Mating systems

In sociobiology and behavioural ecology, the term mating system is used to describe the ways in which animal societies are structured in relation to sexual behaviour. The mating system specifies what males mate with what females under what circumstances. For organisms such as plants, that do not necessarily have males and females, mating system is used to refer to the degree to which individuals are self fertilising or outcrossing.

The following are some of the mating systems generally recognised in animals:
 * Monogamy, more usually called pair bonding: One male and one female have an exclusive mating relationship.
 * Polygamy: One or more males have an exclusive relationship with one or more females. Three types are recognised:
 * Polygyny (the most common polygamous mating system in vertebrates so far studied): One male has an exclusive relationship with two or more females
 * Polyandry: One female has an exclusive relationship with two or more males
 * Polygynandry: Two or more males have an exclusive relationship with two or more females; the numbers of males and females need not be equal, and in vertebrate species studied so far, the number of males is usually less.
 * Promiscuity: Any male within the social group mates with any female.

These mating relationships may or may not be associated with social relationships, in which the sexual partners stay together to become parenting partners. As the alternative term "pair bonding" implies, this is usual in monogamy. In many polyandrous systems, the males and the female stay together to rear the young. In polygynous systems where the number of females paired with each male is low, the male will often stay with one female to help rear the young, while the other females rear their young on their own. In polygynandry, each of the males may assist one female; if all adults help rear all the young, the system is more usually called "communal breeding". In highly polygynous systems, and in promiscuous systems, paternal care of young is rare, or there may be no parental care at all.

It is important to realise that these descriptions are idealised, and that the social partnerships are often easier to observe than the mating relationships. In particular:
 * the relationships are rarely exclusive for all individuals in a species. DNA fingerprinting studies have shown that even in pair-bonding, matings outside the pair (extra-pair copulations) occur with fair frequency, and a significant minority of offspring result from them.
 * some species show different mating systems in different circumstances, for example in different parts of their geographical range, or under different conditions of food availability
 * mixtures of the simple systems described above may occur.

Virtually all the terms used to describe animal mating systems were taken over from social anthropology, where they had been devised to describe systems of marriage. This shows that human sexual behaviour is unusually flexible, since in most animal species, one mating system dominates. While there are close analogies between animal mating systems and human marriage institutions, these should not be pressed too far, because in human societies, marriages typically have to be recognised by the entire social group in some way, and there is no equivalent process in animal societies. The temptation to draw conclusions about what is "natural" for human sexual behaviour from observations of animal mating systems should be resisted: a sociobiologist observing the kinds of behaviour shown by humans in any other species would conclude that all known mating systems were natural for that species, depending on the circumstances or on individual differences.