Mating behavior involves many kinds of social interactions: they can be one-on-one, two-on-one, or large groups, such as the leks of males that gather to display to females in many bird species (e.g., the black grouse). Mating behavior includes mate choice, intrasexual competition for mates, and parental care. Mate choice can lead to competition among one sex for mates of the other sex. The evolution of mating behavior was first analyzed by Charles Darwin in his 1871 theory of sexual selection. Darwin explained extreme male features, such as elaborate physical traits and behavioral displays, as resulting from female preferences for these traits. Without explaining where the female preference comes from, Darwin pointed out that any trait that increases the likelihood of mating will increase reproductive success and thus be favored by natural selection. Darwin called this process sexual selection, to distinguish it from natural selection. These ideas were developed theoretically by Ronald Fisher in 1930, who suggested how a runaway process might promote such traits even if they had no particular function. If a particular male trait is attractive to females, and both the trait and the female preference are heritable, over many generations the proportion of males with that trait will increase. Though the runaway process explains how sexual selection can occur even if the preferred trait has no function, sexual selection can occur jointly with natural selection. The traits used in mate choice might be related to some other qualities that promote survival or reproductive success. For example, in the 1970s, Amotz Zahavi suggested that a trait such as bright coloration, used in mate choice by birds, could be viewed as a handicap; producing bright coloration has high physiological costs. Thus the presence of bright coloration could indicate that an individual is healthy enough to bear these costs. In practice, it has been difficult to distinguish the contributions of sexual selection, based on an arbitrary preference, and natural selection, when the trait used in mate choice also indicates that the mate will contribute to the chooser's reproductive success.
Parental care has been approached theoretically through the concepts of game theory and evolutionarily stable strategies. Game theory has been widely used in the social sciences, especially economics but also sociology. In 1982, John Maynard Smith introduced the idea of the 'evolutionarily stable strategy' or ESS to biology. The ESS is the distribution of participants into different strategies, such that no new strategy could do better.
Empirical studies have shown that the biology of mating behavior is significantly more complicated than these early theories assumed. Individuals (of both sexes) take advantage of social information to make choices about with whom to mate. Mate choice and sexual selection require new theoretical approaches that take into account the effect of each sex's behavior on the other's reproductive success, so that costs and benefits are not assigned to each individual but to the reproductive success of the pair.
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