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It is also non-random. The urge to have sex is in us because we are all descended from people who had an urge to have sex with each other; those that felt no urge left behind no descendants. A woman who has sex with a man (or vice versa) is running the risk of choosing a set of genes to partner hers in the next generation. Little wonder that she is prepared to pick those partner genes carefully. Even the most promiscuous woman does not have sex indiscriminately with anyone who comes along.

The goal for every female animal is to find a mate with sufficient genetic quality to make a good husband, a good father or a good sire. The goal for every male animal is often to find as many wives as possible, and sometimes to find good mothers and dams, only rarely to find good wives. In 1972 Robert Trivers noticed the reason for this asymmetry, which runs right through the animal kingdom; the rare exceptions to his rule prove why it generally holds. The sex that invests most in rearing the young – by carrying a foetus for nine months in its belly, for example – is the sex that makes least marginal profit from an extra mating. The sex that invests least has time to spare to seek other mates. Therefore, broadly speaking, males invest less and seek quantity of mates, while females invest more and seek quality of mates.2

The result is that males compete for the attention of females, which means that males have both a greater opportunity to leave large numbers of offspring than females and a greater risk of fathering none. Males act as a kind of genetic sieve: only the best males get to breed and the constant reproductive extinction of bad males constantly purges bad genes from the population.3 From time to time, it has been suggested that this is the ‘purpose’ of males, but that commits the fallacy of assuming that evolution designs what is best for the species.

The sieve works better in some species than in others. Elephant seals are sieved so severely that in each generation a handful of males fathers all of the offspring. Male albatrosses are so faithful to their single wives that virtually every male that reaches the right age will breed. None the less, it is fair to state that, in the matter of choosing mates, males are usually after quantity and females after quality. In the case of a bird like the peacock, males will go through their ritual courtship display for any passing female; females will mate with only one male, usually the one with the most elaborately decorated tail. Indeed, according to sexual selection theory, it is the female’s fault that the male has such a ridiculous tail at all. Males evolved long tails to charm females. Females evolved the ability to be charmed to be sure of picking the best males.

This chapter is about a different kind of Red Queen contest, one that resulted in the invention of beauty. For, in human beings, when all practical criteria for choosing a mate – wealth, health, compatibility, fertility – are ignored, what is left is the apparently arbitrary criterion of beauty. It is much the same in other animals. In species where the females get nothing useful from their mates, they seem to choose on aesthetic criteria alone.

Ornaments and Choosiness

To put it in human terms we are asking of animals (as we later will of human beings themselves): are they marrying for money, for breeding or for beauty? Sexual selection theory suggests, simply, that much of the behaviour and some of the appearance of an animal is adapted not to helping it survive, but to helping it acquire the best or the most mates. Sometimes these two – survival and acquiring a mate – are conflicting goals. The idea goes back to Charles Darwin, though his thinking on the matter was uncharacteristically fuzzy. He first touched on the subject in The Origin of Species but later wrote a book all about it: The Descent of Man and Selection in Relation to Sex.4

Darwin’s aim was to suggest that the reason human races differed from each