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Sexual selection of mammals

About mammals > Reproductive processes
Two bison bulls sparring

The pressures caused by females choosing males will lead to two types of evolutionary selection: inter-sexual selection (adaptations to win members of the other sex), and intrasexual selection (adaptations to win access to mates over members of the same sex). Both vary in importance according to species and environments.

Inter-sexual selection leads to the development of adaptations, morphological, physiological, or behavioral, to seduce mates. Typically, females are choosers, so most of the intersexual selection targets males. Adaptations resulting from inter-sexual selection are meant to reveal genetic quality, so males will harbor features, or perform behaviors that indicate quality. Examples of inter-sexual selection are numerous in mammals, and the best known morphological examples are probably the growth of horns in ungulates such as giraffe (Giraffa camelopardalis), or bighorn sheep (Ovis canadensis).

In contrast to inter-sexual selection, which arises from pressures imposed by the other sex, intra-sexual selection leads to adaptations as a result of pressures imposed by the same sex. Displays of strength or resources may serve to indicate dominance and establish hierarchy among males, allowing better individuals to access more mates.

Best examples in the mammals are the sparring competitions of ungulates such as deer (genus Odocoileus), or the banging of heads in musk oxen (Ovibos moschatus). In these examples, some of the morphological attributes such as large antlers may be used for both inter-sexual selection (seduction of mates), intra-sexual selection (indication of dominance), and even individual selection (defense against predators). But multiple functions do not lessen their attractiveness as large antlers indicate that the males that harbor them are able to find resources to grow and carry heavy antlers, and thus indicate that they are in good health and have good genes. The extinct Irish elk possessed the largest antlers ever. They were up to 6 ft (1.8 m) in length. However, the purpose of these was not to fight. It is unlikely they could be used for this purpose as they were too big. These massive antlers seem to have evolved because of runaway sexual selection. Females prefered males with larger and larger antlers, so the antlers got bigger and bigger.

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