stylistic:tone
Evolution in animals

Socially monogamous species are scattered throughout the animal kingdom: A few insects, a few fish, about nine-tenths of birds, and a few mammals are socially monogamous. There is even a parasitic worm, Schistosoma mansoni, that in its female-male pairings in the human body is monogamous. The diversity of species with social monogamy suggests that it is not inherited from a common ancestor but instead evolved independently in many different species.

The low occurrence of social monogamy in placental mammals has been claimed to be related to the presence or absence of estrus—or oestrus—the duration of sexual receptivity of a female. This, however, does not explain why estrus females generally mate with any proximate male nor any correlation between sexual and social monogamy. Birds, which are notable for a high incidence of social monogamy, do not have estrus.