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Extra-pair copulation in animals
As well as humans, EPC has been found in many other socially monogamous species.

When EPC occurs in animals which show sustained female-male social bonding, this can lead to extra-pair paternity (EPP), in which the female reproduces with an extra-pair male, and hence produces EPO (extra-pair offspring).

Due to the obvious reproductive success benefits for males, it used to be thought that males exclusively controlled EPCs. However, it is now known that females also seek EPC in some situations.

EPC in birds
Extra-pair copulation is common in birds. For example, zebra finches, although socially monogamous, are not sexually monogamous and hence do engage in extra-pair courtship and attempts at copulation. In a laboratory study, female zebra finches copulated over several days, many times with one male and only once with another male. Results found that significantly more eggs were fertilised by the extra-pair male than expected proportionally from just one copulation verses many copulations with the other male. EPO proportion varies between different species of birds. For example, in eastern bluebirds, studies have shown that around 35% of offspring is due to EPC.

In socially polygynous birds, EPP is only half as common as in socially monogamous birds. Some ethologists consider this finding to be support for the 'Female Choice' hypothesis of mating systems in birds.

EPC in mammals
Additionally, EPC has been shown in monogamous mammals, such as the white-handed gibbon. A study of one group found 88% in-pair copulation and 12% extra-pair copulation. However, there is much variability in rates of EPP in mammals. One study found that this disparity in EPP is better predicted by the differing social structures of different mammals, rather than differing types of pair bonding. For example, EPP was lower in species who live in pairs compared to those who live in solitary or family structures.

Benefits of EPC
Some argue that EPC is one way in which sexual selection is operating for genetic benefits which is why the extra-pair males involved in EPC seem to be a non-random subset. There is some evidence for this in birds. For example, in swallows, males with longer tails are involved in EPC more than those with shorter tails. Also female swallows with a shorter-tailed within-pair mates are more likely to conduct EPC than those whose mates have longer tails. A similar pattern has been found for black-capped chickadees, in which all extra-pair males had higher rank than the within-pair males. But some argue that genetic benefits for offspring is not the reason females participate in EPC. A meta-analysis of genetic benefits of EPC in 55 bird species found that extra-pair offspring were not more likely to survive than within-pair offspring. Also, extra-pair males did not show significantly better ‘good-genes’ traits than within-pair males, except for being slightly larger overall.