The initial stage of the reproductive cycle, in which a member of the opposite sex is singled out for attention. Mate selection is often followed by courtship. Normally, mate selection occurs among sexually mature individuals, but there are some exceptions to the rule. Among some lovebirds (Agapornis) heterosexual pairs form when the birds are about two months old and still have their juvenile plumage. In the African waxbill (Uraeginthus granatinus) monogamous pairs are established before the partners are 35 days old, and are still being fed by their parents.
An important aspect of mate selection is selecting the right species and sex. When parents are of different species, the offspring are generally infertile and incapable of adapting to the ecological niche of either parent. Hence hybridization in nature is invariably disadvantageous, and the pressures of natural selection generate mechanisms designed to avoid such misidentification. The barriers to hybridization, whether geographic, climatic, mechanical, or behavioural, are known as reproductive isolating mechanisms.
Mates must be selected according to sex as well as species. While in many species males and females differ in a number of ways, sexual identification sometimes depends upon the identification of relatively few features. For example, in many species of pigeon (Columbidae) the main distinguishing feature is behavioural. The male performs a bowing display when he encounters a member of the same species. The sexes differ in their response to this behaviour.
In many promiscuous species, it is important to select a mate that is physiologically ready for mating, otherwise much time and effort can be wasted. Females often indicate their readiness for sexual behaviour by means of pheromones, but this does not necessarily mean that they will cooperate. Females are often ‘coy’, testing the male by prolonging the courtship. Even when the female permits copulation, it does not necessarily follow that the male will father her offspring. In many species the female harbours the sperm from a number of males, and is able to exercise some degree of control of the fertilization process. Thus sperm competition is the final arbiter of mate selection.