Animal sexual behaviour

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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, rape, necrophilia, sexual orientation (heterosexuality, homosexuality, bisexuality 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 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 believed monogamous have now been proven to be promiscuous or opportunistic in nature, a wide range of species appear to both 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 homosexual behavior has now been documented in over 450 species.

ORANGUTANS MAKE ME HORNEY

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.

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:
    • 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 and female will mate within the social group.

Monogamy

In past research, many zoologists and writers succumbed to fantasy myths and observer bias, and extolled the virtues of many species said to be faithful, for a season or a lifetime, and which were held up as role models of 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 [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] This includes previous exemplars such as swans and (depending upon circumstances) wolves. 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 having sex with an extra-pair male partner. [3] [4] [16] [17] These discoveries have led biologists to adopt new ways of talking about 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)[18]

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. [12] [19] 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. [12] [19] 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. Gowaty has estimated that, out of 180 different species of socially monogomous songbirds, only 10 percent are sexually monogamous.[20]

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 spendens 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)[19]

Such low levels of genetic monogamy has 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.

Readers interested in the evolution of monogamy may wish to visit the Wikipedia article on the Evolution of Monogamy.

Polygyny

In some species, notably those with harem male structures, only one of few males in a group of females will mate. This is also known as polygyny in sociobiology.[21] Should the active male be driven out, then a number of species, the new male will ensure that breeding resources are not wasted on another males' young.[22] These can include:

  • Competitive infanticide - in lions and some monkeys the new male will kill other young, and the females, 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.

Promiscuity

Two examples of promiscuous mating systems in primates are chimpanzees and bonobos. These species live in social groups consisting of multiple males and multiple females. Each male copulates with many females, and vice versa. In bonobos, the amount of promoscuity is particularly striking because bonobos use sex to alleviate social conflict as well as to reproduce.

Sex for pleasure

It is a common urban myth that animals do not (as a rule) have sex for pleasure, or alternatively that humans (and perhaps dolphins and one or two species of primate) are the only species which do. This is sometimes formulated "animals mate only for reproduction".

Science cannot say at present conclusively what animals do or do not find "pleasurable", a question considered in more depth under Emotion in animals. The urban myth site Snopes.com considers this particular view in depth. Its conclusions are broadly that the statement is true, but only using a very specific definition of "sex for pleasure" [italics in original], in which sexual acts tied to a reproductive cycle or for which an alternative explanation can be asserted, are ignored, as is all sexual activity that does not involve penetration. Animals put themselves at risk to engage in sex, and as a result, most species have evolved sexual signals (usually scent and behavior) to indicate the presence of receptive periods. During these, sex is sought, and outside these it is usually not sought (or is sought but not permitted). Snopes comments that this is not in fact a reflection of whether sex is pleasurable or not, but rather a reflection of whether individuals have sex at arbitrary times. They conclude [1]:

"Of course, we have to make many seemingly artificial distinctions to arrive at our conclusion. Animals other than humans have no awareness that their sexual activities are connected with reproduction: They engage in sex because they're biologically driven to do so, and if the fulfillment of their urges produces a physical sensation we might appropriately call 'pleasure,' it isn't the least bit affected by the possibility (or impossibility) of producing offspring. We are also discounting cases in which animals do engage in sex even though reproduction is an impossibility because we claim there are other 'purposes' (of which the animals themselves are unaware) at play. (For example, the females of some species of birds will invite males to mate with them even after they have laid their eggs, but we ascribe a purpose to this behavior: this is a biological "trick" to fool males into caring for hatchlings they didn't father.) We also employ subjective terms such as 'willingly' and 'regularly' in claiming that bonobos and dolphins are the only other animals who "willingly (and regularly) engage in sex with each other" ... and even then it may be the case that these species have some other 'purpose' for doing so that we haven't yet discovered..."

Notes on specific species

Bonobos

The Bonobo, which has a matriarchal society, is a fully bisexual species -- both males and females engage in sexual behaviour with the same and the opposite sex, with females being particularly noted for engaging in sexual behaviour with each other.

Birds

File:Gay penguins NY Zoo.jpg
Squawk and Milou
Male chinstrap penguins, one of several homosexual pairs at the Central Park Zoo in Manhattan.
(Picture:Nicole Bengiveno/The New York Times)

Some black swans of Australia form sexually active male-male mated pairs and steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs. More of their cygnets survive to adulthood than those of different-sex pairs possibly due to their superior ability to defend large portions of land.

In early February 2004 the New York Times reported that a male pair of chinstrap penguins in the Central Park Zoo in New York City were partnered and had successfully hatched a female chick from an egg. Other penguins in New York have also been reported to be forming same-sex pairs.

Zoos in Japan and Germany have also documented male penguin couples. The couples have been shown to build nests together and use a stone to replace an egg in the nest. Researchers at Rikkyo University in Tokyo, found twenty such pairs at sixteen major aquariums and zoos in Japan. Bremerhaven Zoo in Germany attempted to break up the male couples by importing female penguins from Sweden and separating the male couples; they were unsuccessful. The zoo director stated the relationships were too strong between the couples.

Recently, a mated pair of swans in Boston were found to both be female. They too had attempted to raise eggs together. (365gay.com)

Studies have shown that ten to fifteen percent of female western gulls in some populations in the wild prefer other females.

Lizards

Whip-tailed lizard females have the ability to reproduce through parthenogenesis and as such males are rare and sexual breeding non-standard. Females engage in sexual behavior to stimulate ovulation, with their behavior following their hormonal cycles; during low levels of estrogen, these (female) lizards engage in "masculine" sexual roles. Those animals with currently high estrogen levels assume "feminine" sexual roles.

Lizards that perform the courtship ritual have greater fecundity than those kept in isolation due to an increase in hormones triggered by the sexual behaviors. So, even though asexual whiptail lizards populations lack males, sexual stimuli still increase reproductive success.

From an evolutionary standpoint these females are passing their full genetic code to all of their offspring rather than the 50% of genes that would be passed in sexual reproduction. Certain species of gecko also reproduce by parthenogenesis.

Sheep

An October 2003, study by Dr. Charles E. Roselli et al. (Oregon Health & Science University) states that homosexuality in male sheep (found in eight percent of rams) is associated with a region in the rams' brains which the authors call the "ovine Sexually Dimorphic Nucleus" (oSDN) which is two times smaller than the corresponding region in other male sheep.

However, some view this study to be flawed in that the determination of homosexuality within the sheep, (sample population of twenty-seven for the study), was to have animals who were unable to mount female ewes placed in a cage with two stanchioned males and two unstanchioned females (that is, the males could not move or struggle while the females could). Given the aggressive nature of the sheep copulation, the uneven treatment of males and females, many see this as simply evidence that the sheep in question were unable to be aggressive enough to mount females. Some say that the results were situational sexuality, unlike the bonds seen in human homosexuality. However the physical brain anatomy of the rams that preferred males were different.

The scientists found that, "The oSDN in rams that preferred females was significantly larger and contained more neurons than in male-oriented rams and ewes. In addition, the oSDN of the female-oriented rams expressed higher levels of aromatase, a substance that converts testosterone to estradiol so that the androgen hormone can facilitate typical male sexual behaviors. Aromatase expression was no different between male-oriented rams and ewes."

"The dense cluster of neurons that comprise the oSDN express cytochrome P450 aromatase. Aromatase mRNA levels in the oSDN were significantly greater in female-oriented rams than in ewes, whereas male-oriented rams exhibited intermediate levels of expression." These results suggest that "...naturally occurring variations in sexual partner preferences may be related to differences in brain anatomy and its capacity for estrogen synthesis." Read the abstract of the study. As noted previously, given the potential unagressiveness of the male population in question, the differing aromatase levels may also have been evidence of aggression levels, not sexuality. The results of this study have not been confirmed by others.

Spotted Hyena

The female Spotted Hyena has a unique urinary-genital system, closely resembling the penis of the male. The family structure is matriarchal and dominance relationships with strong sexual elements are routinely observed between related females.

They are notable for using visible sexual arousal as a sign of submission and not dominance, in males as well as females (females have a sizable erectile clitoris), to the extent that biologist Robert Sapolsky speculates that in order to facilitate this, their sympathetic and parasympathetic nervous systems may be partially reversed in respect to their reproductive organs.[23]

Bottlenose Dolphins

Bottlenose Dolphin males have been observed working in pairs to follow and/or restrict the movement of a female for weeks at a time, waiting for her to become sexually receptive. The same pairs have also been observed engaging in intense sexual play with each other.

Janet Mann, a professor of biology and psychology at Georgetown University, argues (1) that the common same-sex behavior among male dolphin calves is about bond formation and benefits the species evolutionarily. They cite studies that have shown the dolphins later in life are bisexual and the male bonds forged from homosexuality work for protection as well as locating females to reproduce with.

Other evidence of interspecies sexual activity

Main article: Chuman

Template:Primarysources

Looking back in history, current research into human evolution tends to confirm that in some cases, interspecies sexual activity may have been responsible for the evolution of entire new species. Analysis of human and animal genes in 2006 provides strong evidence that after humans had diverged from apes, interspecies mating none the less occurred regularly enough to change certain genes in the new gene pool:

"A new comparison of the human and chimp genomes suggests that after the two lineages separated, they may have begun interbreeding. [...] A principal finding is that the X chromosomes of humans and chimps appear to have diverged about 1.2 million years more recently than the other chromosomes."

The research suggests that:

"There were in fact two splits between the human and chimp lineages, with the first being followed by interbreeding between the two populations and then a second split. The suggestion of a hybridization has startled paleoanthropologists, who nonetheless are "treating the new genetic data seriously."

The Washington Post comments, "If this theory proves correct, it will mean modern people are descended from something akin to chimp-human hybrids."

Source: ny times (May 2006).

References

  1. ↑ Ågren, G., Zhou, Q. & Zhong, W. (1989). Ecology and social behaviour of Mongolian gerbils Meriones unguiculatus, at Xiliuhot, Inner Mongolia, China. Animal Behaviour, 37, 11-27.
  2. ↑ Barash, D.P. (1981). Mate guarding and gallivanting by male hoary marmots (Marmota caligata). Behavioral Ecology and Sociobiology, 9, 187-193.
  3. ↑ 3.0 3.1 Birkhead, T.R. & Møller, A.P. (1995). Extra-pair copulations and extra-pair paternity in birds. Animal Behaviour, 49, 843-848.
  4. ↑ 4.0 4.1 Birkhead, T.R. & Møller, A.P. (1996). Monogamy and sperm competition in birds. In J. M. Black (Ed.), Partnerships in Birds: The Study of Monogamy (pp. 323-343). Oxford: Oxford University Press.
  5. ↑ Foltz, D.W. (1981). Genetic evidence for long-term monogamy in a small rodent, Peromyscus polionotus. American Naturalist, 117, 665-675.
  6. ↑ Gursky, S.L. (2000). Sociality in the spectral tarsier, Tarsius spectrum. American Journal of Primatology, 51, 89-101.
  7. ↑ Hasselquist, D. S. & Sherman, P.W. (2001). Social mating systems and extrapair fertilizations in passerine birds. Behavioral Ecology, 12, 457-466.
  8. ↑ Hubrecht, R.C. (1985). Home range size and use and territorial behavior in the common marmoset, Callithrix jacchus jacchus, at the Tapacura Field Station, Recife, Brazil. International Journal of Primatology, 6, 533-550.
  9. ↑ Mason, W.A. (1966). Social organization of the South American monkey, Callicebus moloch: a preliminary report. Tulane Studies in Zoology, 13, 23-28.
  10. ↑ McKinney, F., Derrickson, S.R., & Mineau, P. (1983). Forced copulation in waterfowl. Behaviour, 86, 250-294.
  11. ↑ Reichard, U. (1995). Extra-pair Copulations in a Monogamous Gibbon (Hylobates lar). Ethology, 100, 99-112.
  12. ↑ 12.0 12.1 12.2 Reichard, U.H. (2002). Monogamy—A variable relationship. Max Planck Research, 3, 62-67.
  13. ↑ Richardson, P.R.K. (1987). Aardwolf mating system: overt cuckoldry in an apparently monogamous mammal. South African Journal of Science, 83, 405-412.
  14. ↑ Welsh, D. & Sedinger, J.S. (1990). Extra-Pair copulations in Black Brant. The Condor, 92, 242-244.
  15. ↑ Westneat, D.F. & Stewart, I.R.K. (2003). Extra-pair paternity in birds: causes, correlates, and conflict. Annual Review of Ecology, Evolution, and Systematics, 34, 365-396.
  16. ↑ Owens, I.P.F. & Hartley, I.R. (1998). Sexual dimorphism in birds: why are there so many different forms of dimorphism? Proceedings of the Royal Society, London, B265, 397–407.
  17. ↑ Solomon, N.G., Keane, B., Knoch, L.R., & Hogan, P.J. (2004). Multiple paternity in socially monogamous prairie voles (Microtus ochrogaster). Canadian Journal of Zoology, 82, 1667-1671.
  18. ↑ Reichard, U.H. (2003). Monogamy: Past and present. In U.H. Reichard and C. Boesch (Eds.), Monogamy: Mating strategies and parnternships in birds, humans, and other mammals (pp.3-25).Cambridge: Cambridge University Press.
  19. ↑ 19.0 19.1 19.2 Barash, D.P. & Lipton, J.E. (2001). The Myth of Monogamy. New York, NY: W.H. Freeman and Company.
  20. ↑ Research conducted by Patricia Adair Gowaty. Reported by Morell, V. (1998). Evolution of sex: A new look at monogamy. Science, 281, 1982-1983.
  21. ↑ Technically, polygyny in sociobiology and zoology is defined as a system in which a male has a relationships with more than one female, but the females are predominately bonded to a single male.
  22. ↑ This section and examples taken from:Robert Sapolsky Why Zebras Don't get Ulcers, p.140-141.
  23. ↑ Sapolsky, Why Zebras don't get Ulcers, p.127-129.

See also

Wikimedia Commons has media related to Animal sex.

fr:Sexualité animale

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