Homosexual behavior in animals
Homosexual behavior in animals is sexual behavior among non-human species that is interpreted as homosexual or bisexual. This may include same-sex sexual activity, courtship, affection, pair bonding, and parenting among same-sex animal pairs. Research indicates that various forms of this are found in every major geographic region and every major animal group. The sexual behavior of non-human animals takes many different forms, even within the same species, though homosexual behavior is best known from social species.
Scientists perceive homosexual behavior in animals to different degrees. The motivations for and implications of these behaviors have yet to be fully understood, since most species have yet to be fully studied. According to Bruce Bagemihl, the animal kingdom engages in homosexual behavior "with much greater sexual diversity – including homosexual, bisexual and nonreproductive sex – than the scientific community and society at large have previously been willing to accept." Bagemihl adds, however, that this is "necessarily an account of human interpretations of these phenomena". Simon LeVay introduced caveat that "[a]lthough homosexual behavior is very common in the animal world, it seems to be very uncommon that individual animals have a long-lasting predisposition to engage in such behavior to the exclusion of heterosexual activities. Thus, a homosexual orientation, if one can speak of such thing in animals, seems to be a rarity." One species in which exclusive homosexual orientation occurs, however, is that of domesticated sheep (Ovis aries). "About 10% of rams (males), refuse to mate with ewes (females) but do readily mate with other rams."
According to Bagemihl (1999), same-sex behavior (comprising courtship, sexual, pair-bonding, and parental activities) has been documented in over 450 species of animals worldwide.
In relation to humans
Applying the term homosexual to animals
The term homosexual was coined by Karl-Maria Kertbeny in 1868 to describe same-sex sexual attraction and sexual behavior in humans. Its use in animal studies has been controversial for two main reasons: animal sexuality and motivating factors have been and remain poorly understood, and the term has strong cultural implications in western society that are irrelevant for species other than humans. Thus homosexual behavior has been given a number of terms over the years. According to Bruce Bagemihl, when describing animals, the term homosexual is preferred over gay, lesbian, and other terms currently in use, as these are seen as even more bound to human homosexuality.
Bailey et al. says: "Homosexual: in animals, this has been used to refer to same-sex behavior that is not sexual in character (e.g. ‘homosexual tandem running’ in termites), same-sex courtship or copulatory behavior occurring over a short period of time (e.g. ‘homosexual mounting’ in cockroaches and rams) or long-term pair bonds between same-sex partners that might involve any combination of courting, copulating, parenting and affectional behaviors (e.g. ‘homosexual pair bonds’ in gulls). In humans, the term is used to describe individual sexual behaviors as well as long-term relationships, but in some usages connotes a gay or lesbian social identity. Scientific writing would benefit from reserving this anthropomorphic term for humans and not using it to describe behavior in other animals, because of its deeply rooted context in human society".
Animal preference and motivation is always inferred from behavior. In wild animals, researchers will as a rule not be able to map the entire life of an individual, and must infer from frequency of single observations of behavior. The correct usage of the term homosexual is that an animal exhibits homosexual behavior or even same-sex sexual behavior; however, this article conforms to the usage by modern research,[page needed] applying the term homosexuality to all sexual behavior (copulation, genital stimulation, mating games and sexual display behavior) between animals of the same sex. In most instances, it is presumed that the homosexual behavior is but part of the animal's overall sexual behavioral repertoire, making the animal "bisexual" rather than "homosexual" as the terms are commonly understood in humans.[page needed], but cases of homosexual preference and exclusive homosexual pairs are known.
Nature or nurture
The observation of homosexual behavior in animals can be seen as both an argument for and against the acceptance of homosexuality in humans, and has been used especially against the claim that it is a peccatum contra naturam ("sin against nature"). For instance, homosexuality in animals was cited by the American Psychiatric Association and other groups in their amici curiae brief to the United States Supreme Court in Lawrence v. Texas, which ultimately struck down the sodomy laws of 14 states.
A majority of the research available concerning homosexual behavior in animals lacks specification between animals that exclusively exhibit same-sex tendencies and those that participate in heterosexual and homosexual mating activities interchangeably. This lack of distinction has led to differing opinions and conflicting interpretations of collected data amongst scientists and researchers. For instance, Bruce Bagemihl, author of the book Biological Exuberence: Animal Homosexuality and Natural Diversity, emphasizes that there are no anatomical or endocrinological differences between exclusively homosexual and exclusively heterosexual animal pairs.[page needed] However, if the definition of "homosexual behavior" is made to include animals that participate in both same-sex and opposite-sex mating activities, hormonal differences have been documented among key sex hormones, such as testosterone and estradiol, when compared to those who participate solely in heterosexual mating.
Many of the animals used in laboratory-based studies of homosexuality do not appear to spontaneously exhibit these tendencies often in the wild. Such behavior is often elicited and exaggerated by the researcher during experimentation through the destruction of a portion of brain tissue, or by exposing the animal to high levels of steroid hormones prenatally.[page needed] Information gathered from these studies is limited when applied to spontaneously occurring same-sex behavior in animals outside of the laboratory.
Homosexual behaviour in animals has been discussed since classical antiquity. The earliest written mention of animal homosexuality appears to date back to 2,300 years ago, when Aristotle (384–322 BC) described copulation between pigeons, partridges and quails of the same sex. The Hieroglyphics of Horapollo, written in the 4th century AD by the Egyptian writer Horapollo, mentions "hermaphroditism" in hyenas and homosexuality in partridges. The first review of animal homosexuality was written by the zoologist Ferdinand Karsch-Haack in 1900.
Until recent times, the presence of same-sex sexual behavior was not "officially" observed on a large scale, possibly due to observer bias caused by social attitudes to same-sex sexual behavior, innocent confusion, lack of interest, distaste, scientists fearing loss of their grants or even from a fear of "being ridiculed by their colleagues". Georgetown University biologist Janet Mann states "Scientists who study the topic are often accused of trying to forward an agenda, and their work can come under greater scrutiny than that of their colleagues who study other topics." They also noted "Not every sexual act has a reproductive function ... that's true of humans and non-humans." It appears to be widespread amongst social birds and mammals, particularly the sea mammals and the primates. The true extent of homosexuality in animals is not known. While studies have demonstrated homosexual behavior in a number of species, Petter Bøckman, the scientific advisor of the exhibition Against Nature? in 2007, speculated that the true extent of the phenomenon may be much larger than was then recognized:
No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at all, such as sea urchins and aphis. Moreover, a part of the animal kingdom is hermaphroditic, truly bisexual. For them, homosexuality is not an issue.
An example of overlooking homosexual behavior is noted by Bagemihl describing mating giraffes where nine out of ten pairings occur between males:
Every male that sniffed a female was reported as sex, while anal intercourse with orgasm between males was only "revolving around" dominance, competition or greetings.
Some researchers believe this behavior to have its origin in male social organization and social dominance, similar to the dominance traits shown in prison sexuality. Others, particularly Bagemihl, Joan Roughgarden, Thierry Lodé and Paul Vasey suggest the social function of sex (both homosexual and heterosexual) is not necessarily connected to dominance, but serves to strengthen alliances and social ties within a flock. Others have argued that social organization theory is inadequate because it cannot account for some homosexual behaviors, for example, penguin species where male individuals 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 confirms that permanent homosexuality occurs not only in species with permanent pair bonds, but also in non-monogamous species like 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...
A definite physiological explanation or reason for homosexual activity in animal species has not been agreed upon by researchers in the field. Numerous scholars are of the opinion that varying levels (either higher or lower) of the sex hormones in the animal, in addition to the size of the animal's gonads, play a direct role in the sexual behavior and preference exhibited by that animal. Others firmly argue no evidence to support these claims exists when comparing animals of a specific species exhibiting homosexual behavior exclusively and those that do not. Ultimately, empirical support from comprehensive endocrinological studies exist for both interpretations. Researchers found no evidence of differences in the measurements of the gonads, or the levels of the sex hormones of exclusively homosexual western gulls and ring-billed gulls. 
Additional studies pertaining to hormone involvement in homosexual behavior indicate that when administering treatments of testosterone and estradiol to female heterosexual animals, the elevated hormone levels increase the likelihood of homosexual behavior. Additionally, boosting the levels of sex hormones during an animal's pregnancy appears to increase the likelihood of it birthing a homosexual offspring.
Researchers found that disabling the fucose mutarotase (FucM) gene in laboratory mice – which influences the levels of estrogen to which the brain is exposed – caused the female mice to behave as if they were male as they grew up. "The mutant female mouse underwent a slightly altered developmental programme in the brain to resemble the male brain in terms of sexual preference" said Professor Chankyu Park of the Korea Advanced Institute of Science and Technology in Daejon, South Korea, who led the research. His most recent findings have been published in the BMC Genetics journal on July 7, 2010. Another study found that by manipulating a gene in fruit flies (Drosophila), homosexual behavior appeared to have been induced. However, in addition to homosexual behavior, several abnormal behaviors were also exhibited apparently due to this mutation.
In March 2011, research showed that serotonin is involved in the mechanism of sexual orientation of mice. A study conducted on fruit flies found that inhibiting the dopamine neurotransmitter inhibited lab-induced homosexual behavior.
Some selected species and groups
An estimated one-quarter of all black swans pairings are of males. They steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs. The males spent time in each other's society, guarded the common territory, performed greeting ceremonies before each other, and (in the reproductive period) pre-marital rituals, and if one of the birds tried to sit on the other, an intense fight began. 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. The same reasoning has been applied to male flamingo pairs raising chicks.
Female albatross, on the north-western tip of the island of Oahu, Hawaii, form pairs for co-growing offspring. On the observed island, the number of females considerably exceeds the number of males (59% N=102/172), so 31% of females, after mating with males, create partnerships for hatching and feeding chicks. Compared to male-female couples female partnerships have a lower hatching rate (41% vs 87%) and lower overall reproductive success (31% vs. 67%).
Research has shown that the environmental pollutant methylmercury can increase the prevalence of homosexual behavior in male American white ibis. The study involved exposing chicks in varying dosages to the chemical and measuring the degree of homosexual behavior in adulthood. The results discovered was that as the dosage was increased the likelihood of homosexual behavior also increased. The endocrine blocking feature of mercury has been suggested as a possible cause of sexual disruption in other bird species.
Mallards form male-female pairs only until the female lays eggs, at which time the male leaves the female. Mallards have rates of male-male sexual activity that are unusually high for birds, in some cases, as high as 19% of all pairs in a population.[page needed] Kees Moeliker of the Natural History Museum Rotterdam has observed one male mallard engage in homosexual necrophilia.
Penguins have been observed to engage in homosexual behaviour since at least as early as 1911. George Murray Levick, who documented this behaviour in Adélie penguins at Cape Adare, described it as "depraved". The report was considered too shocking for public release at the time, and was suppressed. The only copies that were made available privately to researchers were translated into Greek, to prevent this knowledge becoming more widely known. The report was unearthed only a century later, and published in Polar Record in June 2012.
In early February 2004 the New York Times reported that Roy and Silo, a male pair of chinstrap penguins in the Central Park Zoo in New York City had successfully hatched and fostered a female chick from a fertile egg they had been given to incubate. Other penguins in New York zoos have also been reported to have formed same-sex pairs.
In Odense Zoo in Denmark, a pair of male king penguins adopted an egg that had been abandoned by a female, proceeding to incubate it and raise the chick. Zoos in Japan and Germany have also documented homosexual male penguin couples. The couples have been shown to build nests together and use a stone as a substitute for an egg. Researchers at Rikkyo University in Tokyo found 20 homosexual pairs at 16 major aquariums and zoos in Japan.
The Bremerhaven Zoo in Germany attempted to encourage reproduction of endangered Humboldt penguins by importing females from Sweden and separating three male pairs, but this was unsuccessful. The zoo's director said that the relationships were "too strong" between the homosexual pairs. German gay groups protested at this attempt to break up the male-male pairs but the zoo's director was reported as saying "We don't know whether the three male pairs are really homosexual or whether they have just bonded because of a shortage of females ... nobody here wants to forcibly separate homosexual couples."
A pair of male Magellanic penguins who had shared a burrow for six years at the San Francisco Zoo and raised a surrogate chick, split when the male of a pair in the next burrow died and the female sought a new mate.
In 2014 Jumbs and Hurricane, two Humboldt penguins at Wingham Wildlife Park became the center of international media attention as two male penguins who had pair bonded a number of years earlier and then successfully hatched and reared an egg given to them as surrogate parents after the mother abandoned it halfway through incubation.
In 2018 Thelma and Louise, two female King Penguins at Kelly Tarltons in Auckland, New Zealand, have been in a relationship for 8 years, when most of the other eligible penguins switch partners each mating season, regardless of their orientation, are both taking care of an egg that Thelma recently hatched, but is unknown whether it was fertilized.
In 1998 two male griffon vultures named Dashik and Yehuda, at the Jerusalem Biblical Zoo, engaged in "open and energetic sex" and built a nest. The keepers provided the couple with an artificial egg, which the two parents took turns incubating; and 45 days later, the zoo replaced the egg with a baby vulture. The two male vultures raised the chick together. A few years later, however, Yehuda became interested in a female vulture that was brought into the aviary. Dashik became depressed, and was eventually moved to the zoological research garden at Tel Aviv University where he too set up a nest with a female vulture.
Two male vultures at the Allwetter Zoo in Muenster built a nest together, although they were picked on and their nest materials were often stolen by other vultures. They were eventually separated to try to promote breeding by placing one of them with female vultures, despite the protests of German homosexual groups.
Both male and female pigeons sometimes exhibit homosexual behavior. In addition to sexual behavior, same-sex pigeon pairs will build nests, and hens will lay (infertile) eggs and attempt to incubate them.
The Amazon river dolphin or boto has been reported to form up in bands of 3–5 individuals engaging in sexual activity. The groups usually comprise young males and sometimes one or two females. Sex is often performed in non-reproductive ways, using snout, flippers and genital rubbing, without regard to gender. In captivity, they have been observed to sometimes perform homosexual and heterosexual penetration of the blowhole, a hole homologous with the nostril of other mammals, making this the only known example of nasal sex in the animal kingdom. The males will sometimes also perform sex with males from the tucuxi species, a type of small porpoise.
Courtship, mounting, and full anal penetration between bulls has been noted to occur among American bison. The Mandan nation Okipa festival concludes with a ceremonial enactment of this behavior, to "ensure the return of the buffalo in the coming season". Also, mounting of one female by another (known as "bulling") is extremely common among cattle. The behaviour is hormone driven and synchronizes with the emergence of estrus (heat), particularly in the presence of a bull.
- the grey-headed flying fox (Pteropus poliocephalus)
- the Bonin flying fox (Pteropus pselaphon)
- the Indian flying fox (Pteropus giganteus)
- Rafinesque's big-eared bat (Corynorhinus rafinesquii)
- the common bent-wing bat (Miniopterus schreibersii)
- the serotine bat (Eptesicus serotinus)
- Bechstein's bat (Myotis bechsteinii)
- the long-fingered bat (Myotis capaccinii)
- Daubenton's bat (Myotis daubentonii)
- the little brown bat (Myotis lucifugus)
- the greater mouse-eared bat (Myotis myotis)
- the whiskered bat (Myotis mystacinus)
- Natterer's bat (Myotis nattereri)
- the common noctule (Nyctalus noctula)
- Leisler's bat (Nyctalus leisleri)
- the common pipistrelle (Pipistrellus pipistrellus)
- the brown long-eared bat (Plecotus auritus)
- the barbastelle (Barbastella barbastellus)
- the greater horseshoe bat (Rhinolophus ferrumequinum)
- the lesser horseshoe bat (Rhinolophus hipposideros)
Bat species that have been observed engaging in homosexual behavior in captivity include the Comoro flying fox (Pteropus livingstonii), the Rodrigues flying fox (Pteropus rodricensis) and the common vampire bat (Desmodus rotundus).
Homosexual behavior in bats has been categorized into 6 groups: mutual homosexual grooming and licking, homosexual masturbation, homosexual play, homosexual mounting, coercive sex, and cross-species homosexual sex.
In the wild, the grey-headed flying fox (Pteropus poliocephalus) engages in allogrooming wherein one partner licks and gently bites the chest and wing membrane of the other partner. Both sexes display this form of mutual homosexual grooming and it is more common in males. Males often have erect penises while they are mutually grooming each other. Like opposite-sex grooming partners, same-sex grooming partners continuously utter a “pre-copulation call,” which is described as a "pulsed grating call," while engaged in this activity.
In wild Bonin flying foxes (Pteropus pselaphon), males perform fellatio or 'male-male genital licking' on other males. Male–male genital licking events occur repeatedly several times in the same pair, and reciprocal genital licking also occurs. The male-male genital licking in these bats is considered a sexual behavior. Allogrooming in Bonin flying foxes has never been observed, hence the male-male genital licking in this species does not seem to be a by-product of allogrooming, but rather a behavior of directly licking the male genital area, independent of allogrooming. In captivity, same-sex genital licking has been observed among males of the Comoro flying fox (Pteropus livingstonii) as well as among males of the common vampire bat (Desmodus rotundus).
In wild Indian flying foxes (Pteropus giganteus), males often mount one another, with erections and thrusting, while play-wrestling. Males of the long-fingered bat (Myotis capaccinii) have been observed in the same position of male-female mounting, with one gripping the back of the other’s fur. A similar behavior was also observed in the common bent-wing bat (Miniopterus schreibersii).
In wild little brown bats (Myotis lucifugus), males often mount other males (and females) during late autumn and winter, when many of the mounted individuals are torpid. 35% of matings during this period are homosexual. These coercive copulations usually include ejaculation and the mounted bat often makes a typical copulation call consisting of a long squawk. Similarly, in hibernacula of the common noctule (Nyctalus noctula), active males were observed to wake up from lethargy on a warm day and engage in mating with lethargic males and (active or lethargic) females. The lethargic males, like females, called out loudly and presented their buccal glands with opened mouth during copulation.
Vesey-Fitzgerald (1949) observed homosexual behaviours in all 12 British bat species known at the time: “Homosexuality is common in the spring in all species, and, since the males are in full possession of their powers, I suspect throughout the summer...I have even seen homosexuality between Natterer's and Daubenton's bats (Myotis nattereri and M. daubentonii)."
Dolphins of several species engage in homosexual acts, though it is best studied in the bottlenose dolphins.[page needed] Sexual encounters between females take the shape of "beak-genital propulsion", where one female inserts her beak in the genital opening of the other while swimming gently forward. Between males, homosexual behaviour includes rubbing of genitals against each other, which sometimes leads to the males swimming belly to belly, inserting the penis in the others genital slit and sometimes anus.
Janet Mann, Georgetown University professor of biology and psychology, argues that the strong personal behavior 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. Confrontations between flocks of bottlenose dolphins and the related species Atlantic spotted dolphin will sometimes lead to cross-species homosexual behaviour between the males rather than combat.
African and Asian males will engage in same-sex bonding and mounting. Such encounters are often associated with affectionate interactions, such as kissing, trunk intertwining, and placing trunks in each other's mouths. Male elephants, who often live apart from the general herd, often form "companionships", consisting of an older individual and one or sometimes two younger males with sexual behavior being an important part of the social dynamic. Unlike heterosexual relations, which are always of a fleeting nature, the relationships between males may last for years. The encounters are analogous to heterosexual bouts, one male often extending his trunk along the other's back and pushing forward with his tusks to signify his intention to mount. Same-sex relations are common and frequent in both sexes, with Asiatic elephants in captivity devoting roughly 45% of sexual encounters to same-sex activity.
Male giraffes have been observed to engage in remarkably high frequencies of homosexual behavior. After aggressive "necking", it is common for two male giraffes to caress and court each other, leading up to mounting and climax. Such interactions between males have been found to be more frequent than heterosexual coupling. In one study, up to 94% of observed mounting incidents took place between two males. The proportion of same sex activities varied between 30 and 75%, and at any given time one in twenty males were engaged in non-combative necking behavior with another male. Only 1% of same-sex mounting incidents occurred between females.
Homosexual behavior is quite common in wild marmots. In Olympic marmots (Marmota olympus) and Hoary Marmots (Marmota caligata), females often mount other females as well as engage in other affectionate and sexual behaviors with females of the same species. They display a high frequency of these behaviors especially when they are in heat. A homosexual encounter often begins with a greeting interaction in which one female nuzzles her nose on the other female’s cheek or mouth, or both females touch noses or mouths. Additionally, a female may gently chew on the ear or neck of her partner, who responds by raising her tail. The first female may sniff the other's genital region or nuzzle that region with her mouth. She may then proceed to mount the other female, during which the mounting female gently grasps the mounted female's dorsal neck fur in her jaws while thrusting. The mounted female arches her back and holds her tail to one side to facilitate their sexual interaction.
Both male and female lions have been seen to interact homosexually. Male lions pair-bond for a number of days and initiate homosexual activity with affectionate nuzzling and caressing, leading to mounting and thrusting. About 8% of mountings have been observed to occur with other males. Pairings between females are held to be fairly common in captivity but have not been observed in the wild.
European polecats Mustela putorius were found to engage homosexually with non-sibling animals. Exclusive homosexuality with mounting and anal penetration in this solitary species serves no apparent adaptive function.[page needed]
Bonobos, which have a matriarchal society, unusual among apes, are a fully bisexual species—both males and females engage in heterosexual and homosexual behavior, being noted for female–female homosexuality in particular, including between juveniles and adults. Roughly 60% of all bonobo sexual activity occurs between two or more females. While the homosexual bonding system in bonobos represents the highest frequency of homosexuality known in any primate species, homosexuality has been reported for all great apes (a group which includes humans), as well as a number of other primate species.[page needed]
Dutch primatologist Frans de Waal on observing and filming bonobos noted that there were two reasons to believe sexual activity is the bonobo's answer to avoiding conflict. Anything that arouses the interest of more than one bonobo at a time, not just food, tends to result in sexual contact. If two bonobos approach a cardboard box thrown into their enclosure, they will briefly mount each other before playing with the box. Such situations lead to squabbles in most other species. But bonobos are quite tolerant, perhaps because they use sex to divert attention and to defuse tension.
Bonobo sex often occurs in aggressive contexts totally unrelated to food. A jealous male might chase another away from a female, after which the two males reunite and engage in scrotal rubbing. Or after a female hits a juvenile, the latter's mother may lunge at the aggressor, an action that is immediately followed by genital rubbing between the two adults.
With the Japanese macaque, also known as the "snow monkey", same-sex relations are frequent, though rates vary between troops. Females will form "consortships" characterized by affectionate social and sexual activities. In some troops up to one quarter of the females form such bonds, which vary in duration from a few days to a few weeks. Often, strong and lasting friendships result from such pairings. Males also have same-sex relations, typically with multiple partners of the same age. Affectionate and playful activities are associated with such relations.
Homosexual behavior forms part of the natural repertoire of sexual or sociosexual behavior of orangutans. Male homosexual behavior occurs both in the wild and in captivity, and it occurs in both adolescent and mature individuals. Homosexual behavior in orangutans is not an artifact of captivity or contact with humans.
Ovis aries has attracted much attention due to the fact that around 8–10% of rams have an exclusive homosexual orientation. Furthermore, around 18–22% of rams are bisexual.
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 half the size of the corresponding region in heterosexual male sheep. 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, a form of estrogen which is believed to 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." As noted before, 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.
Homosexual courtship and sexual activity routinely occur among rams of wild sheep species, such as Bighorn sheep (Ovis canadensis), Thinhorn sheep (Ovis dalli), mouflons and urials (Ovis orientalis). Usually a higher ranking older male courts a younger male using a sequence of stylized movements. To initiate homosexual courtship, a courting male approaches the other male with his head and neck lowered and extended far forward in what is called the 'low-stretch' posture. He may combine this with the 'twist,' in which the courting male sharply rotates his head and points his muzzle toward the other male, often while flicking his tongue and making grumbling sounds. The courting male also often performs a 'foreleg kick,' in which he snaps his front leg up against the other male’s belly or between his hind legs. He also occasionally sniffs and nuzzles the other male’s genital area and may perform the flehmen response. Thinhorn rams additionally lick the penis of the male they are courting. In response, the male being courted may rub his cheeks and forehead on the courting male’s face, nibble and lick him, rub his horns on the courting male’s neck, chest, or shoulders, and develop an erection. Males of another wild sheep species, the Asiatic Mouflons, perform similar courtship behaviors towards fellow males.
Sexual activity between wild males typically involves mounting and anal intercourse. In Thinhorn sheep, genital licking also occurs. During mounting, the larger male usually mounts the smaller male by rearing up on his hind legs and placing his front legs on his partner’s flanks. The mounting male usually has an erect penis and accomplishes full anal penetration while performing pelvic thrusts that may lead to ejaculation. The mounted male arches his back to facilitate the copulation. Homosexual courtship and sexual activity can also take place in groups composed of three to ten wild rams clustered together in a circle. These non-aggressive groups are called 'huddles' and involve rams rubbing, licking, nuzzling, horning, and mounting each other. Female Mountain sheep also engage in occasional courtship activities with one another and in sexual activities such as licking each other’s genitals and mounting.
The family structure of the spotted hyena is matriarchal, and dominance relationships with strong sexual elements are routinely observed between related females. Due largely to the female spotted hyena's unique urogenital system, which looks more like a penis rather than a vagina, early naturalists thought hyenas were hermaphroditic males who commonly practiced homosexuality.[not in citation given] Early writings such as Ovid's Metamorphoses and the Physiologus suggested that the hyena continually changed its sex and nature from male to female and back again. In Paedagogus, Clement of Alexandria noted that the hyena (along with the hare) was "quite obsessed with sexual intercourse". Many Europeans associated the hyena with sexual deformity, prostitution, deviant sexual behavior, and even witchcraft.
The reality behind the confusing reports is the sexually aggressive behavior between the females, including mounting between females. Research has shown that "in contrast to most other female mammals, female Crocuta are male-like in appearance, larger than males, and substantially more aggressive," and they have "been masculinized without being defeminized".[not in citation given]
Study of this unique genitalia and aggressive behavior in the female hyena has led to the understanding that more aggressive females are better able to compete for resources, including food and mating partners. Research has shown that "elevated levels of testosterone in utero" contribute to extra aggressiveness; both males and females mount members of both the same and opposite sex, who in turn are possibly acting more submissive because of lower levels of testosterone in utero.
Parthenogenesis. Several species of whiptail lizard (especially in the genus Aspidoscelis) consist only of females that have the ability to reproduce through parthenogenesis. 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. Some parthenogenetic 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.
Jonathan, the world's oldest tortoise (an Aldabra giant tortoise), had been mating with another tortoise named Frederica since 1991. In 2017, it was discovered that Frederica was actually probably male all along, and was renamed Frederic.
Insects and arachnids
There is evidence of same-sex sexual behavior in at least 110 species of insects and arachnids. Scharf et al. says: "Males are more frequently involved in same-sex sexual (SSS) behavior in the laboratory than in the field, and isolation, high density, and exposure to female pheromones increase its prevalence. SSS behavior is often shorter than the equivalent heterosexual behavior. Most cases can be explained via mistaken identification by the active (courting/mounting) male. Passive males often resist courting/mating attempts".
Scharf et al. continues: "SSS behavior has been reported in most insect orders, and Bagemihl (1999) provides a list of ~100 species of insects demonstrating such behavior. Yet, this list lacks detailed descriptions, and a more comprehensive summary of its prevalence in invertebrates, as well as ethology, causes, implications, and evolution of this behavior, remains lacking".
Male homosexuality has been inferred in several species of dragonflies (the order Odonata). The cloacal pinchers of male damselflies and dragonflies inflict characteristic head damage to females during sex. A survey of 11 species of damsel and dragonflies has revealed such mating damages in 20 to 80% of the males too, indicating a fairly high occurrence of sexual coupling between males.
Male Drosophila melanogaster flies bearing two copies of a mutant allele in the fruitless gene court and attempt to mate exclusively with other males. The genetic basis of animal homosexuality has been studied in the fly Drosophila melanogaster. Here, multiple genes have been identified that can cause homosexual courtship and mating. These genes are thought to control behavior through pheromones as well as altering the structure of the animal's brains. These studies have also investigated the influence of environment on the likelihood of flies displaying homosexual behavior.
Male bed bugs (Cimex lectularius) are sexually attracted to any newly fed individual and this results in homosexual mounting. This occurs in heterosexual mounting by the traumatic insemination in which the male pierces the female abdomen with his needle-like penis. In homosexual mating this risks abdominal injuries as males lack the female counteradaptive spermalege structure. Males produce alarm pheromones to reduce such homosexual mating.
|Wikimedia Commons has media related to Homosexual behavior in animals.|
- Against Nature?
- Biology and sexual orientation
- Cross species sex
- Hermaphroditism in animals
- Homosexuality and psychology
- Innate bisexuality
- List of animals displaying homosexual behavior
- Non-human primate experiments
- Norms of reaction
- Plant sexuality
- Sexual orientation
- Sexual orientation and medicine (disambiguation)
- Braithwaite, L. W., "Ecological studies of the Black Swan III – Behaviour and social organization", Australian Wildlife Research 8, 1981: 134–146
- Braithwaite, L. W. (1970). "The Black Swan". Australian Natural History. 16: 375–379.
- "Same-sex Behavior Seen In Nearly All Animals, Review Finds". sciencedaily.com. Retrieved 25 March 2018.
- "Same-sex behavior seen in nearly all animals". Physorg.com. 2009-06-16. Retrieved 2010-11-17.
- Gordon, Dr Dennis (10 April 2007). "'Catalogue of Life' reaches one million species". National Institute of Water and Atmospheric Research. Archived from the original on 13 July 2007. Retrieved 2007-09-10.
- "Book Reviews, Bestselling Books & Publishing Business News - Publishers Weekly". publishersweekly.com. Retrieved 25 March 2018.
- Bagemihl, 1999, page 2
- Levay, Simon (1996). Queer Science: The Use and Abuse of Research into Homosexuality. Cambridge, Massachusetts: MIT Press. p. 207.
- Aldo Poiani; A. F. Dixson (2010). Animal Homosexuality: A Biosocial Perspective. Cambridge University Press. p. 179. ISBN 9781139490382.
This makes O. aries (ram) only the second mammal known, apart from humans, capable of displaying exclusive homosexuality.
- Levay, Simon (2011). Gay, Straight, and The Reason Why The Science of Sexual Orientation. Cambridge, Massachusetts: Oxford University Press. pp. 70–71.
- Bagemihl, Bruce (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity (Stone Wall Inn ed.). New York City: St. Martin's Press. ISBN 9780312253776.
Homosexual behavior occurs in more than 450 different kinds of animals worldwide, and is found in every major geographic region and every major animal group.
- The first known use of the word Homoseksuäl is found in Benkert Kertbeny, K. M. (1869): Paragraph 143 des Preussichen Strafgesetzebuches vom 14/4-1851 und seine Aufrechterhaltung als Paragraph 152 im Entwurf eines Strafgesetzbuches fur den Norddeutschen Bundes, Leipzig, 1869. Reprinted in Jahrbuch fur sexuelle Zwischenstufen 7 (1905), pp. 1–66
- Dorit, Robert (September–October 2004). "Rethinking Sex". American Scientist. Retrieved 2007-09-11.
- Bagemihl 1999, pp. 122–166.
- Bailey, Nathan W.; Zuk, Marlene (2009). "Same-sex sexual behavior and evolution". Trends in Ecology & Evolution. 24 (8): 439–446. doi:10.1016/j.tree.2009.03.014. PMID 19539396.
- Joan Roughgarden, Evolutions rainbow: Diversity, gender and sexuality in nature and people, University of California Press, Berkeley, 2004; pp. 13–183
- Vasey, Paul L (1995). "Homosexual behaviour in primates: A review of evidence and theory". International Journal of Primatology. 16 (2): 173–204. doi:10.1007/bf02735477.
- Sommer, Volker & Paul L. Vasey (2006), Homosexual Behaviour in Animals, An Evolutionary Perspective. Cambridge University Press, Cambridge. ISBN 0-521-86446-1
- Douglas, Kate (December 7, 2009). "Homosexual selection: The power of same-sex liaisons". New Scientist. Retrieved 2009-12-21.
- Gailey, D. A.; Hall, J.C. (April 1989). "Behavior and Cytogenetics of fruitless in Drosophila melanogaster: Different Courtship Defects Caused by Separate, Closely Linked Lesions". Genetics. 121 (4): 773–785. PMC 1203660. PMID 2542123. Retrieved 2008-12-14.
- Smith, Dinitia (February 7, 2004). "Love That Dare Not Squeak Its Name". The New York Times. Retrieved 2007-09-10.
- "Brief for Amici Curiae in Support of Petitioners, Lawrence v. Texas"
- Bagemihl, Bruce (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity. New York: St. Martin's Press.
- Bagemihl, Bruce (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity. New York, New York: St. Martin's Press. p. 164.
- Somer, Volker; Vasey, Paul L., eds. (2006). Homosexual Behavior In Animals: An Evolutionary Perspective. Cambridge: Cambridge Press. ASIN 0521182301.
- Riccucci M (2011). "Same-sex sexual behaviour in bats". Hystrix It. J. Mammal. 22 (1): 139–47. doi:10.4404/hystrix-22.1-4478.
- Joan Roughgarden, Evolution's rainbow: Diversity, gender and sexuality in nature and people, University of California Press, Berkeley, 2004
- "1,500 Animal Species Practice Homosexuality". News-medical.net. 2006-10-23. Archived from the original on May 28, 2007. Retrieved 2007-09-10.
- "Homosexuality in the Animal kingdom". nhm.uio.no. Natural History Museum, University of Oslo. February 2009. Retrieved October 6, 2017.
- Moskowitz, Clara (19 May 2008). "Homosexuality Common in the Wild, Scientists Say". Fox News. Retrieved 2008-07-02.
- Bagemihl, citing a study by Leuthold, W. (1977): African Ungulates: A Comparative Review of Their Ethology and Behavioural Ecology: Springer Verlag, Berlin, cited in Biological Exuberance: Animal Homosexuality and Natural Diversity, 1999;
- Thierry Lodé La guerre des sexes chez les animaux Eds O Jacb, Paris, 2006, ISBN 2-7381-1901-8
- "Cold Shoulder for Swedish Seductresses | Germany". Deutsche Welle. 2005-02-10. Retrieved 2010-11-17.
- "Gay penguin couple adopts abandoned egg in German zoo". CBC News. 2009-06-05.
- Roselli, Charles E.; Kay Larkin; John A. Resko; John N. Stellflug; Fred Stormshak (2004). "The Volume of a Sexually Dimorphic Nucleus in the Ovine Medial Preoptic Area/Anterior Hypothalamus Varies with Sexual Partner Preference". Endocrinology. 145 (2): 478–483. doi:10.1210/en.2003-1098. PMID 14525915. Archived from the original on 2012-12-09. Retrieved 2007-09-10.
- Bagemihl, Bruce (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity. St. Martin's Press. ISBN 978-0312253776.
- Adler, Tina (4 Jan 1997). "Animal's Fancies". Society for Science and the Public. 151 (1): 8–9. JSTOR 3980720.
- Somer, Volker; Vasey, Paul L., eds. (2006). Homosexual Behavior In Animals: An Evolutionary Perspective. Cambridge: Cambridge Press. pp. 5–6. ASIN 0521182301.
- Bagemihl, Bruce (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity. New York, New York: St. Martin's Press. p. 164.
- Moore, Matthew (2010-07-08). "Female mice 'can be turned lesbian by deleting gene'". The Telegraph. London. Retrieved 2010-11-17.
- "Male-like sexual behavior of female mouse lacking fucose mutarotase". BioMed Central. 2010-07-07. Retrieved 2010-11-17.
- Terry, J. (2000). "'Unnatural Acts' in Nature: The Scientific Fascination with Queer Animals". GLQ: A Journal of Lesbian and Gay Studies. 6 (2): 151–193. doi:10.1215/10642684-6-2-151.
- "Sexual preference chemical found in mice". BBC News. 2011-03-23. Retrieved 2011-03-24.
- Liu, Yan; Jiang, Yun'Ai; Si, Yunxia; Kim, Ji-Young; Chen, Zhou-Feng; Rao, Yi (2011-03-23). "Molecular regulation of sexual preference revealed by genetic studies of 5-HT in the brains of male mice". Nature. 472 (7341): 95–99. doi:10.1038/nature09822. PMC 4094133. PMID 21441904.
- Zuk, Marlene (22 November 2011). "Same-sex insects: what do bees-or at least flies-have to tell us about homosexuality?". Natural History. 119 (10): 22.
- Bagemihl 1999, pp. 487–491.
- "Oslo gay animal show draws crowds". BBC News. 19 October 2006. Retrieved 2009-06-15.
- Young, Lindsay C.; Zaun, Brenda J.; VanderWerf, Eric A. (2008-08-23). "Successful same-sex pairing in Laysan albatross". Biology Letters. 4 (4): 323–325. doi:10.1098/rsbl.2008.0191. ISSN 1744-9561. PMC 2610150. PMID 18505710.
- Milton, Joseph (1 December 2010). "Mercury causes homosexuality in male ibises". Nature. doi:10.1038/news.2010.641.
- Frederick, Peter; Jayasena, Nilmini (1 December 2010). "Altered pairing behaviour and reproductive success in white ibises exposed to environmentally relevant concentrations of methylmercury". Proceedings of the Royal Society B. 278 (1713): 1851–1857. doi:10.1098/rspb.2010.2189. PMC 3097836. PMID 21123262.
- Moeliker, C. W. (9 November 2001). "The First Case of Homosexual Necrophilia in the Mallard Anas platyrhynchos (Aves: Anatidae)" (PDF). Deinsea. 8 (2001): 243–247. Retrieved 6 October 2014.
- "Unearthed study on 'sexual depravity' in penguins". abc.net.au. 10 June 2012. Retrieved 25 March 2018.
- "They're in love. They're gay. They're penguins... And they're not alone". Columbia University. Columbia News Service. June 10, 2002. Archived from the original on January 23, 2009.
- "Central Park Zoo's gay penguins ignite debate", 2002-02-07, San Francisco Chronicle
- "Denmark's Gay Penguins Become Fathers". Huffington Post. November 11, 2012.
- "Gay Penguins Become Dads". BuzzFeed. November 9, 2012.
- "Ananova Article on "Tempting Gay Penguins Straight"". Ananova.com. Retrieved 2010-11-17.
- "Followup Ananova Article on German authorities stopping trying to change the penguins' sexual orientation, after GLBTQI organizations protest". Ananova.com. Retrieved 2010-11-17.
- "Gay outrage over penguin sex test". BBC News. 2005-02-14. Retrieved 2010-05-04.
- May, Meredith (2009-07-14). "Widow a wedge between zoo's male penguin pair". San Francisco Chronicle. Retrieved 2010-11-17.
- McCormack, Simon (14 November 2011), "Gay Penguins Reunion: Buddy And Pedro Will Be Back Together By Spring", Gay Voices, The Huffington Post, retrieved 16 November 2011
- "Same-sex penguin pair pursue female partners". CBC News. 2011-12-12.
- Schuster, Ruth (December 19, 2013). "Lesbian Penguins Shack Up at Israeli Zoo". The Jewish Daily Forward.
- Wingham Wildlife Park
- "Auckland's gay penguin couple prepare for special Christmas bundle of joy". Newshub. Retrieved 2018-12-24.
- Eric Silver (2 August 1999). "Gay vulture couple raise surrogate chicks". The Independent. London. Retrieved 2009-09-21.
- Jonathan Lis (21 September 2009). "'Gay' vulture couple split up at Jerusalem zoo, then become fathers". Haaretz. Retrieved 2009-09-21.
- "Gay vultures split up to go straight | Showbiz: Latest News | STV Entertainment". Entertainment.stv.tv. Retrieved 2010-11-17.
- Bagemihl 1999, pp. 339–348.
- Sylvestre, J.-P. (Some Observations on Behavior of Two Orinoco Dolphins (Inia geoffrensis humboldtiaba [Pilleri and Gihr 1977]), in Captivity, at Duisburg Zoo. Aquatic mammals no 11, pp. 58–65 article
- Bagemihl, Bruce (May 2000). "Left-Handed Bears & Androgynous Cassowaries: Homosexual/transgendered animals and indigenous knowledge". Whole Earth Magazine (archived by archive.org). Archived from the original on August 15, 2000. Retrieved 13 December 2013.
- Sugita N (Nov 2016). "Homosexual Fellatio: Erect Penis Licking between Male Bonin Flying Foxes Pteropus pselaphon". PLOS ONE. 11 (11): e0166024. doi:10.1371/journal.pone.0166024. PMC 5100941. PMID 27824953.
- Havens A (2006). Dewey, Tanya; Shefferly, Nancy, eds. "Myotis lucifugus". Animal Diversity Web.
- Norris, K. S.; Dohl, T. P. (1980). "Behaviour of the Hawaiian spinner dolphin, Stenella longirostris". Fishery Bulletin. 77: 821–849.
- Wells, R. S. (1995). "Community structure of Bottlenose Dolphins near Sarasota, Florida". Paper presented at the 24th International Ethological Conference, Honoluly, Hawaii.
- "Same Sex Couples Common in the Wild". livescience.com. Retrieved 25 March 2018.
- Herzing, D. L.; Johnson, C. M. (1997). "Interspecific Interaction between Spotted Dolphins (Stenella frontalis) and Bottlenose Dolphins (Tursiops truncatus) in the Bahamas, 1985–1995". Aquatic Mammals. 23: 85–99.
- Bagemihl 1999, pp. 427–430.
- Coe M. J. (1967). ""Necking" behavior in the giraffe". Journal of Zoology. 151 (3): 313–321. doi:10.1111/j.1469-7998.1967.tb02117.x.
- Bagemihl 1999, pp. 391–393.
- Bagemihl 1999, pp. 525-7.
- Barash DP (1973). "The Social Biology of the Olympic Marmot". Animal Behaviour Monographs. 6 (3): 171–245. doi:10.1016/0003-3472(73)90002-X.
- Barash DP (Feb 1974). "The social behaviour of the hoary marmot (Marmota caligata)". Anim. Behav. 22 (1): 256–61. doi:10.1016/S0003-3472(74)80077-1.
- Bruce Bagemihl, Biological Exuberance: Animal Homosexuality and Natural Diversity, St. Martin's Press, 1999; pp. 302–305. In his discussion of lion same-sex relations, Bagemihl is making use of published work by: J.B. Cooper, "An Exploratory Study on African Lions" in Comparative Psychology Monographs 17:1–48; R.L. Eaton, "The Biology and Social Behavior of Reproduction in the Lion" in Eaton, ed. The World's Cats, vol. II; pp.3–58; Seattle, 1974; G.B. Schaller, The Serengeti Lion; University of Chicago Press, 1972
- Srivastav, Suvira (15–31 December 2001). "Lion, Without Lioness". TerraGreen: News to Save the Earth. Terragreen. Archived from the original on 14 September 2007. Retrieved 2007-09-02.
- Thierry Lodé La guerre des sexes chez les animaux Eds O Jacob, Paris, 2006.ISBN 2-7381-1901-8
- Clay, Zanna; Klaus, Zuberbühler (1 March 2012). "Communication during sex among female bonobos: effects of dominance, solicitation and audience". Scientific Reports. 2 (291). ISSN 2045-2322. PMC 3291041. Retrieved 13 January 2019.
- Frans de Waal, "Bonobo Sex and Society", Scientific American (March 1995), p. 82ff
- Frans B. M. de Waal (2001). "Bonobos and Fig Leaves". The ape and the sushi master : cultural reflections by a primatologist. Basic Books. ISBN 978-84-493-1325-7.
- Bonobos at the Columbus Zoo Archived August 17, 2007, at the Wayback Machine
- Dawkins, Richard (2004). "Chimpanzees". The Ancestor's Tale. Houghton Mifflin. ISBN 978-1-155-16265-2.
- Frans B. M. de Waal (March 1995). "Bonobo Sex and Society". Scientific American. pp. 82–88. Retrieved 2006-07-17.
- "The behavior of a close relative challenges assumptions about male supremacy in human evolution". Primates.com. Retrieved 2010-11-17.
- "Courtney Laird, "Social Organization"". Bio.davidson.edu. Archived from the original on 2011-05-19. Retrieved 2010-11-17.
- Stanford, C. B. (1998). "The social behavior of chimpanzees and bonobos". Current Anthropology. 39 (4): 399–407. doi:10.1086/204757.
- Kano, Takayoshi (1992). The Last Ape: Pygmy Chimpanzee Behavior and Ecology. Stanford, CA: Stanford University Press.
- Frans B. M. de Waal, "Bonobo Sex and Society" Scientific American, March 1995, pp. 82–88
- Grueter, Cyril C.; Stoinski, Tara S. (May 11, 2016). "Homosexual Behavior in Female Mountain Gorillas: Reflection of Dominance, Affiliation, Reconciliation or Arousal?". PLOS One. 11 (5): e0154185. doi:10.1371/journal.pone.0154185. ISSN 1932-6203. PMC 4864209. PMID 27167861. e0154185.
- Bagemihl 1999, pp. 302–305.
- Fox, E. A. (November 2001). "Homosexual behavior in wild Sumatran orangutans (Pongo pygmaeus abelii)". American Journal of Primatology. 55 (3): 177–181. doi:10.1002/ajp.1051. ISSN 0275-2565. PMID 11746281.
- Lionel Tiger (1992). The Pursuit of Pleasure. Transaction Publishers. pp. 66ff. ISBN 978-0-7658-0696-3.
- Roselli CE; Stormshak F (2009). "The Neurobiology of Sexual Partner Preferences in Rams". Hormones and Behavior. 55 (5): 611–620. doi:10.1016/j.yhbeh.2009.03.013. PMC 2684522. PMID 19446078.
- Roselli CE; Stormshak F (2009). "Prenatal Programming of Sexual Partner Preference: The Ram Model". Journal of Neuroendocrinology. 21 (4): 359–364. doi:10.1111/j.1365-2826.2009.01828.x. PMC 2668810. PMID 19207819.
- A Perkins; J A Fitzgerald; E O Price (1992). "Luteinizing hormone and testosterone response of sexually active and inactive rams". Journal of Animal Science. 70 (7): 2086–93. doi:10.2527/1992.7072086x. PMID 1644682.
- Roselli CE; Larkin K; Schrunk JM; Stormshak F (2004). "Sexual partner preference, hypothalamic morphology and aromatase in rams". Physiology & Behavior. 83 (2): 233–45. doi:10.1016/j.physbeh.2004.08.017. PMID 15488542.
- Scrivener, C. J. (2008): "Behavioural problems", Merck Manual of Veterinary Medicine.
- Bruce Bagemihl (1999). "Hoofed Mammals". Biological Exuberance: Animal Homosexuality and Natural Diversity (illustrated ed.). St. Martin's Press. pp. 456–61. ISBN 978-0312192396.
- "Like mother, like cubs: Hyena alpha moms jumpstart cubs with hormonal jolt". Michigan State University. 2006. Archived from the original on 2007-06-29. Retrieved 2007-09-11.
- Holekamp, Kay E. (2003). "Research: Spotted Hyena - Introduction and Overview". Michigan State University, Department of Zoology. Archived from the original on 2007-06-29. Retrieved 2007-09-11.
- Selim, Jocelyn (2006-04-28). "Top Dogs: Steroid hormones give hyenas a head start". Discover Magazine. Retrieved 2007-09-11.
- Forger, Nancy G.; Laurence G. Frank; S. Marc Breedlove; Stephen E. Glickman (6 December 1998). "Sexual Dimorphism of Perineal Muscles and Motoneurons in Spotted Hyenas". The Journal of Comparative Neurology. 375 (2): 333–43. doi:10.1002/(SICI)1096-9861(19961111)375:2<333::AID-CNE11>3.0.CO;2-W. PMID 8915834.
- "Wilson, Sexing the Hyena - "The males mount each other"". University of Chicago Press. Retrieved 2007-09-11.
- Cole, Charles J.; Townsend, Carol R. (9 May 2005). "Parthenogenetic lizards as vertebrate systems". Journal of Experimental Zoology. 256 (S4): 174–176. doi:10.1002/jez.1402560436. Retrieved 2008-12-14.
- Anon (9 June 2005). "Geckos: It's Not Always About Sex". Science Daily. Lewis & Clark College. Retrieved 2008-12-14.
- Budzinski, R.-M. (1997). "Homosexuelles Verhalten bei Geckos der Gattung Phelsuma". Sauria. 19 (3): 33–34.
- Binyon, Michael (October 19, 2017). "St Helena tortoise has a gay old time". The Times. Retrieved October 20, 2017.
- Scharf, I.; & Martin, O. Y. (2013). "Same-sex sexual behavior in insects and arachnids: prevalence, causes, and consequences". Behavioral Ecology and Sociobiology. 67 (11): 1719–1730. doi:10.1007/s00265-013-1610-x.
- Dunkle S. W. (1991). "Head damage from mating attempts in dragonflies (Odonata:Anisoptera)". Entomological News. 102 (1): 37–41.
- Utzeri C.; Belfiore C. (1990). "Anomalous tandems in Odonata". Fragmenta Entomologica. 22 (2): 271–288.
- Yamamoto D, Nakano Y (1999). "Sexual behavior mutants revisited: molecular and cellular basis of Drosophila mating". Cell. Mol. Life Sci. 56 (7–8): 634–46. doi:10.1007/s000180050458. PMID 11212311.
- Yamamoto D, Ito H, Fujitani K (1996). "Genetic dissection of sexual orientation: behavioral, cellular, and molecular approaches in Drosophila melanogaster". Neurosci. Res. 26 (2): 95–107. doi:10.1016/S0168-0102(96)01087-5. PMID 8953572.
- Ferveur JF, Savarit F, O'Kane CJ, Sureau G, Greenspan RJ, Jallon JM (1997). "Genetic feminization of pheromones and its behavioral consequences in Drosophila males". Science. 276 (5318): 1555–8. doi:10.1126/science.276.5318.1555. PMID 9171057.
- Yamamoto D, Fujitani K, Usui K, Ito H, Nakano Y (1998). "From behavior to development: genes for sexual behavior define the neuronal sexual switch in Drosophila". Mech. Dev. 73 (2): 135–46. doi:10.1016/S0925-4773(98)00042-2. PMID 9622612.
- Zhang SD, Odenwald WF (1995). "Misexpression of the white (w) gene triggers male–male courtship in Drosophila". Proc. Natl. Acad. Sci. U.S.A. 92 (12): 5525–9. doi:10.1073/pnas.92.12.5525. PMC 41728. PMID 7777542.
- Svetec N, Ferveur JF (2005). "Social experience and pheromonal perception can change male–male interactions in Drosophila melanogaster". J. Exp. Biol. 208 (Pt 5): 891–8. doi:10.1242/jeb.01454. PMID 15755887.