What separates man from the animals? Four inches and eleven minutes

Jared Diamond asks why the sex life of humans differs so substantially from that of even our nearest relations in the animal world

Sex preoccupies us. It is the source of our most intense pleasures. Often it also causes misery, much arising from built-in conflicts between the evolved roles of women and men. Most of us do not realise how unusual human sexual practices are, compared with those of all other living animals. Scientists infer that the sex life of even recent ape-like ancestors was very different from ours today. Some distinct evolutionary forces must have operated on our ancestors to make us different. What were those forces, and what really is so bizarre about us?

Understanding how our sexuality evolved also helps us to understand other distinctively human features. Those features include culture, speech, parent-child relations and mastery of complex tools. While palaeontologists usually attribute the evolution of these features to large brains and upright posture, I believe that our bizarre sexuality was equally essential for their evolution.

Unusual aspects of our sexuality include female menopause, the role of men in human societies, having sex in private - often for fun rather than for procreation - and the expansion of women's breasts even before use in lactation. To the layperson, these seem almost too natural to require explanation. But they are surprisingly difficult to account for. Also worth considering are the function of men's penises and the reasons why women, not men, nurse babies.

Most wild animals remain fertile until they die, or until close to that time. So do human males. Yet human females undergo a steep decline in fertility from around 40, leading to universal complete sterility within a decade or so. While some women have regular menstrual cycles up to the age of 55, conception after the age of 50 was rare until the recent development of medical technologies using hormone therapy and artificial fertilisation.

To laypeople, menopause is an inevitable fact of life. But to evolutionary biologists, it is an aberration in the animal world. The essence of natural selection is that it promotes genes for traits that increase the number of one's descendants bearing those genes. How could natural selection possibly result in every female member of a species carrying genes that throttle her ability to leave more descendants?

Theorising about the evolutionary basis of human female menopause must explain how a woman's apparently counterproductive evolutionary strategy of making fewer babies could result in her making more.

The evolutionary chain of reasoning rests on several cruel facts. One is the human child's long period of parental dependence, longer than in any other animal species. A baby chimpanzee starts gathering and preparing its own food as soon as it is weaned. But human hunter-gatherers acquire and prepare most of their food with tools, such as digging sticks, nets, spears and baskets. Even to wield all those tools is completely beyond the manual dexterity of babies. Tool use and tool making are transmitted not just by imitation but by language, which takes over a decade for a child to master.

So human children are not, in most societies, capable of adult economic function until their teenage years. Until then, the child stays dependent on parents, especially the mother. Parents are important not only for gathering food and teaching tool-making, but also for providing protection and status within the tribe. In traditional societies the early death of either parent prejudiced a child's life, even if the surviving parent remarried, because of possible conflicts with the step-parent's genetic interests.

Hence a mother who already has several children risks losing some of her genetic investment if she does not survive until the youngest is at least a teenager. That cruel fact becomes more ominous in the light of another: the birth of each child immediately jeopardises a mother's previous children because of the risk of death in childbirth. In most other animal species, that risk is insignificant. In a study of 401 pregnant female rhesus macaques, only one died in childbirth. For humans in traditional societies, the risk was much higher and increased with age. Even in affluent western societies, the risk of dying in childbirth is seven times higher for a mother over the age of 40 than for a 20-year-old. But each new child puts the mother's life at risk, not only because of the risk of death in childbirth, but also because of the delayed risks of death by exhaustion, carrying a young child and working harder to feed more mouths.

Yet another cruel fact is that infants of older mothers are themselves increasingly unlikely to survive or be healthy because of age-related increases in the risks of abortion, stillbirth, low foetal weight, and genetic defects. The risk of a foetus carrying Down's syndrome increases with the mother's age, from one in 2,000 births under 30 to the grim odds of one in ten for a mother in her late forties.

So, as a woman gets older, she is likely to have accumulated more children, has been caring for them longer and is putting a bigger investment at risk with each successive pregnancy. But her chances of dying in or after childbirth, or that her foetus or infant will die or be damaged, also increase. In effect, the older mother is taking on more risk for less potential gain.

These factors favour human female menopause and would paradoxically result in a woman ending up with more surviving children by giving birth to fewer children. Natural selection has not programmed menopause into men because of three more cruel facts: men never die in childbirth, rarely die while copulating and are less likely than mothers to exhaust themselves caring for infants. These are the major driving forces behind the evolution of human female menopause.

Consider also the evolution of body signals of exaggerated size and confined to one sex, which serve as social signals. Examples include a peacock's tail or a lion's mane. At first sight, we seem devoid of such structures. But perhaps we do sport one: a man's penis.

One might object that a human penis serves a non-signalling function and is nothing more than well-designed reproductive machinery. But that is not a serious objection to my speculation: women's breasts simultaneously constitute signals and reproductive machinery. Comparisons with our ape relatives hint that the size of the human penis similarly exceeds bare functional requirements and that the excess size may serve as a signal.

The erect penis is about one-inch long in gorillas, one inch in orangutans, but five inches in humans, even though male apes have much bigger bodies. Are those extra inches a functionally unnecessary luxury?

One counter-interpretation is that a large penis might be useful in the wide variety of our copulatory positions compared with many other mammals. But the one-inch penis of the male orangutan permits it to perform a comparable variety of positions, and to outperform us by executing them all while hanging from a tree. Orangutans also top us in sustaining intercourse - mean duration 15 minutes against a mere four minutes for the average American man.

A hint that the large human penis serves as some sort of signal is offered when men have the opportunity to design their own penises, rather than remaining content with their evolutionary legacy. Men in the highlands of New Guinea do that by enclosing the penis in a decorative sheath called a phallocarp. The sheath is up to two-foot long and four inches in diameter, often bright red or yellow in colour, and variously decorated at the tip with fur, leaves, or a forked ornament. Each man owns several, varying in size, ornamentation and angle of erection, and selects a different one each day, much as we select a shirt to wear. Asked why they wore phallocarps, the tribesmen replied that they felt naked and immodest without them - although they were otherwise naked and left even their testes exposed.

The phallocarp is in effect a conspicuous erect pseudo-penis representing what a man would like to be endowed with. The evolved size of the penis was limited by the length of a woman's vagina. A phallocarp shows us what it would be like if not subject to that constraint. The penis, while more modest than a phallocarp, is immodestly large by the standards of our ape ancestors, although the chimpanzee penis has also become enlarged over the inferred ancestral state and rivals human penises in size. Penis evolution evidently illustrates the operation of runaway selection just as the geneticist Sir Ronald Fisher postulated.

Starting from a one-inch ancestral ape penis similar to the penis of a modern gorilla or orangutan, the human penis increased in length by a runaway process, conveying an advantage to its owner as an increasingly conspicuous signal of virility, until its length became limited by counterselection as difficulties of fitting into a woman's vagina became imminent.

The human penis may also illustrate zoologist Amotz Zahavi's handicap theory as a structure costly to its owner. Granted, it is smaller and less costly than a peacock's tail. If the same quantity of tissue were instead devoted to the extra cerebral cortex, that brainy redesigned man would gain a big advantage.

In effect, a man is boasting, "I'm already so smart and superior that I don't need to devote more ounces of protoplasm to my brain, but I can instead afford and flaunt the handicap of packing the ounces uselessly into my penis."

What remains debatable is the intended audience of this proclamation of virility. Contrary to most male assumptions, women say that they are more turned on by other features and that the sight of a penis is, if anything, unattractive. Instead, the ones really fascinated by the penis and its dimensions are men.

Even if some women are also impressed by a large penis or satisfied by its stimulation during intercourse (as is very likely), it cannot be assumed that the signal is directed at only one sex. Zoologists studying animals regularly discover that sexual ornaments serve a dual function: to attract potential mates of the opposite sex and establish dominance over rivals of the same sex. In that respect, as in many others, we humans still carry the legacy of hundreds of millions of years of vertebrate evolution engraved deeply into our sexuality. Over that legacy, our art, language and culture have only recently added a veneer.

Jared M. Diamond is professor of physiology, UCLA school of medicine, Los Angeles. Why Is Sex Fun? The Evolution of Human Sexuality, Weidenfeld and Nicolson, Pounds 11.99.