Many animals have a highly developed social organization through which resources are apportioned among dominant members of the population. Such competitive interactions may involve social dominance, in which the dominant individuals exclude subdominant individuals from the resource. One resource that is frequently effected by the social hierarchy is mating opportunity. When this occurs, the conditions are ripe for sexual selection.
Sexual selection is a special type of natural selection in which reproductive success among individuals is determined by the way in which mating occurs. Competition between individuals of the same sex for mates can favor individuals with certain hereditary traits. Because these characters are perpetuated only if reproduction is successful, their frequency tends to increase. Sexual selection results in morphological distinction of the sexes (sexual dimorphism), because one sex is selected for traits that are not required by the other. See image below.
An example of sexual dimorphism as a result of sexual selection. A young male buck (left) displays antlers, while they are absent on the female doe (right).
Two main forms of sexual selection occur: intrasexual selection and epigamic selection. In intrasexual selection males (or more rarely females) compete through display or physical contest for mates. In epigamic selection females accept males (or occasionally the reverse) with certain traits. In some instances, males (and sometimes females) can monopolize many individuals of the other sex during the breeding season for mating purposes-a practice called polygyny--which creates especially intense sexual selection.
The role of male fitness and social dominance in mating has been studied extensively in baboons. Researchers found that high position in a social hierarchy did not necessarily result in high copulatory success (Bercovitch 333). If females fertilize their eggs with sperm received during only a few days of their estrous cycle, then males that copulate outside this time cannot produce offspring. Studies have shown that dominant males copulate more frequently during the days of high female fertility than do their subordinate counterparts.
Among primates, the trend of socially dominant males having more success copulating with females during her fertile periods is quite clear. The barrier imposed by the traditional relationship between rank and reproductive success is significant, but not insurmountable. To counteract this relationship, subordinate males in most species in which social dominance plays a role in mating success have developed alternative mating techniques.
Alternative Mating Practices
Subordinate males of many species have developed techniques to increase the likelihood of mating with females during the fertile period of her estrous cycle. Some of these tactics include subordinate males deceiving larger rivals with female-mimicking scents, the "satellite" behavior of some males that remain near to other males with highly attractive calls or displays, defending a resource which is highly attractive to females, or directly defending the females themselves. The male horseshoe crab, for example, will patrol water near the beach and grasp females moving toward shore to lay their eggs. The male will proceed to fertilize the female's eggs as she arrives on the shore. Note the special claws that male horseshoe crabs possess to make it difficult for the female to dislodge her male suitor in the link above.