Mating system

A mating system is a way in which a group is structured in relation to sexual behaviour. The precise meaning depends upon the context. With respect to higher animals, it specifies which males mate with which females, under which circumstances; recognised animal mating systems include monogamy, polygamy (which includes polygyny, polyandry, and polygynandry) and promiscuity. In plants, it refers to the degree and circumstances of outcrossing. In human sociobiology, the terms have been extended to encompass the formation of relationships such as marriage.

In animals

The following are some of the mating systems generally recognized in animals:

• Monogamy: One male and one female have an exclusive mating relationship. The term "pair bonding" often implies this. It could also mean one or more animals have an exclusive relationship with one or more opposite-sex animals.

• Polygamy: 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 realize that these descriptions are idealized, 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.

In plants

The primary mating systems in plants are outcrossing (cross-fertilisation), autogamy (self-fertilisation) and apomixis (asexual reproduction without fertilisation, but only when it arising by modification of sexual function). However, mixed mating systems, in which plants use two or even all three mating systems, are not uncommon.

There are a number of models have been used to estimate the parameters of plant mating systems. The basic model is the mixed mating model, which is based on the assumption that every fertilisation is either self-fertilisation or completely random cross-fertilisation. More complex models relax this assumption; for example the effective selfing model recognises that mating may be more common between pairs of closely related plants that between pairs of distantly related plants.

In humans

Main article: Mating (human)

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 socio-biologist 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.

As culture increasingly affects human mating choices, it becomes correspondingly difficult to ascertain what is the 'natural' mating system of the human animal from a zoological perspective. But we can take some clues from our own anatomy, which is essentially unchanged from our prehistoric past:

• humans have a very large relative size of testes to body mass versus most primates
• humans have a comparatively large ejaculate and sperm count versus other primates
• as compared to most primates, humans spend more time in copulation
• as compared to most primates, humans copulate with greater frequency
• the human female's estrous is hidden, compared to most mammals that have outward signs of ovulation
• for most mammals, the estrous cycle and its outward signs bring on mating activity, but due to the hidden estrous, humans copulate throughout the reproductive cycle
• after ejaculation in males and orgasm in females, human males release a hormone that has a sedative effect, while human females remain sexually receptive

These anatomical factors combine to suggest that from a zoological standpoint the human animal has a reproductive strategy based at least to some degree on sperm competition and that females enhance their genetic reproductive success by making every egg a contest, and males by participating in as many contests as possible. While such a strategy was conducive to the cooperative competition and solidary bonds of tribal existence, in the face of carnal knowledge and the rise of culture, new more complex behavioral choices are seemingly superseding our physiology.

See also

• r/K selection theory

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Further reading

• Marlowe, F.W. (2003). The Mating System of Foragers in the Standard Cross-Cultural Sample. Cross-Cultural Research, 37, 282-306. Full text