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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 (and primate sexuality especially) is a rapidly developing field. It used to be believed that only humans and a handful of species performed sexual acts other than for procreation, and that animals sexuality was instinctive and a simple response to the "right" stimulation (sight, scent). Current understanding is that many species 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 observed among 1,500 species, and in 500 of those it is well documented
Types of activity
It appears that many animals, both male and female, masturbate, both when partners are available and otherwise.
For example, http://www.petplace.com comments in its guide on assessing potential breeding stock purchases: "Masturbation is a normal behavior in all stallions that does not reduce semen production or performance in the breeding shed, and thus the use of devices to prevent such behavior is strongly discouraged and can be harmful to the stallion."  Likewise the paper "Sexual Behavior - Current Topics in Applied Ethology and Clinical Methods" by Sue McDonnell of the Equine Behavior Laboratory, University of Pennsylvania School of Veterinary Medicine states:
- "One example is the behavior known within the horse breeding industry as masturbation. This involves normal periodic erections and penile movements. This behavior, both from the descriptive field studies cited above and in extensive study of domestic horses, is now understood as normal, frequent behavior of male equids.
Attempting to inhibit or punish masturbation, which is still a common practice of horse managers regionally around the world, often leads to increased masturbation and disturbances of normal breeding behavior (McDonnell and Hinze, in preparation)." 
Sexual release seeking is common in both domestic and non-domestic species. For example, a video (non-explicit) showing a kangaroo masturbating, inadvertently caught during a TV broadcast, can be found here. The female porcupine will use a stick as a vibrator, holding one end of a stick between her paws and walk around, straddling the stick as it bumps against the ground and vibrates against her genitalia. 
Sexologist Havelock Ellis in his 1927 "Studies in the Psychology of Sex" identified bulls, goats, sheep, camels and elephants as species known to practice masturbation, adding of some other species:
- I am informed by a gentleman who is a recognized authority on goats, that they sometimes take the penis into the mouth and produce actual orgasm, thus practicing auto-fellatio. As regards ferrets ... "if the bitch, when in heat, cannot obtain a dog [ie, male ferret] she pines and becomes ill. If a smooth pebble is introduced into the hutch, she will masturbate upon it, thus preserving her normal health for one season. But if this artificial substitute is given to her a second season, she will not, as formerly, be content with it." [...] Blumenbach observed a bear act somewhat similarly on seeing other bears coupling, and hyenas, according to Ploss and Bartels, have been seen practicing mutual masturbation by licking each other's genitals.
In his 1999 book, Biological exuberance, Bruce Bagemihl PhD documents (p.71, 209-210) that:
- Masturbation also occurs widely among animals, both male and female. A variety of creative techniques are used, including genital stimulation using the hand or front paw (primates, Lions), foot (Vampire Bats, primates), flipper (Walruses), or tail (Savanna Baboons), sometimes accompanied by stimulation of the nipples (Rhesus Macaques, Bonobos); auto-fellating or licking, sucking and/or nuzzling by a male of his own penis (Common Chimpanzees, Savanna Bonobos, Vervet Monkeys, Squirrel Monkeys, Thinhorn Sheep, Bharal, Aovdad, Dwarf Cavies); stimulation of the penis by flipping or rubbing it against the belly or in its own sheath (White-tailed and Mule Deer, Zebras and Takhi); spontaneous ejaculations (Mountain Sheep, Warthogs, Spotted Hyenas); and stimulation of the genitals using inanimate objects (found in several primates and cetaceans). 
- Many birds masturbate by mounting and copulating with tufts of grass, leaves or mounds of earth, and some mammals such as primates and Dolphins also rub their genitals against the ground or other surfaces to stimulate themselves. 
- Masturbation in female mammals, as well as heterosexual and homosexual intercourse (especially in primates), often involves direct or indirect stimulation of the clitoris [...]. This organ is present in the females of all mammalian species and several other animal groups. 
- Apes and Monkeys use a variety of objects to masturbate with and even deliberately create implements for sexual stimulation [...] often in highly creative ways. 
Animals of several species are documented as engaging in both auto-fellatio and oral sex. Although easily confused by lay-people, this is a separate and sexually oriented behavior, distinct from non-sexual grooming or scent-investigation.
- Main article: List of animals displaying homosexual behavior
The presence of same-sex sexual behavior was not scientifically observed on a large scale until recent times, possibly due to observer bias caused by social attitudes to same-sex sexual behavior. Homosexual behaviour does occur in the animal kingdom, especially in social species, particularly in marine birds and mammals, monkeys and the great apes. Homosexual behaviour has been observed among 1,500 species, and in 500 of those it is well documented (there are approximately 1,250,000 species of animal in the world). Georgetown University professor Janet Mann has specifically theorised that homosexual behaviour, at least in dolphins, is an evolutionary advantage that minimises intraspecies aggression, especially among males.
- Male penguin couples have been documented to mate for life, build nests together, and to use a stone as a surrogate egg in nesting and brooding. In 2004, the Central Park Zoo in the United States replaced one male couple's stone with a fertile egg, which the couple then raised as their own offspring. German and Japanese zoos have also reported homosexual behaviour among their penguins. This phenomenon has also been reported at Kelly Tarlton's Aquarium in Auckland, New Zealand.
- 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 behaviour, to "ensure the return of the buffalo in the coming season." [How to reference and link to summary or text] Also, mounting of one female by another is common among cattle. (See also, Freemartin. Freemartins occur because of clearly causal hormonal factors at work during gestation.)
- Homosexual behaviour in male sheep (found in 6-10% of rams) is associated with variations in cerebral mass distribution and chemical activity. A study reported in Endocrinology concluded that biological and physiological factors are in effect. These findings are similar to human findings studied by Simon LeVay.
- "Approximately eight percent of [male] 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..."
- Male bighorn sheep are divisible into two kinds, the typical males among whom homosexual behavior is common and "effiminate sheep" or "behavioral transvestites" which are not known to engage in homosexual behavior.  
Same-sex sexual behavior should only be identified as a sexual orientation with caution. In humans the behavior is considered distinct from the orientation - many heterosexuals engage in same-sex behavior at times, and many homosexuals have heterosexual lifestyles. In animals this distinction is still being explored.
Cross species sex
Although many people believe animal sexuality is instinctive and therefore (it is implied) almost mechanistic, in fact research regularly records that many animals are sexual opportunists, and may show an interest in partners other than their own or related species. [How to reference and link to summary or text] 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. Such cross-species sex has been observed more with animals in captivity than in the wild, probably due to ease of observation, although attempts by wild moose to obtain sex from domestic horses are apparently well known by wildlife specialists.
In the wild, where observation is harder, genetic studies have shown a "large number" of inter-species hybrids, and other investigations describe productive and non-productive inter-species mating as a "natural occurrence". Recent genetic evidence strongly suggesting this has occurred even within the history of the human species, and that early humans often had sexual activity with other primate species, is considered below.
Hybrid offspring can result if the two species are related. However, this offspring may not necessarily be fertile itself. The mule, for example (a 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."
Due to the difficulties of observation, interspecies sex of this kind between two top-level predators, occurring in the wild, was only conclusively documented with the finding of a grizzly-polar bear hybrid in April 2006. Again, as with lions and tigers, the two species would normally not share enough common territory to provide adequate opportunity for much cross-species sexual activity. In other words, whether both species were 0% or 100% promiscuous and sought sex with the same species only or the nearest bear of any species, the overwhelming number of matings would still of necessity be with the same species.
Animal sexual advances on, and attempted interactions with, humans and other species, have been documented by ethologists such as Kohler, Gerald Durrell and Desmond Morris, as well as authoritative researchers such as Birute Galdikas who studied orangutans in Borneo. Peter Singer, the philosopher and animal rights activist reports: "While walking through the camp with Galdikas, my informant was suddenly seized by a large male orangutan, his intentions made obvious by his erect penis. Fighting off so powerful an animal was not an option, but Galdikas called to her companion not to be concerned, because the orangutan would not harm her, and adding, as further reassurance, that 'they have a very small penis.'" (though "the orangutan lost interest before penetration took place") 
Although not often reported, it appears animals, or primates at the least, are able to sexualize inanimate objects in a manner similar to human sexual fetishes.
Not only will an animal that has a habitual object for masturbation sometimes appear to sexualize that object, but in some cases primates have generalized that to sexualize kinds of objects in a class where no prior sexual connection exists -- similar to human fetishes.
Thus Gabriel, a chimpanzee at the Southwest National Primate Research Center, is said to have a shoe fetish (or possibly a leather fetish) according to caretaker Bert Barrera, and it is reported (probably referring to the same individual) that:
- "A male chimpanzee raised in captivity developed a bit of a shoe fetish, masturbating obsessively by rubbing his caretaker's leather boot." mysanantonio.com drsusanblock.com.
The sexualization of objects or locations is also well recognized in the breeding world. So for example, stallions may often 'drop' (become sexually aroused) upon visiting a location where they have been allowed to have sex before, or upon seeing a stimulus previously associated with sexual activity such as an artificial vagina. [How to reference and link to summary or text]
In this case however, the primary structure is Pavlovian conditioning, and the fetishistic association is due to a conditioned response (or association) formed with a distinctive 'reward'. Human fetishism can very also be traced back to similar or near-identical conditioning: likewise based upon the Pavlovian association between an erotic sensation or anticipation, and objects which become immediately associated with that activity. (See also: operant conditioning)
Washoe, a chimpanzee who has been taught American Sign Language, has been reported to frequently make the sign for "tickle me" to researchers. Although not a sexual act per se, tickling is none the less recognized as a fetish in some contexts. (See: Tickling fetish)
Sexual imagery viewing
A study by Platt, Khera and Deaner at Duke University North Carolina (reported in Current Biology and online here), showed that monkeys will give up privileges (in this case, juice which was highly valued), to be allowed to see a female monkey's hindquarters.
Deaner and his team reported that monkeys would take a juice cut to look at powerful males' faces or the perineum of a female, but to persuade the monkeys to stare at subordinate males, the researchers had to bribe them with larger drinks. "Virtually all [male] monkeys will give up juice to see female hindquarters ... they really value the images."
The researchers stress that in monkey society, such sights have great social utility, and therefore it should not be considered simply "monkeys enjoy pornographic pictures". There is no evidence at this point that viewable pictures or movies of sexual activity are valued for their sexual enjoyment, although as noted above (#Masturbation) there are reports that watching sex in real life, may have such an effect. The subject of animals and sexual imagery is not yet well researched.
Rape and apparently coercive sex
Controversial interpretations and implications aside (see Sociobiological theories of rape), sex in a forceful or apparently coercive context has also been documented in a variety of species. A notable example is bottlenose dolphins, where at times, gangs of bachelor males 'corner' females. The behavior is also common in some arachnids (spiders), notably those whose females eat the males during sex if not tricked with food and tied down with threads, and in some herbivorous herd species or species where males and females are very different in size, where the male dominates sexually by sheer force and size. Other animals which appear to combine sexual intercourse with apparent violent assault, also include some species of bird such as ducks and geese.
Female brown trout fake orgasm in around 60% (69 of 117) of sexual encounters, according to a researcher at the Swedish National Board of Fisheries. A similar phenomenon has been known in Atlantic salmon since 1954. It is speculated that the female fakes completion of orgasm if she is unsure as to the male being correctly positioned for fertilization of eggs, and that the male is fooled by this into completing his shedding of sperm. It is believed that the female, who has refrained from shedding her eggs, saves them for the next time, shortly after.
A second source reports a slightly different theory, that she does this to encourage a less than satisfactory male to finish the sex act quicker and believe it has been successful, whilst the female withholds her eggs to seek a better partner afterwards. 
Sex between adults and juveniles
It has also been recorded that certain species of mole will impregnate newborns of their own species. [How to reference and link to summary or text] It is not clear if this is forceful or not. Similarly, the male stoat (Mustela erminea) will mate with infant females of their species. [How to reference and link to summary or text] This apparently is a natural part of their reproductive biology - there is a delayed gestation period, so these females give birth the following year when they are fully grown.
Necrophilia in animals is where a living animal engages in a sexual act with a dead animal. In one case, it is stated  that following a bird impact with a window, Kees Moeliker of Rotterdam, Netherlands, observed sexual activities outside between a live duck and a dead one. He states that the living drake picked at the corpse of the dead one (also male) for a few minutes and then, without provocation, it mounted the corpse and began copulating with it. The act of necrophilia lasted for about 75 minutes, in which time, according to Moeliker, the living drake took two short breaks before resuming with copulating behavior. There is currently no scientific research able to verify certain other assumptions as to motive, which were made by Mr. Moeliker.
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.
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                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.     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)
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.   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.   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.
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)
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.
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. 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. 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.
Two examples of promiscuous mating systems in primates are chimpanzees and bonobos. These species live in social groups consisting of several males and several females. Each male copulates with many females, and vice versa. In bonobos, the amount of promiscuity is particularly striking because bonobos use sex to alleviate social conflict as well as to reproduce.
Seasonal nature of animal sexuality
Many animal species have specific mating (or breeding) seasons. These are often associated with changes to herd or group structure, and behavioral changes, including territorialism amongst individuals. These may be annual (eg wolves), biannual (eg dogs) or more frequently (eg horses). During these periods, females of most species are more mentally and physically receptive to sexual advances, a period often described as being "in season" or "in heat", but outside them animals still engage in sexual behaviors, and such acts as do occur are not necessarily harmful.
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 :
- "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..."
A 2006 Danish Animal Ethics Council report which examined current knowledge of animal sexuality in the context of legal queries concerning sexual acts by humans, has the following comments, primarily related to domestically common animals:
- "Even though the evolution-related purpose of mating can be said to be reproduction, it is not actually the creating of offspring which originally causes them to mate. It is probable that they mate because they are motivated for the actual copulation, and because this is connected with a positive experience. It is therefore reasonable to assume that there is some form of pleasure or satisfaction connected with the act. This assumption is confirmed by the behavior of males, who in the case of many species are prepared to work to get access to female animals, especially if the female animal is in oestrus, and males who for breeding purposes are used to having sperm collected become very eager, when the equipment they associate with the collection is taken out."
- "There is nothing in female mammals’ anatomy or physiology, that contradicts that stimulation of the sexual organs and mating is able to be a positive experience. For instance, the clitoris acts in the same way as with women, and scientific studies have shown that the success of reproduction is improved by stimulation of clitoris on (among other species) cows and mares in connection with insemination, because it improves the transportation of the sperm due to contractions of the inner genitalia. This probably also concerns female animals of other animal species, and contractions in the inner genitals are seen e.g. also during orgasm for women. It is therefore reasonable to assume that sexual intercourse may be linked with a positive experience for female animals."
Notes on specific species
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.
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.
Studies have shown that ten to fifteen percent of female western gulls in some populations in the wild prefer other females.
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.
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.
The female Spotted Hyena has a unique urinary-genital system, closely resembling the penis of the male called a pseudopenis. 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.
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
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).
- McDonnell, S.M., Henry, M., & Bristol, F. (1991). Spontaneous erection and masturbation in equids. Proceedings Vth International Equine Reproduction Symposium. J Reprod Fert Suppl, 44, 664-665.
- Bruce Bagemihl: Biological Exuberance: Animal Homosexuality and Natural Diversity. St. Martin's Press, 1999. ISBN 0-312-19239-8
- Oslo gay animal show draws crowds. BBC News. URL accessed on 2006-10-19.
- "Central Park Zoo's gay penguins ignite debate" by Dinitia Smith, San Francisco Chronicle, February 7, 2004
- "The Volume of a Sexually Dimorphic Nucleus in the Ovine Medial Preoptic Area/Anterior Hypothalamus Varies with Sexual Partner Preference" by Charles E. Roselli, et al., The Endocrine Society, October 2, 2003
- Source: Idaho Fish and Game Department, as cited on WTOP news, November 13 2006. 
- Haeberle (1978) states that sexual intercourse is not so very unusual between animals of different species as it is between humans and animals. Kinsey et al. (1948, p. 668) states "When one examines the observed cases of such crosses, and especially the rather considerable number of instances in which primates, including man, have been involved, one begins to suspect that the rules about intraspecific mating are not so universal as tradition would have it". Kinsey et al. (1953) further point out that genetic studies have shown the existence of a "large number" of inter-specific hybrids, that have occurred in the wild, and investigations (eg, Cauldwell, 1968; Ford & Beach, 1951; Harris, 1969; Masters, 1962; Ullerstam, 1966, etc) have found that interspecies mating is a "natural occurrence".' (Cited by Miletski, in her anthrozoological study of animal-human sexuality, 1999, p.51)
- Deaner M. O., Khera A. V. & Platt M. L. Curr. Biol. published online http://www.current-biology.com/content/article/abstract?uid=PIIS0960982205001041 (2005).
- Source for both: CBC News, May 17 2002. Apparently cited from Moths That Drink Elephant's Tears and Other Zoological Curiosities by Matt Walker.
- includeonly>MacLeod, Donald. "Necrophilia among ducks ruffles research feathers", The Guardian, 2005-03-08. Retrieved on 2006-04-05.
- C.W. Moeliker. The first case of homosexual necrophilia in the mallard (page 243-247)PDF
- 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.
- Barash, D.P. (1981). Mate guarding and gallivanting by male hoary marmots (Marmota caligata). Behavioral Ecology and Sociobiology, 9, 187-193.
- Birkhead, T.R. & Møller, A.P. (1995). Extra-pair copulations and extra-pair paternity in birds. Animal Behaviour, 49, 843-848.
- 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.
- Foltz, D.W. (1981). Genetic evidence for long-term monogamy in a small rodent, Peromyscus polionotus. American Naturalist, 117, 665-675.
- Gursky, S.L. (2000). Sociality in the spectral tarsier, Tarsius spectrum. American Journal of Primatology, 51, 89-101.
- Hasselquist, D. S. & Sherman, P.W. (2001). Social mating systems and extrapair fertilizations in passerine birds. Behavioral Ecology, 12, 457-466.
- 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.
- Mason, W.A. (1966). Social organization of the South American monkey, Callicebus moloch: a preliminary report. Tulane Studies in Zoology, 13, 23-28.
- McKinney, F., Derrickson, S.R., & Mineau, P. (1983). Forced copulation in waterfowl. Behaviour, 86, 250-294.
- Reichard, U. (1995). Extra-pair Copulations in a Monogamous Gibbon (Hylobates lar). Ethology, 100, 99-112.
- Reichard, U.H. (2002). Monogamy—A variable relationship. Max Planck Research, 3, 62-67.
- Richardson, P.R.K. (1987). Aardwolf mating system: overt cuckoldry in an apparently monogamous mammal. South African Journal of Science, 83, 405-412.
- Welsh, D. & Sedinger, J.S. (1990). Extra-Pair copulations in Black Brant. The Condor, 92, 242-244.
- 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.
- 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.
- 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.
- 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.
- Barash, D.P. & Lipton, J.E. (2001). The Myth of Monogamy. New York, NY: W.H. Freeman and Company.
- Research conducted by Patricia Adair Gowaty. Reported by Morell, V. (1998). Evolution of sex: A new look at monogamy. Science, 281, 1982-1983.
- 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.
- This section and examples taken from:Robert Sapolsky Why Zebras Don't get Ulcers, p.140-141.
- For example, masturbation, trial mounting, and other behaviors are regularly seen in male animals out of season
- 2006 Danish Animal Ethics Council report: "The mucous membrane in the female animal’s vagina and the animal’s behavior is under influence of its rut cycle. That means that the animal is physically and mentally more ready for sexual activities at some times than at others. But this does not mean that sexual activity will lead to injuries, fear or suffering, if it happens outside the rut period." (Danish: "Slimhinden i hundyrets vagina og dyrets adfærd er under indflydelse af dets brunstcyklus. Det betyder, at dyret er fysisk og mentalt mere parat til seksuelle aktiviteter på nogle tidspunkter end på andre. Men dette er ikke ensbetydende med, at den seksuelle aktivitet vil være forbundet med skader, angst og lidelse, hvis den foregår udenfor brunstperioden.") Report November 2006 (PDF)
- Danish Animal Ethics Council report Udtalelse om menneskers seksuelle omgang med dyr published November 2006. Council members included two academics, two farmers/smallholders, and two veterinary surgeons, as well as a third veterinary surgeon acting as secretary.
- Danish: "Selv om det evolutionsmæssige formål med at parre sig kan siges at være reproduktion, er det ikke selve det, at dyrene får afkom, der i første omgang får dem til at parre sig. Det er til gengæld sandsynligt, at de parrer sig, fordi de er motiverede for selve parringsakten, og at denne er forbundet med en positiv oplevelse. Det er derfor rimeligt at antage, at der er en eller anden form for behag eller tilfredsstillelse forbundet med akten. Denne antagelse bekræftes af adfærden hos handyr, der for mange arters vedkommende er parate til at arbejde for at få adgang til hundyr, især hvis hundyret er i brunst, og handyr der i avlsøjemed er vant til at få tappet sæd – de viser stor ivrighed, når det udstyr, de forbinder med sædopsamlingen, tages frem." Report November 2006 (PDF)
- Danish: "Der er intet ved hunpattedyrenes anatomi eller fysiologi, der modsiger, at stimulation af kønsorganerne og parring skulle kunne være en positiv oplevelse – fx fungerer klitoris på samme måde som hos kvinder. Videnskabelige undersøgelser har vist, at reproduktionssuccesen forbedres ved stimulation af klitoris på bl.a. køer og hopper i forbindelse med insemination, fordi det forbedrer sædtransporten pga. sammentrækninger af de indre kønsdele. Dette gælder sandsynligvis også hundyr af andre dyrearter, og sammentrækninger i de indre kønsdele ses fx også under orgasme hos kvinder. Det er derfor rimeligt at antage, at det seksuelle samvær kan være forbundet med en positiv oplevelse for hundyrene." Report November 2006 (PDF)
- Sapolsky, Why Zebras don't get Ulcers, p.127-129.
- List of animals displaying homosexual behavior
- Biology and sexual orientation
- Choice and sexual orientation
- Animal behaviour
- Pair bonding
- Freemartin (a female bovine with a masculinized brain which acts sexually as a male)
Key texts – Books
Additional material – Books
Reviews of the area
Key texts – Papers
Additional material – Papers
- Gay flamingos celebrate fifth anniversary with their children
- Gay penguins resist split attempts
- biologists' view
- National Geographic
- San Francisco Zoo has run a "sex tour" covering animal sexuality, on Valentine's Day
- Dog masturbation recorded video "Horny Dog"
- A wild, and gay, kingdom World Science
- fr:Sexualité animale
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