Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, leading to evolutionary arms race between males and females. It has primarily been studied in animals, though it can in principle apply to any sexually reproducing organism, such as plants.
This can be in two forms:
Sexual conflict may lead to sexually antagonistic co-evolution, in which one sex (usually males) evolves a "manipulative" trait which is countered by a "resistance" trait in the other sex. For example, male bean weevils (Callosobruchus maculatus) have spiny genitalia that are thought to allow them to copulate for a longer time without getting dislodged and hence transfer more sperm. However, this damages the female and reduces her fitness, so females have evolved the counteradaptation of kicking at males during mating, which reduces the time spent in copulation .
Some regard sexual conflict as a subset of sexual selection (which was traditionally regarded as mutualistic, while others suggest it is a separate evolutionary phenomenon.
There are a wide variety of manifestations of sexual conflict, occurring in a broad range of taxa. One way of sorting these is by temporal relation to a given reference point, e.g. mating or fertilization.
Infanticide is a behavior that occurs in many species where an adult kills younger individuals, including eggs. Sexual conflict is one of the most common causes, although other cases are seen, such as male bass eating their own juvenile descendants. It is usually the males who perpetrate such behavior, though it is not unknown for females to behave in the same way.
Vertebrates have received the most research, with cases such as hanuman langurs, lions, house sparrows and mice being studied. This behavior also occurs in the invertebrates, however. One such case is the spider Stegodyphus lineatus, where males invade female nests and toss out their egg sacs. Females only have one clutch in their lifetime, and experience reduced reproductive success if they lose them. This results in vicious battles in which injuries and even death are not uncommon. Jacana jacana, a tropical wading bird, offers one example of infanticide by the female sex. Females guard a territory while males care for their young. As males are a limiting resource, other females will commonly displace or kill their young. Males then mate with them and care for the young of the female which destroyed their previous offspring.
This behavior is costly to both sides, and counter-adaptations have evolved in the victim sex ranging from cooperative defense of their young to loss minimization strategies such as aborting existing offspring upon the arrival of a new male (the Bruce effect).
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