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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, the term describes which males and which females mate, under which circumstances; recognised animal mating systems include monogamy, polygamy (which includes polygyny, polyandry, and polygynandry), and promiscuity. In plants, the term 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.
The primary mating systems in plants are outcrossing (cross-fertilisation), autogamy (self-fertilisation) and apomixis (asexual reproduction without fertilization, but only when arising by modification of sexual function). Mixed mating systems, in which plants use two or even all three mating systems, are not uncommon.
A number of models have been used to describe 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.
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.
- Polygamy: Three types are recognized:
- 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:Polygynandry is a slight variation of this, where two or more males have an exclusive relationship with two or more females; the numbers of males and females don't have to be equal, and in vertebrate species studied so far, the number of males is usually less.
- Promiscuity: A member of one sex within the social group mates with any member of the opposite sex.
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 and 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.
These descriptions are idealized, and 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.
Compared to other vertebrates, where a species usually has a single mating system, human display great variety. Humans also differ by having formal marriages which in many cultures involve negotiation and arrangement between elder relatives. Regarding sexual dimorphism (see the section about animals above), humans falls in the intermediate group with moderate sex differences in body size but with relatively large testes. This indicates relatively frequent sperm competition which is supported by reports of extrapair paternity of 2-22% in socially monogamous and polygynous human societies. One estimate is that 83% of human societies are polygynous, 0.05% are polyandrous, and the rest are monogamous. Even the last group may at least in part be genetically polygynous.
Polygyny is associated with an increased sharing of subsistence provided by women. This is consistent with the theory that if women raise the children alone, men can concentrate on the mating effort. Polygyny is also associated with greater environmental variability in the form of variability of rainfall. This may increase the differences in the resources available to men. An important association is that polygyny is associated with a higher pathogen load in an area which may make having good genes in a male increasingly important. A high pathogen load also decreases the relative importance of sororal polygyny which may be because it becomes increasingly important to have genetic variability in the offspring (See Major histocompatibility complex and sexual selection).
Virtually all the terms used to describe animal mating systems were adopted from social anthropology, where they had been devised to describe systems of marriage. This shows that human sexual behavior 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 analogies should not be pressed too far, because in human societies, marriages typically have to be recognized 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 behavior from observations of animal mating systems should be resisted: a socio-biologist observing the kinds of behavior 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, ascertaining what is the 'natural' mating system of the human animal from a zoological perspective becomes increasingly difficult. Some clues can be taken from human anatomy, which is essentially unchanged from the prehistoric past:
- humans have a large relative size of testes to body mass in comparison to most primates;
- humans have a large ejaculate volume and sperm count in comparison to other primates;
- as compared to most primates, humans spend more time in copulation;
- as compared to most primates, humans copulate with greater frequency;
- the outward signs of estrous in women (i.e. higher body temperature, breast swelling, sugar cravings, etc.), are often perceived to be less obvious in comparison to the outward signs of ovulation in most other mammals;
- for most mammals, the estrous cycle and its outward signs bring on mating activity; the majority of female-initiated matings in humans coincides with estrous, but humans copulate throughout the reproductive cycle;
- after ejaculation/orgasm in males and females, humans release a hormone that has a sedative effect; however human females may remain sexually receptive and may remain in the plateau stage of orgasm if their orgasm has not been completed.
Some have suggested that these anatomical factors signify some degree of sperm competition, though as levels of genetic and societal promiscuity are highly varied across cultures, this evidence is far from conclusive.
- Brown, A. H. D.; et al. (1989). "Isozyme analysis of plant mating systems". In Soltis, D. E.; Soltis, P. S. (eds). Isozymes in Plant Biology. Portland: Dioscorides Press. pp. 73–86.
- The Oxford Handbook of Evolutionary Psychology, Edited by Robin Dunbar and Louise Barret, Oxford University Press, 2007, Chapter 30 Ecological and socio-cultural impacts on mating and marriage systems by Bobbi S. Low
- Marlowe, F.W. (2003). The Mating System of Foragers in the Standard Cross-Cultural Sample. Cross-Cultural Research, 37, 282-306. Full text