Introduction

Darwin proposed sexual selection to explain why the males of so many species have conspicuous bright colours or long tail feathers even though such extravagant features or behaviour might reduce survival by natural selection (Darwin 1874). He included chemical signals alongside visual and sound signals, describing the distinctive odours of breeding male crocodiles and many mammals, as well as male moths. He concluded that the development of elaborate odour glands in male mammals is ‘intelligible through sexual selection, if the most odoriferous males are the most successful in winning the females, and in leaving offspring to inherit their gradually perfected glands and odours’ (Darwin 1874, p. 809). The notorious odours of male goats in the breeding season, for example, might be as spectacular as a peacock's tail, if only we had the nose to appreciate them.

Darwin suggested that sexually selected signals in animals would have many features in common, including elaboration or expression of the signals in only one sex, development only in adults, often only in the breeding season, and use primarily or exclusively in mating (Darwin 1874, p. 807). Chemical signals, and the scent glands that create them, commonly show all these characteristics (in humans too, Chapter 13). After long neglect, studies of chemical signals in sexual selection are now providing some of the most exciting results of all.

Sexual selection is shorthand for selection that arises through competition over mates or for matings. It is a subcategory of natural selection and operates in fundamentally the same way, by hereditary variation among individuals which translates into different numbers of offspring produced by each individual (Alcock 1998).