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Mating systems

About mammals > Reproduction of mammals

Mammals exhibit a wide variety of mating systems, which can be basically divided into promiscuous, monogamous, polygynous, and multi-male. In mammals that live in gregarious social groups, the mating system is commonly (but not always) reflected by the composition of those groups, whereas in dispersed mammals patterns of mating must be determined from observations of interactions between separately ranging, “solitary” individuals. In all cases, however, it must be remembered that social systems and mating systems do not necessarily coincide. Even with mammals that are seemingly monogamous, genetic tests of paternity are quite likely to produce surprises just as they have already done for several bird species.

Monogamy, which is the predominant pattern of social organization and widely assumed to be the dominant mating system among birds, is relatively rare among mammals. It is somewhat more common in carnivores and primates than in other mammals, but even in those groups it is found in only a minority of species. It seems likely that promiscuous mating was present in ancestral mammals in association with their likely nocturnal, dispersed habits, as it is in various relatively primitive nocturnal mammals today that lack any obvious social networks (e.g. many marsupials, insectivores, carnivores, and rodents). Another common mating pattern among mammals is polygyny, in which a single male has exclusive or almost exclusive mating access to a number of females.

Polygyny is commonly found, for example, in hoofed mammals, pinnipeds, and elephants. By contrast, it is relatively rare to find multi-male systems in which several adult males are present in a social network or group competing for mating access to females, although it is widespread among higher primates. A key issue here is the potential occurrence of competition between sperm from different males. In mating systems in which a single male has clear priority of access to one or more females (monogamous and polygynous systems), the probability of sperm competition is presumably low, whereas in promiscuous and multi-male systems there is likely to be a high incidence of sperm competition. This expectation has been confirmed by studies of the relative size of the testes in mammals. Species with promiscuous or multi-male mating systems generally have significantly larger testes, relative to body size, than species with monogamous or polygynous systems. This indicates that males show increased levels of sperm production in cases where sperm competition is relatively intense.

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