Forms of sexual selection
Competition between individuals of the same sex can assume very diverse forms. This can take the form of a physical battle between members of the same sex, which might even lead to the death of one of the combatants (elk, elephants, some species of crickets). Mostly, however, the opponent is not killed, as the species has created ethological inhibitions that do not allow the competitors to use their frequently fatal weapons (caribou, some snakes, spiders, canine carnivores). The battle thus becomes a sort of ritualized combat, which lasts only until the stronger combatant is decided, It should be pointed out that, under unnatural conditions, for example when individuals are kept in captivity, these ethological mechanisms cannot come into action and the competitor is finally killed. Ritualization of the battle is certainly advantageous from the point of view of the species, because young or temporarily weak individuals are not killed. Ritualization is also advantageous from the viewpoint of the individual: if combatants do not fight to the death, the winner saves a lot of energy and avoids potential injuries (Lumsden 1983).s
However, competition between individuals of the same sex can also take the form of more or less passive submission to the selection made by individuals of the opposite sex. This situation is very frequently encountered amongst vertebrates (Westcott 1994). In 232 works concerned with sexual competition, selection performed by females was observed for 186 animal species in 167 works, selection by males in 30 works, battles between males in 58 works and, in 14 cases, a sort of competition was observed amongst males as to who could last the longest (stamina competition) (Andersson & Iwasa 1996). In contrast to competition in the form of a battle or stamina competition, in which the chance of success depends on size, dexterity or strength, and thus on the viability of the individual, in cases where the choice is made by a member of the opposite sex, the criteria for selection can be quite arbitrary. Thus, sexual selection can lead to evolution of various structures and patterns of behavior, from bright colors and remarkable body organs to the complicated songs of birds and intricate courtship dances of some species of insects.
Both types of selection can occur simultaneously in a single species and can even be directed against one another. Males can compete together for access to females while, at the same time, females can attempt, with greater or lesser success, to choose the optimal male on the basis of completely different criteria.