Extra-pair copulation

Monogamy is the practice of having only one sexual partner at any one time, forming a long-term bond and combining efforts to raise offspring together; mating outside this pairing is extra-pair copulation.

One such hypothesis is that males maximise their reproductive success by copulating with as many females as possible outside of a pair bond relationship because their parental investment is lower, meaning they can copulate and leave the female with minimum risk to themselves.

[8] It has been suggested that,[8] due to having such low parental investment, it is evolutionarily adaptive for men to copulate with as many women as possible.

Mitigating against improved reproductive success from EPC is the loss of enhanced child-rearing environments for any resulting offspring.

[1] There is also the increased risk of sexually transmitted infections,[1] which is suggested as a possible evolutionary reason for the transition from polygamous to monogamous relationships in humans.

[15] Despite this, females do seek out extra-pair copulation, with some research finding that women's levels of infidelity are equal to that of men's, although this evidence is mixed.

[16] Due to the increased risk, there is more confusion about the evolutionary benefits of extra-pair copulation for females.

The most common theory is that women mate outside of the monogamous relationship to acquire better genetic material for their offspring.

[14] A second theory is that a woman will engage in extra-pair copulation to seek additional resources for herself or her offspring.

[1] This is based on observations from the animal world in which females may copulate outside of their pair-bond relationship with neighbours to gain extra protection, food or nesting materials.

Finally, evolutionary psychologists have theorized that extra-pair copulation is an indirect result of selection on males.

The alleles in males that promote extra-pair copulation as an evolutionary strategy to increase reproductive success is shared between sexes leading to this behaviour being expressed in females.

Known or suspected EPC by women may result in lowered life chances for any offspring, or the woman being at higher risk of violence and homicide, for example.

[3] Some of the highest levels of EPP are found in the New Zealand hihi or stitchbird (Notiomystis cincta), in which up to 79% of offspring are sired by EPC.

[24] EPC can have significant consequences for parental care, as shown in azure-winged magpie (Cyanopica cyanus).