Animal sexual behaviour
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For human sexual practices, see Human sexual behavior
Animal sexual behavior takes many different forms, even within the same species. Researchers have observed monogamy, promiscuity, sex between species, sexual arousal from objects or places, sex apparently via duress or coercion, copulation with dead animals, sexual, heterosexual and bisexual sexual behaviour, and situational sexual behaviour and a range of other practices among animals other than humans. Related studies have noted diversity in sexed bodies and gendered behaviour, such as intersex and transgender animals.
The study of animal sexuality (and primate sexuality especially) is a rapidly developing field. It used to be believed that only humans and a handful of species performed sexual acts other than for procreation, and that animals' sexuality was instinctive and a simple response to the "right" stimulation (sight, scent). Current understanding is that many species that were formerly believed monogamous have now been proven to be promiscuous or opportunistic in nature; a wide range of species appear both to masturbate and to use objects as tools to help them do so; in many species animals try to give and get sexual stimulation with others where procreation is not the aim; and sexual behavior has now been observed among 1,500 species and in 500 of those it is well do ented.[1]
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 cir stances.
The following are some of the mating systems generally recognised in humans and other animals:
* Monogamy: 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:
o Polygyny (the most common polygamous mating system in vertebrates so far studied): One male has an exclusive relationship with two or more females
o Polyandry: One female has an exclusive relationship with two or more males.
o 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, there are usually fewer males.
* Promiscuity: Any male and female will mate within the social group.
Monogamy
Zoologists and biologists now have solid evidence that monogamous pairs of animals are not always sexually exclusive. Many animals that form pairs to mate and raise offspring regularly engage in sexual activities with extra-pair partners[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] This includes previous exemplars such as swans. Sometimes these extra-pair sexual activities lead to offspring. Genetic tests frequently show that some of the offspring raised by a monogamous pair come from the female mating with an extra-pair male partner.[4][5][17][18] These discoveries have led biologists to adopt new ways of talking about monogamy:
Social monogamy
"Social monogamy refers to a male and female's social living arrangement (e.g., shared use of a territory, behaviour indicative of a social pair, and/or proximity between a male and female) without inferring any sexual interactions or reproductive patterns. In humans, social monogamy equals monogamous marriage. Sexual monogamy is defined as an exclusive sexual relationship between a female and a male based on observations of sexual interactions. Finally, the term genetic monogamy is used when DNA analyses can confirm that a female-male pair reproduce exclusively with each other. A combination of terms indicates examples where levels of relationships coincide, e.g., sociosexual and sociogenetic monogamy describe corresponding social and sexual, and social and genetic monogamous relationships, respectively." (Reichard, 2003, page 4)[19]
Whatever makes a pair of animals socially monogamous does not necessarily make them sexually or genetically monogamous. Social monogamy, sexual monogamy, and genetic monogamy can occur in different combinations.
Social monogamy is relatively rare in the animal kingdom. The actual incidence of social monogamy varies greatly across different branches of the evolutionary tree. Over 90 percent of avian species are socially monogamous.[13][20] This stands in contrast to mammals. Only 3 percent of mammalian species are socially monogamous, although up to 15 percent of primate species are socially monogamous.[13][20] Social monogamy has also been observed in reptiles, fish, and insects.
Sexual monogamy is very rare among animals. The great majority of socially monogamous species engage in extra-pair copulations, making them sexually non-monogamous. For example, while over 90% of birds are socially monogamous, "on average, 30 percent or more of the baby birds in any nest [are] sired by someone other than the resident male."[21] Gowaty has estimated that, out of 180 different species of socially monogomous songbirds, only 10 percent are sexually monogamous.[22]
The incidence of genetic monogamy, determined by DNA fingerprinting, varies widely across species. For a few rare species, the incidence of genetic monogamy is 100 percent, with all offspring genetically related to the socially monogamous pair. But genetic monogamy is strikingly low in other species. Barash and Lipton note:
"The highest known frequency of extra-pair copulations are found among the fairy-wrens, lovely tropical creatures technically known as Malurus splendens and Malurus cyaneus. More than 65 percent of all fairy-wren chicks are fathered by males outside the supposed breeding group." (Barash & Lipton, 2001, page 12)[20]
Such low levels of genetic monogamy have surprised biologists and zoologists, forcing them to rethink the role of social monogamy in evolution. They can no longer assume social monogamy determines how genes are distributed in a species. The lower the rates of genetic monogamy among socially monogamous pairs, the less of a role social monogamy plays in determining how genes are distributed among offspring. See also Evolution of Monogamy.
Polygamy
Polygamy is defined as a mating structure in which a single individual of one gender has exclusive access to several individuals of the opposite gender. It takes two main forms - polygyny and polyandry. As polygyny is the most common form of polygamy among vertebrates (including humans, to some extent), it has been studied far more extensively than polyandry.
Polygyny
In some species, notably those with harem male structures, only one of a few males in a group of females will mate. This is also known as polygyny in sociobiology.[23] Should the active male be driven out, killed, or otherwise removed from the group, in a number of species the new male will ensure that breeding resources are not wasted on another male's young.[24] The new male may achieve this in many different ways, including:
* Compe ive infanticide - in lions and some monkeys the new male will kill other young; therefore, the females, which are no longer nursing, rapidly become receptive again.
* Harassment to miscarriage - amongst wild horses and baboons, the male will "systematically harass" pregnant females until they miscarry.
* Pheromone based spontaneous abortion - in some rodents such as mice, a new male with a different scent will cause females who are pregnant to spontaneously fail to implant recently fertilized eggs. This does not require contact; it is mediated by scent alone. It is known as the Bruce-Parkes effect.