More than a bit of monkey business

The sexual decorations and propensities of other species often strike us as bizarre. How weird would be the man excited by the pendulous pink bottom of a sexually receptive female chimpanzee? How outrageous the woman who emulated such females by soliciting sex from all her male colleagues for a few days every month. Our revulsion is understandable, but, says Jared Diamond, intellectually misplaced. If we stand back with a Martian biologist's eye, then the natural target for Diamond's opening chapter - the animal with the weirdest sex life - turns out to be our own species.

We are peculiar in living as mated couples within a society of other mated couples. We have sex all the time, rather than only in very restricted periods of female lust (oestrus). Unusually, males contribute to rearing their offspring. Sex is usually private and, finally, the menopause strangely cuts off women's reproductive output, but sex may nevertheless continue.

Diamond treats us to a snappy and engaging analysis of the evolutionary causes and consequences of being what Desmond Morris dubbed "the sexiest primate alive". Diamond argues that our unique sexuality was crucial for the emergence of other aspects of our humanity including culture, speech and technology.

Why is Sex Fun? will probably be of the greatest fascination to readers new to the evolutionary puzzles of human sexuality. Diamond cunningly weaves in lessons about fundamental ideas in evolutionary biology and the way in which scientists tackle these topics. We are told that women do not emulate female spiders who consume their mate during copulation because "a woman could not consume enough of a man's body at one sitting to improve significantly the nutritional basis for her pregnancy". In other words, evolutionary explanations for behavioural dispositions rest on their cost-benefit consequences, rather than on the motivations that cause them. Sexual tastes and other dispositions will be shaped to produce the most beneficial reproductive consequences.

The best science lessons are in the form of intriguing detective stories. One concerns "concealed ovulation". Why, unlike most female mammals, do women not advertise their monthly peak of fertility? Interestingly, concealed ovulation is found in several other primates, and Diamond describes how a stealthy analysis of the social correlates of these cases leads to a two-step scenario for its evolution, pointing counter-intuitively to origins in female promiscuity and male infanticidal tendencies.

But part of the fun of Why is Sex Fun? is that there will be much to argue over. For instance, how concealed is women's ovulation? There is evidence that ovulation does have behavioural correlates, from the frequency of sex itself (particularly if it is extramarital sex, when conception is more likely) to the synchronisation of women's menstrual cycles. Second, there is surely more to the elaborate nature of human sexual enjoyment than extending sex through the month: we still have to explain the extraordinary quality of it, as much as the quantity. One hypothesis is Geoffrey Miller's suggestion that the unprecedented evolutionary expansion of the human brain is of a magnitude consistent with it being a product of sexual selection, elaborate displays of imaginative love-making being one of the many signals selected for. Another point of contention is provided by the bonobo, for whom sex is clearly fun: sex for 75 per cent of the cycle, a Kama Sutra of positions, obvious female orgasms, female-female clitoral stimulation and masturbation. Of the three close relatives, human, chimp and bonobo, it looks as if the odd one out is the poor old chimpanzee, with its relatively restricted sex life. Like human males, the bonobo has an unusually long penis (a gorilla's is just over an inch long when erect). We are not sure if citing this fact causes a detumescence in the argument on which Diamond finishes - that the big human penis itself represents a special, sexually selected signal - or if it illustrates some more general principle about what can happen when sex is fun.

Andrew Whiten is professor of evolutionary and developmental psychology, Susie Whiten lecturer in biomedical science, University of St Andrews.