Mating system
Encyclopedia
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
.
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:
(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.
, 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:
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 complex culture, new more complex behavioral choices are seemingly superseding our physiology.
Monogamy
Monogamy /Gr. μονός+γάμος - one+marriage/ a form of marriage in which an individual has only one spouse at any one time. In current usage monogamy often refers to having one sexual partner irrespective of marriage or reproduction...
, polygamy
Polygamy
Polygamy is a marriage which includes more than two partners...
(which includes polygyny
Polygyny
Polygyny is a form of marriage in which a man has two or more wives at the same time. In countries where the practice is illegal, the man is referred to as a bigamist or a polygamist...
, polyandry
Polyandry
Polyandry refers to a form of marriage in which a woman has two or more husbands at the same time. The form of polyandry in which a woman is married to two or more brothers is known as "fraternal polyandry", and it is believed by many anthropologists to be the most frequently encountered...
, and polygynandry
Polygynandry
Polygynandry occurs when 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 lower.-In Bonobos:...
) and promiscuity
Promiscuity
In humans, promiscuity refers to less discriminating casual sex with many sexual partners. The term carries a moral or religious judgement and is viewed in the context of the mainstream social ideal for sexual activity to take place within exclusive committed relationships...
. In plants, it refers to the degree and circumstances of outcrossing
Outcrossing
Outcrossing is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of all individuals being subject to disease or reducing genetic abnormalities...
. In human sociobiology
Sociobiology
Sociobiology is a field of scientific study which is based on the assumption that social behavior has resulted from evolution and attempts to explain and examine social behavior within that context. Often considered a branch of biology and sociology, it also draws from ethology, anthropology,...
, the terms have been extended to encompass the formation of relationships such as marriage
Marriage
Marriage is a social union or legal contract between people that creates kinship. It is an institution in which interpersonal relationships, usually intimate and sexual, are acknowledged in a variety of ways, depending on the culture or subculture in which it is found...
.
In animals
The following are some of the mating systems generally recognized in animals:- MonogamyMonogamyMonogamy /Gr. μονός+γάμος - one+marriage/ a form of marriage in which an individual has only one spouse at any one time. In current usage monogamy often refers to having one sexual partner irrespective of marriage or reproduction...
: One male and one female have an exclusive mating relationship. The term "pair bondPair bondIn biology, a pair bond is the strong affinity that develops in some species between the males and females in a pair, potentially leading to breeding. Pair-bonding is a term coined in the 1940s that is frequently used in sociobiology and evolutionary psychology circles...
ing" often implies this. It could also mean one or more animals have an exclusive relationship with one or more opposite-sex animals.
- PolygamyPolygamyPolygamy is a marriage which includes more than two partners...
: Three types are recognized:- PolygynyPolygynyPolygyny is a form of marriage in which a man has two or more wives at the same time. In countries where the practice is illegal, the man is referred to as a bigamist or a polygamist...
(the most common polygamous mating system in vertebrates so far studied): One male has an exclusive relationship with two or more females - PolyandryPolyandryPolyandry refers to a form of marriage in which a woman has two or more husbands at the same time. The form of polyandry in which a woman is married to two or more brothers is known as "fraternal polyandry", and it is believed by many anthropologists to be the most frequently encountered...
: One female has an exclusive relationship with two or more males - PolygynandryPolygynandryPolygynandry occurs when 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 lower.-In Bonobos:...
: 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.
- Polygyny
- PromiscuityPromiscuityIn humans, promiscuity refers to less discriminating casual sex with many sexual partners. The term carries a moral or religious judgement and is viewed in the context of the mainstream social ideal for sexual activity to take place within exclusive committed relationships...
: 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, 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 outcrossingOutcrossing
Outcrossing is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of all individuals being subject to disease or reducing genetic abnormalities...
(cross-fertilisation), autogamy (self-fertilisation) and apomixis
Apomixis
In botany, apomixis was defined by Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization. This definition notably does not mention meiosis...
(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
Mixed mating model
The mixed mating model is a mathematical model that describes the mating system of a plant population in terms of the degree of self-fertilisation present. It is a fairly simplistic model, employing several simplifying assumptions, most notably the assumption that every fertilisation event may be...
, 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
Effective selfing model
The effective selfing model is a mathematical model that describes the mating system of a plant population in terms of the degree of self-fertilisation present....
recognises that mating may be more common between pairs of closely related plants that between pairs of distantly related plants.
In humans
Virtually all the terms used to describe animal mating systems were taken over from social anthropologySocial anthropology
Social Anthropology is one of the four or five branches of anthropology that studies how contemporary human beings behave in social groups. Practitioners of social anthropology investigate, often through long-term, intensive field studies , the social organization of a particular person: customs,...
, where they had been devised to describe systems of marriage
Marriage
Marriage is a social union or legal contract between people that creates kinship. It is an institution in which interpersonal relationships, usually intimate and sexual, are acknowledged in a variety of ways, depending on the culture or subculture in which it is found...
. 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 may 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 complex culture, new more complex behavioral choices are seemingly superseding our physiology.
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