Intrasexual selection, first described by Charles Darwin, is a form of sexual selection where individuals of the same sex—most often males—compete for access to mates. This process explains the evolution of specific traits that enhance competitive ability. The pressure of these contests leads to the development of physical and behavioral characteristics that provide an advantage, resulting in some individuals achieving greater reproductive success than others.
Pre-Mating Competition
A primary form of intrasexual selection is direct physical combat, where males fight for mating opportunities. The victors gain access to females, and this intense competition drives the evolution of weapons and increased body size. The antlers of deer, horns on beetles, and large canine teeth in many primates are examples of traits shaped by male-to-male combat.
In northern elephant seals, males can weigh up to six times more than females. Dominant males, or “harem masters,” engage in violent battles to defend territories on beaches where females congregate. The winners monopolize mating opportunities with large groups of females, ensuring their genes are passed on while losers are often excluded from reproducing. This extreme size difference between the sexes is a direct consequence of these fights.
Competition is not always resolved through violence. Many species engage in ritualized displays, which are non-lethal contests that allow males to assess one another’s fighting ability without risking serious injury. For example, some lizards perform “push-up” displays to signal their physical condition, while certain birds use specific postures or dances to intimidate rivals.
Another form of pre-mating competition is securing and defending a territory. Males compete to control areas with resources females need for breeding, such as nesting sites or food. By holding a high-quality territory and controlling access to these resources, a male significantly increases his chances of mating with the females drawn to the area.
Post-Mating Competition
Competition can continue even after mating. When females mate with multiple males, a contest known as sperm competition occurs, where sperm from different males compete to fertilize the eggs. This pressure has driven the evolution of various male adaptations to give their sperm an advantage.
Adaptations include producing enormous quantities of sperm to overwhelm rivals or developing faster sperm. In some insects, the adaptations are more direct. The penis of a male damselfly is barbed to physically remove the sperm of previous males from the female’s reproductive tract before he deposits his own, increasing his probability of fertilization.
To prevent sperm competition, some males engage in mate guarding. This involves a male staying close to a female after mating to prevent other males from approaching her. This strategy, common in many bird and insect species, helps ensure that his sperm fertilizes her eggs.
A more extreme form of post-mating competition is infanticide, the killing of another male’s dependent offspring. In lion prides, when a new male takes over, he may kill unweaned cubs sired by his predecessor. Since nursing inhibits a female’s return to fertility, eliminating the cubs brings her back into estrus more quickly, allowing the new male to sire his own offspring.
Female Intrasexual Competition
Though often more subtle, competition also occurs among females. This form of intrasexual selection is most common when males provide significant parental care or when reproductive resources are limited. In these situations, females compete to secure the best mates or the resources needed to raise their young.
Female competition often revolves around access to resources rather than mates. Females may compete for the safest nesting sites, territories with the most food, or areas best protected from predators. Securing a superior territory directly impacts a female’s ability to successfully rear her offspring.
In some species with reversed sex roles, females compete directly for mates. In the jacana, a tropical wading bird, males incubate the eggs and care for the young. Consequently, females are larger and more aggressive, competing to defend territories that contain several males. A female mates with all the males in her territory, who then raise her clutches of eggs.
Social dynamics can also be a tool for female competition. In certain social mammal species, dominant females may suppress the reproductive cycles of subordinates through behavioral intimidation. The resulting stress can hormonally prevent lower-ranking females from ovulating. This ensures the dominant female’s offspring face less competition for resources and care.