same is not true of social behaviour. By and large, ethologists have found very little phylogenetic inertia in social systems. Closely related species can have very different social organisation if they live in different habitats or eat different food. Distant relatives can have very similar social systems by convergent evolution if they inhabit similar ecological niches. Where two species show similar behaviour, it tells you less about their common ancestor and more about the pressures of the environment that shaped them.19
A good example is the sex life of the African apes. As primatologists delved further into the lives of apes, they found that alongside the similarities were some intriguing contrasts. These contrasts were thrown into sharper relief by the studies of George Schaller and Diane Fossey on gorillas, Birute Galdikas on orang-utans and the later studies of Takayoshi Kano on bonobos. In the zoo, a chimp looks a bit like a small gorilla. The skeletons of large chimpanzees have been confused with those of small gorillas. In the wild, however, there is a marked difference in their behaviour. It all starts with diet. Gorillas are herbivores, eating the stems and leaves of green plants such as nettles or reeds as well as some fruit. Chimpanzees are principally frugivores, seeking out fruit in trees, but adding ants, termites or monkey meat when they can. This difference in diet dictates a difference in social organisation. Plants are abundant but not very nutritious. To thrive on them, a gorilla must spend nearly all day eating and need not move very far. This makes a group of gorillas rather stable and easy to defend. This in turn has tempted male gorillas into evolving a polygamous mating strategy: each male can monopolise a small harem of females and their immature young, driving away other males.
Fruit, however, appears unpredictably in different places. Chimpanzees need to have large home ranges to be sure of finding a fruiting tree. But when a tree is found there is plenty of food to go round, so the animals can share their home range with many other chimps. But because of the large home range, these groups often split 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,
