Since methods for genetic paternity analyses were introduced it became clear that many pair-living animal species, including humans, do not take partnership fidelity that seriously. In most species there is some proportion of offspring that is not sired by their social father. Coppery titi monkeys living in the Amazon lowland rainforest seem to be an exception. Scientists could not find evidence for extra-pair paternity in their study population in Peru. Mate choice seems to be so successful that a potential genetic advantage does not outweigh the social costs of infidelity.
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Source: ScienceDaily