobo sisterhood lies in sex. The bond between two female best friends is cemented by frequent and intense bouts of ‘hoka-hoka’, which scientists unromantically translate into genito-genital rubbing. Under the benign rule of cooperative and loving sisterhoods, the society of the bonobo reads more like a feminist fantasy than something real. That it should come to be understood only in the 1980s, when male-biased science was under challenge, is an uncanny coincidence. (The mind boggles at how the Victorians would have described hoka-hoka.)

      As predicted by feminist doctrine, male bonobos have reacted to the new female-dominated regime by evolving kinder, gentler natures. There is much less fighting and shouting, and so far murderous raids on members of other troops are unknown. Since female bonobos are even more sexually active than chimps and have sex nearly ten times as often (and a thousand times as often as gorillas), the ambitious male bonobo’s best strategy for attaining fatherhood is to save his energy for the bedchamber, not the boxing ring. I would like to be able to tell you that bonobo testicles are even bigger than chimpanzee testicles, but – although they are certainly very large – nobody has yet managed to weigh them.²⁴ In her book Sexual Selections, Marlene Zuk describes how the timely discovery of bonobo sex lives has made them into the latest animal celebrities,

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