Homosexuelles Verhalten bei Tieren
Homosexuality and animals
Applying the term homosexual to animals
The term homosexual was first used in the 1870's, to describe same-sex sexual attraction and sexual behaviour in humans. Its use in animal studies has been controversial, mainly for two reasons: Animal sexuality and its motivating factors has been and perhaps still is poorly understood, and the term itself has strong cultural implication in western society, implications that are obviously irrelevant for other species than humans. Same-sex sexuality among animals has been given a number of terms over the years, however this article conform to the usage by modern researchers like Bruce Bagemihl, Joan Roughgarden, Volker Summer and Paul L. Vasey, applying the term homosexuality to all sexual behaviour (copulation, genital stimulation, mating games and sexual display behaviour) between animals of the same sex.
Animal homosexuality as sexuality
Opponents to the above use of the word "homosexual" has pointed to the instinctual nature of sex in animals, arguing that all sexual behaviour in non-human animals is mechanical and do not involve the emotions necessary to class some of it as homosexual. However, modern research has largely left the mechanical view of animal sexuality, arguing that a number of animals, notably mammals and birds show very clear signs of enjoyment during copulation. The physiological and psychological factors involved in human sexual behaviour indicate that a very strong instinctual component is involved in sex in our species too, indeed that instincts and emotions are not mutually exclusive. In their work Homosexual Behaviour in Animals, An Evolutionary Perspective (2006), Volker Sommer & Paul L. Vasey argues that the pleasure principle govern animal sexual behaviour, and that this pleasure is actively sought and used by animals in non-reproductive social situations, accounting for much of the homosexual behaviour.
Moral implications of animal homosexuality
The discovery that homosexual behaviour is quite common among animals has thrown new light on homosexuality as “unnatural”. A commonly used argument against taking animal homosexuality into account is that moral as such is a strictly human phenomenon, and that animal behaviour have no implication for what is morally right or wrong in humans. Another argument is that if homosexual acts among animals are in accordance with animal nature and thus moral, then parental killing of offspring and intra-species devouring are also in accordance with animal nature and thus moral. [1]. Others again argue that as homosexuality is frequently found in apes, it is probably a natural part of human biology as well. The debate is not likely to settle soon.
Animal homosexual behavior in science
The presence of same-sex sexual behavior was not 'officially' observed on a large scale until recent times, possibly due to observer bias caused by social attitudes to same-sex sexual behavior. It appears to be widespread amongst social birds and mammals, particularly the sea mammals and the primates. Some researchers believe it to have its origin in male social organization and social dominance, similar to the dominance traits shown in prison sexuality. Others, particularly Joan Roughgarden, Bruce Bagemihl and Paul Vasey points to that the social function of sex (bout homosexual and heterosexual) is not necessarily connected to dominance, but servers to strengthen alliances and social ties within a flock. Others cast doubt on social organization theory, pointing to citing gay penguins that mate for life and refuse to pair with females when given the chance. While reports on many such mating scenarios are still only anecdotal, a growing body of scientific work confirm that permanent homosexuality occur in species with permanent pair bonds, but also in non-monogamous species like sheep. A review by Bruce Bagemihl (see references) shows that homosexual behavior has been observed in close to 1500 species, ranging from primates to gut worms, and is well documented for 500 of them.
Cross species sex
Although a commonly held conception is that animals' sexuality is instinctive almost to the point of being mechanistic, research regularly records that many animals are sexual opportunists, and may show an interest in other partners than their own or related species. This is more visible in domesticated species, as domestication commonly selects for increased breeding rate (and so an accelerated breeding cycle has commonly arisen in domesticated species over the centuries), and also because these species are easier to witness by humans. Nevertheless non-domesticated animals have been observed to attempt sexual activity with other species, or indeed inanimate objects. This form of cross-species sex has occasionally been observed in the wild, however most observations are from animals in captivity (such as the zoo).
If the pair are a male and a female, hybrid offspring can result if the two species are related. However, this offspring may not be able to breed itself. The mule, for example (horse/donkey cross) is normally sterile, whilst the liger (lion/tiger cross) is sometimes fertile. Novosibirsk zoo director Rostislav Shilo says on the liger (born in its zoo); “It’s just that the lion and the tiger live in neighboring caves in the Novosibirsk zoo, and got used to each other. It’s practically impossible in the wild.”. Cross species sex in the wild has been observed between several species, among them blue tit and great tit, Chimpanzees and olive baboons and Amazon River Dolphin and the tucuxi dolphin.
Specific species
Bonobo apes
The bonobo, which has a matriarchal society (unusual amongst apes), is a fully bisexual species -- both males and females engage in heterosexual and homosexual behavior, being noted for lesbianism in particular. About 60% of all sexual activity in this species is between two or more females. While the homosexual bonding system in bonobos represent the highest frequency of homosexuality known in any species, homosexuality has been reported for all great apes, including humans, as well as a number of other primate species.
Birds
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 a male pair of chinstrap penguins in the Central Park Zoo in New York City were partnered and even 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 gay 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 20 gay pairs at 16 major aquariums and zoos in Japan. Bremerhaven Zoo in Germany attempted to break up the gay 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 gay couples.
Studies have shown that 10 to 15 percent of female western gulls in some populations in the wild are lesbian.
Lizards
Whiptail lizard (Teiidae genus) 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 fertility 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 and Science University) states that homosexuality in male sheep (found in 8% 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 heterosexual male sheep.
One report on sheep cited below states:
- "Approximately 8% of rams exhibit sexual preferences [that is, even when given a choice] for male partners (male-oriented rams) in contrast to most rams, which prefer female partners (female-oriented rams). We identified a cell group within the medial preoptic area/anterior hypothalamus of age-matched adult sheep that was significantly larger in adult rams than in ewes..."[2]. In fact, apparent homosexual individuals are known from all of the traditional domestic species.
It should be noted however that some view this study to be flawed in that the determination of homosexuality within the sheep (sample population of 27 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 (i.e. the males could not move or struggle while the females could). Given the aggressive nature of 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. As with most animal homosexuality, the results were situational sexuality, unlike the bonds seen in human homosexuality.
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 prior, given the potential unagressiveness of the male population in question, the differing aromatase levels may also have been evidence of aggression levels, not sexuality. It should also be noted that the results of this study have not been confirmed by other studies.
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.
Bottlenose Dolphins
Bottlenose Dolphin males have been observed working in pairs or larger groups 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/groups have also been observed engaging in ardent sexual play with each other.
Janet Mann, a professor of biology and psychology at Georgetown University, argues that the strong personal behaviour among male dolphin calves is about bond formation and benefits the species in an evolutionary context. She cites studies showing that these dolphins later in life as adults are in a sense bisexual, and the male bonds forged earlier in life work together for protection as well as locating females to reproduce with.
Fruit flies
Male Drosophila melanogaster flies bearing two copies of a mutant allele in the fruitless gene court and attempt to mate exclusively with other males.
References
- Bruce Bagemihl: Biological Exuberance: Animal Homosexuality and Natural Diversity. St. Martin's Press, 1999. ISBN 0312192398