Sexual conflict

Sexual conflict occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, leading to an 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 and fungi.

This can be in two forms:


 * 1) Interlocus sexual conflict, where male alleles have conflicting interests with female alleles that are at a different genetic locus. This can be in the form of conflict over parental care: maternal alleles which cause the offspring to require a greater investment from the father, and paternal alleles which cause the offspring to require a greater investment from the mother. Another form is conflict over mating rate. Males frequently have a higher optimal mating rate than females (because in most animal species they invest fewer resources in offspring than females do), and so males have numerous adaptations to induce females to mate with them. Another well-documented example of inter-locus sexual conflict is the seminal fluid of Drosophila melanogaster, which up-regulates females' egg-laying rate and reduces her desire to re-mate with another male (serving the male's interests), but also shortens the female's lifespan reducing her fitness.
 * 2) Intralocus sexual conflict, where the same set of alleles in males and females have different optima. i.e. they are expressed differently in the sexes. A classic example is the human hip, where females need larger hips for childbirth. The genes that affect hip size must reach a compromise that is at neither the male optimum nor the female optimum. In some cases, this conflict may be resolved through the differential expression of such loci in males and females, but evidence indicates that intralocus conflict may be an important constraint in the evolution of many traits.

Sexual conflict may lead to antagonistic co-evolution, in which one sex (usually males) evolves a favorable trait which is countered by a countering 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.

Manifestations
There are a wide variety of manifestations of sexual conflict, occurring in a broad range of taxa, and showing extreme examples such as the well-known phenomenon of sexual cannibalism among insects, usually demonstrated by the female. One way of sorting these manifestations is by temporal relation to a given reference point, e.g. mating, fertilization, or birth.

Infanticide
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 exhibit 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 limited 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 affected 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).

A tripartite (fetus—mother—father) immune conflict in Placentals
During pregnancy, there is a two-way traffic of immunologically active cell lines through the placenta. Fetal lymphocyte lines may survive in women even decades after giving birth. These cells may serve the adaptive interest of the mother, however, they may also serve conflicting interests of the fetus or those of the father. This mixture of shared or contradicting interests have been hypothesized to give rise to diseases like autoimmune diseases, infertility, and habitual spontaneous abortion (i.e., early miscarriage) in humans.