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Sexual reproduction enables genes to be mixed thus offering a few advantages to the species concerned. Although broadly practiced by all forms of living beings, it does not appear to be a necessity since some vertebrate species can renounce their sexuality albeit temporarily. Numerous forms of sexual behaviour are practiced by the species who recourse to sexual reproduction. This variety is certainly an expression of the selective pressure exerted to the reproductive function.
The selective value of the behaviour associated with reproduction stems from the notion of sexual selection already developed by Darwin: more than the survival capability alone, the selection is a means to ensure the transmission of the genetic capital through a large and vigourous lineage. Hence, all the forms of behaviour associated with the reproductive function are part of a fierce competition, from the seduction stage up to the parental behaviour, including the unavoidable fecondation. Solutions emerged from this competition, some ingenious, other disparate, which are specific to each species. The sexual behaviour can in turn influence the evolution of a species: the selection of a partner, usually the femalesıs choice, is a selective act that can orientate the future of a population. Moreover, the incompatibility of seductive behaviour, like geographical isolation, could lead to the advent of two distinct species.
This diversity of reproductive behaviour is matched by certain functional differences within the nervous system which, as a behaviour organ, forms the target of this selective pressure. Under the influence of sexual hormones (which are very well preserved during the phylogenesis), the brain is sexually differentiated during the pre- or perinatal period, then it expresses in due time the reproductive behaviour specific to the gender and the species: selection of partner, seduction, copulation, mono- or polygamy, parental behaviour. Because of their importance with respect to the survival of the species, the reproductive behaviours are the object, in the individual, of powerful reinforcements which correspond to the subjective notions of desire and pleasure. The comparative study of the sexual behaviour and of the properties of the nervous structures makes it possible to understand better the mechanisms by means of which evolution adjusts and optimizes, according to environmental constraints, the reproductive strategy of individuals which could influence the future of the species.