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Animal sexual behaviour takes many different forms, even within the same species. Among animals, researchers have observed monogamy; promiscuity; sex between species; sexual arousal from objects or places; sex apparently via duress or coercion; copulation with dead animals; homosexual, heterosexual, and bisexual sexual behaviour; situational sexual behaviour; and a range of other practices. Related studies have noted diversity in sexed bodies and gendered behaviour, intersex and transgender animals. Animal sexual behaviour relates primarily to reproduction and the perpetuation of species. The study of animal sexuality (especially that of primates) is a rapidly developing field. It used to be believed that only humans and a handful of other species performed sexual acts other than for procreation, and that animals' sexuality was instinctive and a simple response to the "right" stimulation (sight, scent). Current understanding is that many species that were formerly believed monogamous have now been proven to be promiscuous or opportunistic in nature; a wide range of species appear both to masturbate and to use objects as tools to help them do so; in many species animals try to give and get sexual stimulation with others where procreation is not the aim; and homosexual behaviour has now been observed among 1,500 species and in 500 of those it is well documented. Animal sexual behaviour involves struggle between many males. Female animals select males only if they are strong and able to protect themselves or not. For example, if a male animal fights against another male animal of the same species then the animal that wins the fight will have the chance to mate with the higher number of females and also he will pass on his genes to his offspring, who pass on their genes to the next offspring in line.

Mating systems

General

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 which males mate with which females, and under what circumstances.

The following are some of the mating systems generally recognised in humans and other animals:

• Monogamy: Two individuals have an exclusive mating relationship.
• Polygamy: A single individual concurrently carries relationship/mates with one or more of the opposite sex. Three types are recognized:
  • 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 individuals have an exclusive relationship with two or more individuals from the opposite sex; the numbers of males and females need not be equal, and in vertebrate species studied so far, there are usually fewer males.
• Promiscuity: Any male and female will mate within the social group.

Monogamy

Zoologists and biologists now have solid evidence that monogamous pairs of animals are not always sexually exclusive. Many animals that form pairs to mate and raise offspring regularly engage in sexual activities with extra-pair partners. This includes previous examples such as swans. Sometimes these extra-pair sexual activities lead to offspring. Genetic tests frequently show that some of the offspring raised by a monogamous pair come from the female mating with an extra-pair male partner. 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 takes the form of 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, p. 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% of avian species are socially monogamous. This stands in contrast to mammals. Only 3% of mammalian species are socially monogamous, although up to 15% of primate species are. Social monogamy has also been observed in reptiles, fish, and insects.

Sexual monogamy is also rare among animals. Many socially monogamous species engage in extra-pair copulations, making them sexually non-monogamous. For example, while over 90% of birds are socially monogamous, "on average, 30% or more of the baby birds in any nest sired by someone other than the resident male." Patricia Adair Gowaty has estimated that, out of 180 different species of socially monogamous songbirds, only 10% 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%, with all offspring genetically related to the socially monogamous pair. But genetic monogamy is strikingly low in other species. Barash and Lipton note:

The highest known frequency of extra-pair copulations are found among the fairy-wrens, lovely tropical creatures technically known as Malurus splendens and Malurus cyaneus. More than 65% of all fairy-wren chicks are fathered by males outside the supposed breeding group.

— Barash & Lipton, 2001, p. 12

Such low levels of genetic monogamy have surprised biologists and zoologists, forcing them to rethink the role of social monogamy in evolution. They can no longer assume social monogamy determines how genes are distributed in a species. The lower the rates of genetic monogamy among socially monogamous pairs, the less of a role social monogamy plays in determining how genes are distributed among offspring.

Polygamy and polygyny

Polygamy is defined as a mating structure in which a single individual of one sex has exclusive access to several individuals of the opposite sex. It takes two main forms – polygyny and polyandry. As polygyny is the most common form of polygamy among vertebrates (including humans, to some extent), it has been studied far more extensively than polyandry.

In some species, notably those with harem-like structures, only one of a few males in a group of females will mate. Technically, polygyny in sociobiology and zoology is defined as a system in which a male has a relationship with more than one female, but the females are predominantly bonded to a single male. Should the active male be driven out, killed, or otherwise removed from the group, in a number of species the new male will ensure that breeding resources are not wasted on another male's young. The new male may achieve this in many different ways, including:

• Competitive infanticide

in lions, hippopotamuses, and some monkeys, the new male will kill the offspring of the previous alpha male to cause their mothers to become receptive to his sexual advances since they are no longer nursing.

• 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.

Von Haartman specifically described the mating behaviour of the European pied flycatcher as successive polygyny. Within this system, the males leave their home territory once their primary female lays her first egg. Males then create a second territory, presumably in order to attract a secondary female to breed. Even when they succeed at acquiring a second mate, the males typically return to the first female to exclusively provide for her and her offspring.

Promiscuity

Two examples of systems in primates are promiscuous mating chimpanzees and bonobos. These species live in social groups consisting of several males and several females. Each female copulates with many males, 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.

Seasonality

Many animal species have specific mating (or breeding) seasons (seasonal breeding). These are often associated with changes to herd or group structure, and behavioural changes, including territorialism amongst individuals. These may be annual (e.g. wolves), biannual (e.g. dogs) or more frequently (e.g. horses). During these periods, females of most species are more mentally and physically receptive to sexual advances, a period scientifically described as estrous but commonly described as being "in season" or "in heat", but outside them animals still engage in sexual behaviours, and such acts as do occur are not necessarily harmful. Certain other animals (opportunistic breeders) breed dependent upon other conditions in their environment aside from time of year.

Interpretation bias

The field of study of sexuality in non-human species has been a long standing taboo, with researchers either failing to observe or mis-categorizing and mis-describing sexual behaviour which does not meet their preconceptions. (See: Observer bias.) More current research provides views such as that of the Natural History Museum at the University of Oslo, which in 2006 held an exhibition on animal sexuality:

Many researchers have described homosexuality as something altogether different from sex. They must realise that animals can have sex with who they will, when they will and without consideration to a researcher's ethical principles.

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An example of overlooking behaviour relates to descriptions of giraffe mating:

When nine out of ten pairings occur between males, "very male that sniffed a female was reported as sex, while anal intercourse with orgasm between males was only 'revolving around' dominance, competition or greetings.

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Other aspects that are often misinterpreted by humans are the frequency and context in which animals engage in sexual behaviours. For example, domestic or farm animals display behaviours like mounting and head butting during both sex and competition or combats with each other. Careful analysis must be made to interpret what animal activities are implied by those behaviours.

Genetics and sex

A study carried out by Moore et al. suggests that sexual differentiation isn’t dependent only on hormones that are secreted by the gonads. Genetic sex-determining factors also play a critical role in the sexual differentiation process. These genetic factors may later go on to activate hormones secreted by the gonads. More importantly, these genetic factors may be responsible for the differences between an organisms’ sexuality as seen in the animal kingdom. This suggests that these inherited factors may be responsible for the varying degrees of sexuality observed and can be influenced by the environment as well as other physiological factors to develop a unique organism.

Sex for pleasure

It is a common myth that animals do not (as a rule) have sex for pleasure, or alternatively that humans, pigs (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 conclusively say at present what animals do or do not find "pleasurable," a question considered in more depth under Emotion in animals. The urban myth website 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," 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 behaviour) to indicate the presence of receptive periods. During these, sex is sought, and outside these it is usually not sought. 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 behaviour: 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 behaviour 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.

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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.

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Types of activity

Autoeroticism or masturbation

It appears that many animals, both male and female, masturbate, both when partners are available and otherwise. It has also been observed in dogs.

For example, 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" Likewise a review from the University of Pennsylvania School of Veterinary Medicine says:

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, for example by tying a brush to the area of the flank underside where the penis rubs into contact with the underside, which is still a common practice of horse managers regionally around the world, often leads to increased masturbation and disturbances of normal breeding behaviour.

— Sue M. McDonnell, Sexual Behavior – Current Topics in Applied Ethology and Clinical Methods

Castration does not prevent masturbation, as it is observed in geldings. Masturbation is common in both mares and stallions, before and after puberty.

Sexologist Havelock Ellis in his 1927 "Studies in the Psychology of Sex" identified bulls, goats, sheep, camels and elephants as species known to practice autoeroticism, 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 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 documents that:

Autoeroticism 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.

Autoeroticism 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.

and that:

Apes and Monkeys use a variety of objects to masturbate with and even deliberately create implements for sexual stimulation often in highly creative ways.

Petter Bøckman of the Natural History Museum at the University of Oslo commented (in respect of a 2006 exhibition on homosexuality in the animal kingdom) that:

Masturbation is common in the animal kingdom ... We have a Darwinist mentality that all animals only have sex to procreate. But there are plenty of animals who will masturbate when they have nothing better to do. Masturbation has been observed among primates, deer, killer whales and penguins, and we're talking about both males and females. They rub themselves against stones and roots. Orangutans are especially inventive. They make dildos of wood and bark.

Oral sex

Animals of several species are documented as engaging in both autofellatio and oral sex. Although easily confused by laypeople, this is a separate and sexually oriented behaviour, distinct from non-sexual grooming or the investigation of scents.

Auto-fellatio or oral sex in animals is documented in wolves, goats, primates, hyaenas, bats and sheep (see section Masturbation for details).

Contraceptives

Among monkeys, Lionel Tiger and Robin Fox conducted a study on how Depo-Provera contraceptives lead to decreased male attractiveness to females and eventually to male homosexuality. Janet E. Smith summarizes the findings as follows:

study in the early 70s involved a tribe of monkeys. The alpha monkey of this tribe, named Austin, chose three female monkeys to be his exclusive sexual partners. Austin had a grand time with these three female monkeys. Then the researchers injected Austin's three females with the contraceptive Depo-Provera. Austin stopped having sex with them and chose other female monkeys to be his sexual partners. Then they contracepted all of the females in the tribe. The males stopped having sex with the females and started behaving in a turbulent and confused manner.

Homosexual behaviour

The presence of same-sex sexual behaviour was not scientifically observed on a large scale until recent times. Homosexual behaviour does occur in the animal kingdom outside humans, 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.

To turn the approach on its head: 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.

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Georgetown University professor Janet Mann has specifically theorised that homosexual behaviour, at least in dolphins, is an evolutionary advantage that minimizes 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 fertilized 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. "Humans have created the myth that sexuality can be justified only by reproduction, which by definition limits it to hetero sex," says Michael Bronski, author of The Pleasure Principle: Culture, Backlash, and the Struggle for Gay Freedom. "But here is an animal society that uses homosexuality to improve its social life."
• 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.)
• Bonobos in zoos. After studying the primates for his book Bonobo: The Forgotten Ape, primatologist Frans de Waal, a professor of psychology at Emory University in Atlanta, says that such expressions of intimacy are consistent with the homosexual behaviour of what he terms "the erotic champions of the world." "Same-sex, opposite-sex — bonobos just love sex play," de Waal said in an interview. "They have so much sex, it gets boring."
• 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 reported by Simon LeVay.

Approximately eight percent of rams exhibit sexual preferences 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 behaviour, including intercourse, is common and "effeminate sheep", or "behavioural transvestites", which are not known to engage in homosexual behaviour.
• Homosexual behaviour has been observed among bats; recently, in particular, the fruit bat.

Genital-genital rubbing

Genital-genital rubbing, or GG rubbing, among non-human animals is sexual activity in which one animal rubs his or her genitals against the genitals of another animal. The term GG rubbing is frequently used by primatologists to describe this type of sexual intimacy among female bonobos, and is stated to be the "bonobo's most typical sexual pattern, undocumented in any other primate". The term is sometimes used in reference to GG rubbing among male bonobos, under the term "penis fencing," which is the non-human form of frot that human males engage in. Such rubbing between males is thought, according to varying evolutionary theorists, to have existed before the development of hominids into humans and bonobos, and may or may not have occurred in the homosexual activity of both of these genetically related species.

Genital rubbing has been observed among bull manatees, in conjunction with "kissing," and is also common among homosexually active mammals.

Cross species sex

While it is commonly believed that animal sexuality is instinctive and thus somewhat mechanistic, research regularly records that many animals are sexual opportunists, partaking in sexual relations with individuals of visibly distinct species. This is more visible in domesticated species and animals in captivity, 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 more easily observed by humans. Nevertheless, animals have been observed in the wild to attempt sexual activity with other species or indeed inanimate objects.

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 from two organisms of distinct but closely related parent species, although the resulting offspring is not always fertile.

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.

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. Philosopher and animal welfare activist Peter Singer 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.

Prostitution

In some penguin species, the females, even when in a committed relationship, will exchange sexual favours with strange males for the pebbles they need to build their nests. Prostitution was also observed among chimpanzees, who trade food for sex.

Sexual fetishes

Although not often reported, animals, or primates at the least, are able to sexualize inanimate objects similar to the way human beings sexualize the objects of their sexual fetishes. Not only will an animal that has a habitual object for masturbation sometimes appear to sexualize that object, primates have generalized further to sexualize kinds of objects for which no instinctual or prior sexual connection exists. 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 that "he once found an opening in his enclosure that was large enough to grab a caretaker's foot and he held on until she relinquished a boot."

The sexualization of objects or locations is also well recognized in the breeding world. So for example, stallions may often 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.

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 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 mentally associated with that activity.

Sexual imagery viewing

A study by Platt, Khera and Deaner at Duke University (reported in Current Biology and online here), showed that male monkeys will give up privileges (in this case, juice, which is 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 monkeys will give up juice to see female hindquarters ... they really value the images."

The researchers stress that in monkey society, such behaviours have great social utility and we should therefore not simply reach the conclusion that "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.

Problems with encouraging pandas to mate in captivity have been very common. However, showing young male pandas "panda pornography" is widely credited with a recent population boom among pandas in zoos.

Rape

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, a pod of bachelor males will 'corner' a female '...although what happens once the males have herded in a female, and whether she goes for one or all of them, is not yet known: the researchers have yet to witness a dolphin copulation.' The behaviour is also common in some arachnids (spiders), notably those whose females eat the males during sex if not tricked with food and/or 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.

Some species of birds appear to combine sexual intercourse with apparent violent assault; these include ducks, geese, and white-fronted bee-eaters. According to Emlen and Wrege (1986) forced copulations occur in this socially nesting species, and females must avoid the unwelcome attention of males as they emerge from their nest burrows or they are forced to the ground and mated with. Apparently, such attacks are made preferentially on females who are laying and who may thus mother their offspring as a result.

In 2007, research suggested that in the Acilius genus of water beetles (also known as "diving beetles"), an "evolutionary arms race" between the two sexes means that there is no courtship system for these beetles. "It's a system of rape. But the females don't take things quietly. They evolve counter-weapons." Cited mating behaviours include males suffocating females underwater till exhausted, and allowing only occasional access to the surface to breathe for up to six hours (to prevent them breeding with other males), and females which have a variety of body shapings (to prevent males from gaining a grip). Foreplay is "limited to the female desperately trying to dislodge the male by swimming frantically around."

Charles Siebert reports in his New York Times article Elephant Crackup? that:

Since the early 1990s, for example, young male elephants in Pilanesberg National Park and the Hluhluwe-Umfolozi Game Reserve in South Africa have been raping and killing rhinoceroses; this abnormal behaviour, according to a 2001 study in the journal Pachyderm, has been reported in ‘‘a number of reserves’’ in the region.

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This interpretation of the elephants' behaviour is, however, disputed by Rob Slotow, one of the original study's authors. He states there was "nothing sexual about these attacks".

Sex between adults and juveniles

It has also been recorded that certain species of mole will impregnate newborns of their own species. It is not clear if this is forceful or not. Similarly, the male stoat (Mustela erminea) will mate with infant females of their species. This is apparently 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.

A male spotted hyena attempted to mate with a female hyena which succeeded in driving him off. He eventually turned to his ten-month-old cub, repeatedly mounting it and ejaculating on it. The cub sometimes ignored this and sometimes struggled 'slightly as if in play'. The mother did not intervene.

Infants and children in Bonobo societies are often involved in sexual behaviour.

Among insects, there have been recordings of females being forcibly copulated, sometimes before adulthood.

Among primates, interest towards sexually immature varies amongst different species under different circumstances and situations. Amongst chimpanzees, juvenile males (equivalent of human teens) have been recorded mounting and copulating with immature members of the species. Amongst bonobos, immature males have been recorded initiating genital play with female adult or female adolescent bonobos. Copulation-like contact between immature bonobo males and mature female bonobos increases with age and continues until the male bonobo has reached juvenile age. On the other hand, adult gorillas do not show any sexual interest in juvenile or infant members of their species. Primates regularly have sex in full view of infants, juveniles and younger members of their species.

Sexual cannibalism

Sexual cannibalism, which has been documented in arachnids, insects and amphipods, is a phenomenon in which a female organism kills and consumes the male before, during, or after copulation. Although it does confer some known advantages to reproduction, whether or not the male is complicit has not been scientifically determined.

Necrophilia

Necrophilia in animals is where a living animal engages in a sexual act with a dead animal. In one of the most well-known examples, Kees Moeliker of the Rotterdam Natural History Museum, Netherlands observed sexual activities outside his office between a live duck and a dead one. Two male mallards which Moeliker believed were engaged in rape flight, a common motif in duck sexual behaviour, collided with his window. "When one died the other one just went for it and didn't get any negative feedback—well, didn't get any feedback," according to Moeliker, who described the event as "homosexual necrophilia." The case was reported scientifically in Deinsea 8-2001, along with photos, and earned Moeliker an Ig Nobel Prize in biology, awarded for research that cannot or should not be reproduced.

Additionally, male cane toads have been documented (in Cane Toads: An Unnatural History) engaging in copulation with dead toads and inanimate objects.

Neurochemistry

Oxytocin, called the hormone of love, is found in the hypothalamus of the brain and is associated with the ability to maintain healthy interpersonal relationships as well as physiological changes during reproduction. These changes include stimulation of the mammary glands to release milk, and assists in contracting the uterus during the final stages of childbirth. Oxytocin may also be the biological reason why mothers feel a need to cuddle and protect young. Some studies have indicated that women who experience strong positive emotions also have an increase in oxytocin release. Vasopressin, also called antidiuretic hormorne (ADH), is another hormone found in the Hypothalamus. Vasopressin is responsible for regulating blood volume and salt concentration. Oxytocin and vasopressin are also involved in parenting habits as they contribute to feelings of protection and evoke spending time raising young.

Role of neurohormones in two species of vole's mating styles

The mating style of prairie voles is monogamous. After a male and female prairie vole sexually reproduces with one another, they form a lifelong bond. Montane voles, on the other hand, exhibit a polygamous mating style. When montane voles fornicate, they form no attachments—each set-off and go their separate way after copulation. Studies on the brains of these two species of voles have found that it is two neurohormones and their respective receptors that are responsible for these differences in mating strategies. Male prairie voles emit vasopressin after copulating with a female prairie vole. An attachment to the female then ensues. Female prairie voles will release oxytocin after reproducing with a male prairie vole. An attachment to this male prairie vole likewise ensues. In montane voles—both males and females—such a high quantity of oxytocin and vasopressin does not exist in their brains when they mate. Even when injected with oxytocin or vasopressin the mating style of the montane vole does not change; contrast this to the prairie vole who, even without mating, may form a lifelong attachment to another prairie vole of the opposite sex, if oxytocin or vasopressin is injected into him or her. The reason for this is that prairie voles have more oxytocin and vasopressin receptors than do montane voles, and are thus far more receptive to the two neurohormones. It is not the quantity of the hormone that determine social attachment, mating bonds and sexual lust, but rather the number of receptors receptive to that quantity.

Role of oxytocin in rat sexual behaviour

Mating and reproduction

Oxytocin is also referred to as the love hormone because it plays such a large role in all the basic elements of life; copulation, birth, care and bonding. Oxytocin is released during pregnancy and surges after birth of mammalian young. This surge allows the animals to affectively bond with their young, care for them, as well as protect them from harm. Rats experience dual motivations as we will see shortly. Studies have shown that without oxytocin, rats will not experience this maternal behaviour which shows that oxytocin truly plays a role in the motherhood of rats.

Caring for young

Female rats show some interesting characteristics in regards to sexual behaviour. Mother rats may solicit male rats to their nest after the birth of their young. Mother rats show maternal instincts most heavily right after birth very similarly to the way humans do. This is referred to as postpartum estrus in rats. The female mother rats will solicit male rats to the nest but at the same time will become aggressive towards them in protection of her young. This shows that rats can carry on two completely opposite motivations at once and that the male rat is just a neutral stimulus. This is true of typical rats when they experience the normal levels of oxytocin, but if the rat is given injections of an oxytocin antagonist, they will no longer experience these maternal instincts (Kennet 2012). The lack of maternal behaviour points to the idea that oxytocin plays a large role in bonding as well.

Bonding

Studies show that bonding in rats is achieved through the secretion of prolactin. Prolactin also regulates a wide array of activities and feelings from stress to immunity. This prolactin is released largely after birth, during feeding of the young, mating, and the presence of ovarian steroids (Kennet 2012). This increase in prolactin has been shown to be regulated largely by oxytocin.

Role of oxytocin in primate sexual behaviour

Mating/Bonding

Oxytocin plays a similar role in primates as it does in humans. The levels are increased heavily at birth and are maintained through the feeding and caring process. The hormones also play a role in the ability for monkeys to soothe their partners. When the monkey experiences a period of distress, the higher oxytocin monkeys were much more able to soothe their partners than monkeys who had lower levels of oxytocin.

Caring for young

Similar to any human parent child relationship, the role of oxytocin in monkeys is much alike. The similarities between monkeys and humans are generally very obvious, and their care for their young is similar to ours. Oxytocin has much of the same affect, and most mammals experience these actions the same way. Additionally, there have been instances where monkeys have cared for human babies, and humans have cared for monkeys allowing for bonding to occur across species. At Brookfield Zoo in Chicago, a small toddler fell into a pit with the 500 pound gorillas. Much to everyone’s surprise the large gorilla picked up the toddler and protected her from harm until authorities came to help the child. This shows that the care that these apes have for their young is similar to ours, and also that the protection of young in general is a paramount part of their life cycle.

Bonding

The bonding process is also shown through the use of grooming, much like human parents and their children. Grooming, sex, and cuddling frequencies correlate positively with levels of oxytocin. As the level of oxytocin increases so does the interest in sex, and grooming. While oxytocin plays a major role in parent child relationships, it is also found to play a role in adult sexual relationships. Its secretion affects the nature of the relationship or if there will even be a relationship at all.

Promiscuity/lifelong partnership

Studies have shown that oxytocin is much higher in monkeys that are in lifelong monogamous relationships as opposed to monkeys which are single. Similarly the oxytocin levels of the couples correlated positively. When the oxytocin secretion of one increased the other one increased along with it. Higher levels of oxytocin also showed that monkeys exhibit more behaviours such as cuddling, grooming, and sex while lower levels of oxytocin mean less interest in these activities.

Recent turns in research on oxytocin

Recent research on oxytocin's role in the animal brain suggests that it plays less of a role in behaviours of love and affection than previously believed. "When oxytocin was first discovered in 1909, it was thought mostly to influence a mother’s labor contractions and milk let-down. Then, in the 1990s, research with prairie voles found that giving them a dose of oxytocin resulted in the formation of a bond with their future mate (Azar, 40)." Oxytocin has since been treated by the media as the sole player in the “love and mating game” in mammals. This view, however, is proving to be false as, "most hormones don’t influence behaviour directly. Rather, they affect thinking and emotions in variable ways (Azar, 40)." There is much more involved in sexual behaviour in the mammalian animal than oxytocin and vasopressin can explain.

Specific species

Mammals

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 and at up to 75% of sexual activity being nonreproductive. Primatologist Frans de Waal believes that bonobos use sexual activity to resolve conflict between individuals. Sexual activity occurs between almost all ages and sexes of bonobo societies.

Male bottlenose dolphins have been observed working in pairs to follow 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 that the common same-sex behaviour 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 with which to reproduce. In 1991, an English man was prosecuted for allegedly having sexual contact with a dolphin. The man was found not guilty after it was revealed at trial that the dolphin was known to tow bathers through the water by hooking his large penis around them.

Some horses have environment or appearance preferences when selecting mates. There is also anecdotal evidence of limited bisexual behaviour in some stallions, although there is (as of 2008) no conclusive scientific confirmation. The anecdotal evidence claims this is most likely to occur in a single isolated group, with no access to mares.

Male lions often lead their social groups jointly with one or more of their brothers. To ensure loyalty, the male co-leaders will "strengthen the bonds" by often having sex with each other. 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. Most lionesses will have reproduced by the time they are four years of age. Lions do not mate at any specific time of year, and the females are polyestrous. As with other cats' penises, the male lion's penis has spines which point backwards. Upon withdrawal of the penis, the spines rake the walls of the female's vagina, which may cause ovulation. A lioness may mate with more than one male when she is in heat; during a mating bout, which could last several days, the couple copulates twenty to forty times a day, often forgoing eating. Lions reproduce very well in captivity.

As with most tetrapods, canine copulation involves the male mounting the female from behind, a position informally referred to as "doggy style". When a male canine is interested in mounting a female, he will sniff the female's vulva. If the female is unreceptive, she may sit, lie down, snap, retreat, or otherwise be uncooperative. If the female is receptive, she will stand still and hold her tail to the side, a stance referred to as "flagging". The male will often continue examining the female's rear, before mounting her from behind whilst attempting penetration with his penis.

Red foxes reproduce once a year in spring. The vixen's oestrus period lasts 3 weeks, during which the dog-foxes mate with the vixens for several days, often in burrows. Copulation is accompanied by a copulatory tie which may last for more than an hour. The copulatory tie occurs when the bulbus glandis at the base of the male fox's penis enlarges. Though foxes are largely monogamous, DNA evidence from one population indicated large levels of polygyny, incest and mixed paternity litters. Subordinate vixens may become pregnant, but usually fail to whelp, or have their kits killed postpartum by either the dominant female or other subordinates.

River otters typically breed from December to April. Copulation lasts from 16–73 minutes and may occur in water or on land. During the breeding, the male grabs the female by the neck with his teeth. Copulation is vigorous, and is interrupted by periods of rest. Females may caterwaul during or shortly after mating. Female estrus lasts about a month per year, and true gestation lasts 61–63 days. Delayed implantation distinguishes the species from the European otter, which lacks this feature.

Spotted hyenas

The female spotted hyena has a unique urinary-genital system, closely resembling the penis of the male, called a pseudo-penis. 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. The spotted hyena is a non-seasonal breeder, though a birth peak does occur during the wet season. Females are polyestrus, with an estrus period lasting two weeks. Like many felid species, the spotted hyena is promiscuous, and no enduring pair bonds are formed. Members of both sexes may copulate with several mates over the course of several years. Males will show submissive behaviour when approaching females in heat, even if the male outweighs its partner. Females usually favour younger males born or joined into the clan after they were born. Older females show a similar preference, with the addition of preferring males with whom they have had long and friendly prior relationships. Passive males tend to have greater success in courting females than aggressive ones. Copulation in spotted hyenas is a relatively short affair, which typically only occurs at night with no other hyenas present. The mating process is complicated, as the female's reproductive tract is entered and exited through her pseudo-penis rather than directly through the vagina, which is blocked by the false scrotum and testes. Once the female retracts her clitoris, the male enters the female by sliding beneath her, an operation facilitated by the penis' upward angle. Once this is accomplished, a typical mammalian mating posture is adopted.

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. 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.”

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.

Golden jackal

The golden jackal's courtship rituals are remarkably long, during which the breeding pair remains almost constantly together. The mating process may last 26–28 days. In Transcaucasia, estrus begins in early February, and occasionally late January during warm winters. Spermatogenesis in males occurs 10–12 days before the females enter estrus and, during this time, males' testicles triple in weight. Estrus lasts for three to four days, and females failing to mate during this time will undergo a loss of receptivity which lasts six to eight days. Females undergoing their first estrus are often pursued by several males, which will quarrel amongst themselves. Prior to mating, the pair patrols and scent marks its territory. Copulation is preceded by the female holding her tail out and angled in such a way that the genitalia are exposed. The two approach each other, whimpering, lifting their tails and bristling their fur, displaying varying intensities of offensive and defensive behaviour. The female sniffs and licks the male's genitals, whilst the male nuzzles the female's fur. They may circle each other and fight briefly. The male then proceeds to lick the female's vulva, and repeatedly mounts her without erection or hip thrusting. Actual copulation takes place days later, and continues for about a week. The copulatory tie lasts 20–45 minutes in Eurasia, while in Africa it lasts roughly four minutes. Toward the end of estrus, the pair drifts apart, with the female often approaching the male in a more submissive manner than before. In anticipation of the role he will take in raising pups, the male disgorges or surrenders any food he has to the female.

Gray wolf

The gray wolf is generally monogamous, with mated pairs usually remaining together for life, unless one of the pair dies. Upon the death of one mated wolf, pairs are quickly re-established. Since males often predominate in any given wolf population, unpaired females are a rarity. If a dispersing male wolf is unable to establish a territory or find a mate, he mates with the daughters of already established breeding pairs from other packs. Such wolves are termed "Casanova wolves" and, unlike males from established packs, they do not form pair bonds with the females they mate with. Some wolf packs may have multiple breeding females this way, as is the case in Yellowstone National Park.

The age of first breeding in wolves depends largely on environmental factors: when food is plentiful, or when wolf populations are heavily managed, wolves can rear pups at younger ages in order to better exploit abundant resources. This is further demonstrated by the fact that captive wolves have been known to breed as soon as they reach 9–10 months, while the youngest recorded breeding wolves in the wild were 2 years old. Estrus typically occurs in late winter, with older, multiparous females entering estrus 2–3 weeks earlier than younger females. When receptive, the female averts the base of her tail to one side, exposing the vulva. During mating, the pair is locked into a copulatory tie, which may last 5–36 minutes. The copulatory tie is caused by the swelling of the male wolf's penis inside the female's vulva. Wolves are able to separate from a copulatory tie more readily than domestic dogs. Because estrus in wolves lasts only a month, male wolves do not abandon their mates to find other females to inseminate as dogs do.

Sheep

An October 2003 study by Dr. Charles E. Roselli et al. (Oregon Health & 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 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. 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, an estrogen hormone believed to facilitate typical male sexual behaviours. 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 previously, given the potential ingressiveness 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.

Other vertebrates

Seahorses

Seahorses, long upheld as monogamous and mating for life, are identified as "promiscuous, flighty, and more than a little bit gay" according to research published in 2007.

Scientists at 15 aquariums studied 90 seahorses of 3 species. Of 3168 sexual encounters, 37% were same sex acts. Flirting was common (up to 25 potential partners a day of both sexes); only one species (the British Spiny Seahorse) included faithful representatives, and for these 5 of 17 were faithful, 12 were not. Bisexuality was widespread and considered "both a great surprise and a shock", with big-bellied seahorses of both sexes not showing partner preference. 1986 contacts were male-female, 836 were female-female and 346 were male-male.

Birds

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

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

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

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

Studies have shown that ten to 15% of female western gulls in some populations in the wild prefer other females.

As many as 19% of mallard pairs in a given population have been observed to consist of male-male homosexuals.

In 2009 at a zoo in Bremerhaven, Germany, two male adult humboldt penguins adopted an egg that had been abandoned by its biological parents. After the egg hatched, the two penguins raised, protected, cared for, and fed the chick in the same manner that heterosexual penguins raise their own biological offspring.

Lizards

Whip-tailed lizard females have the ability to reproduce through parthenogenesis and as such males are rare and sexual breeding non-standard. Females engage in sexual behaviour to stimulate ovulation, with their behaviour 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 behaviours. 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.

Invertebrates

Penis fencing is a mating behaviour engaged in by certain species of flatworm, such as Pseudobiceros bedfordi. Species which engage in the practice are hermaphroditic, possessing both eggs and sperm-producing testes. The species "fence" using two-headed dagger-like penises which are pointed, and white in color. One organism inseminates the other. The sperm is absorbed through pores in the skin, causing fertilization.

The butterflies spend much time on searching for mates. When the male spots his mate, he will fly closer and closer and release the chemical substance called pheromones. The male makes a special "courtship dance" which may consist of some special dancing postures to attract the female. If the female appreciates his dancing, she may join him. Then they join their bodies together end to end at their abdomens. Here, the male passes the sperm to the female's egg-laying tube which will soon be fertilized by the sperm. The male often has to face death shortly after mating. This is one of the reasons why butterflies are considered to have unusual sexual behaviour.

Many animals—not just spiders—make plugs of mucus to seal the female's orifice after mating. Normally such plugs are secreted by the male, to stymie subsequent suitors. In spiders, though, the female sometimes assists the process. Spider sex is unusual in that males transfer their sperm to the female on small limbs called pedipalps. They use these to pick their sperm up from their genitals and insert it into the female's sexual orifice, rather than copulating directly. On the 14 occasions a sexual plug was made, the female produced it without assistance from the male. On ten of these occasions the male's pedipalps then seemed to get stuck while he was transferring the sperm (which is rarely the case in other species of spider), and he had great difficulty freeing himself. In two of those ten instances, he was eaten as a result.

Other evidence of interspecies sexual activity

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 other apes, interspecies mating nonetheless 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."

Role in discussion of human sexuality

Information about animal sexuality frequently arises as a persuasive device in arguments regarding human sexuality. Originally, the lack of documented animal sexual behaviour other than heterosexual sexual monogamy was used to argue that the dominant heterosexual monogamy of most modern human societies is more natural and acceptable. Likewise, the lack of documented sex between animals for the purpose of pleasure was used to promote the moral standard of reserving sex primarily for procreation. Proponents of alternate sexuality attribute this early lack of documented evidence to an observer bias in researchers, who, they argue, tended to interpret sexual behaviour inconsistent with their values as other behaviour.

With increasing published evidence of different types of sexual behaviour between animals, arguments for heterosexual monogamy in human society have moved towards characterizing these behaviours as resulting from differences between humans and animals, and in particular on ambiguity in motivation and subjective experience in animals, which is difficult to study. Arguments identifying human and animal behaviour are characterized as anthropomorphism, and in some cases an opposite observer bias is attributed to researchers. Supporters of alternate sexuality embrace the new research as confirmation of the naturalness of alternate sexual behaviour and evidence of its long-term feasibility and utility.

The media could also be a key player in helping the destruction of the stereotypical idea that gay, lesbian, bisexual or transgender people represent an unnatural aspect of society. It does so by describing and promoting the variation in sexual behaviour in the animal kingdom, thus displaying that this behaviour is acceptable for humans as well.

In both cases, any argument whose conclusion is that something is good or right because it is natural, or that something is bad or wrong because it is unnatural or artificial is known as the appeal to nature fallacy.

See also

• Against Nature?, An exhibition on animal homosexuality taking place in the Natural History Museum at the University of Oslo.
• Animal behaviour
• Anthropophilia in animals
• Biology and sexual orientation
• Choice and sexual orientation
• Cuckoldry in fish
• Ethology
• Freemartin (a female bovine with a masculinized brain which acts sexually as a male)
• Green Porno, a series of short films about animal mating, enacted by humans, airing on the Sundance Channel
• List of animals displaying homosexual behaviour
• Pair bonding
• r/K selection theory
• Sexual arousal in animals
• Sexual intercourse#In other animals
• Zoology
• Zoophilia

References

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24. This section and examples taken from Robert Sapolsky (1998) Why Zebras Don't Get Ulcers, W.H. Freeman and Co., ISBN 0-7167-3210-6, pp. 140–141.
25. Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1163/156853951X00296, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1163/156853951X00296 instead.
26. Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 583410, please use {{cite journal}} with |pmid=583410 instead.
27. For example, masturbation, trial mounting, and other behaviours are regularly seen in male animals out of season
28. 2006 Danish Animal Ethics Council report: "The mucous membrane in the female animal’s vagina and the animal’s behaviour 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)
29. Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 16727133, please use {{cite journal}} with |pmid=16727133 instead.
30. Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1111/j.1749-6632.1947.tb31725.x, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1111/j.1749-6632.1947.tb31725.x instead.
31. Buried Pleasure. Snopes.com. Retrieved on 2011-12-22.
32. 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.
33. 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)
34. 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)
35. Watson, P. F. (1978). Artificial breeding of non-domestic animals: (the proceedings of a symposium held at the Zoological Society of London on 7 and 8 September 1977). Academic Press for the Zoological Society of London. ISBN 978-0-12-613343-1. Retrieved 9 February 2013.
36. Balcombe, Jonathan P. (2011). The Exultant Ark: A Pictorial Tour of Animal Pleasure. University of California Press. pp. 89–. ISBN 978-0-520-26024-5.
37. Journal of the American Veterinary Medical Association. American Veterinary Medical Association. 1931.
38. Müller, Georg Alfred (1897). Diseases of the dog and their treatment. W.H. Hoskins. pp. 183–.
39. Bedford-Guaus, Dr. Sylvia. "Breeding Soundness Examination of the Stallion". www.petplace.com. Retrieved 2010-06-04. Inspection of the penis may reveal the presence of infection, tumors or scar tissue resulting from the placement of stallion rings to prevent masturbation. Masturbation is a normal behaviour in all stallions that does not reduce semen production or performance in the breeding shed, and thus the use of devices to prevent such behaviour is strongly discouraged and can be harmful to the stallion.
40. 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.
41. McDonnell, S. M.; A. L., AL (2005). Squires, E. (ed.). "Aversive conditioning of periodic spontaneous erection adversely affects sexual behavior and semen in stallions" (PDF). Animal reproduction science. 89 (1–4): 77–92. doi:10.1016/j.anireprosci.2005.06.016. PMID 16112531. Periodic spontaneous erection and penile movements known as masturbation (SEAM) occur normally at approximately 90 min intervals in awake equids. are consistent with suppressed sexual arousal and reduced breeding efficiency. Semen volume and total number of sperm per ejaculate were significantly less
42. McDonnell, S. M. "Specific Normal Behaviors of Domestic Horses That Are Misunderstood as Abnormal". Equine Behavior Laboratory, University of Pennsylvania School of Veterinary Medicine. Retrieved 2010-06-04.
43. McDonnell, S. M.; Diehl, N. K.; Garcia, M. C.; Kenney, R. M. (1989). "Gonadotropin Releasing Hormone (GnRH) Affects Precopulatory Behavior in Testosterone-Treated Geldings" (PDF). Physiology & Behavior. 45 (1): 145–148. doi:10.1016/0031-9384(89)90177-7. PMID 2657816.
44. ^ Bagemihl, pp. 71, 209–210
45. Mech, L. David (2012). Wolf. Random House Digital, Inc. ISBN 978-0-307-81913-0.
46. Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 19862320, please use {{cite journal}} with |pmid=19862320 instead.
47. Tiger, Lionel (1992). The Pursuit of Pleasure. Transaction Publishers. pp. 66 ff. ISBN 0-7658-0696-7.
48. Janet E. Smith (2011). "Contraception: Why Not? (revised)". Archived from the original on 5 June 2011. Retrieved 2011-05-08. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
49. ^ Bagemihl
50. "Oslo gay animal show draws crowds". BBC News. 2006-10-19. Archived from the original on 29 October 2006. Retrieved 2006-10-19. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
51. "Central Park Zoo's gay penguins ignite debate" by Dinitia Smith, San Francisco Chronicle, February 7, 2004
52. ^ Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 14525915, please use {{cite journal}} with |pmid=14525915 instead.
53. In BriefRams Will Be Rams. washingtonpost.com (2004-07-04). Retrieved on 2011-02-15.
54. STANFORD Magazine: May/June 2004 > Feature Story > On the Originality of Species. Stanfordalumni.org (2003-07-02). Retrieved on 2011-02-15.
55. Riccucci, Marco (2011). "Same-sex sexual behaviour in bats". Hystrix It. J. Mamm. (n.s.). 22 (1): 139–147. doi:10.4404/hystrix-22.1-4478.
56. de Waal FB (1995). "Bonobo sex and society". Sci Am. 272 (3): 82–8. doi:10.1038/scientificamerican0395-82. PMID 7871411. Perhaps the bonobo's most typical sexual pattern, undocumented in any other primate, is genito-genital rubbing (or GG rubbing) between adult females. One female facing another clings with arms and legs to a partner that, standing on both hands and feet, lifts her off the ground
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58. Kirkpatrick, RC; Lévi-Strauss, C (2000). "The Evolution of Human Homosexual Behavior" (PDF). Current anthropology. 41 (3): 385–413. doi:10.1086/300145. PMID 10768881. {{cite journal}}: Missing |author2= (help)
59. Bagemihl, Bruce (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity (PDF). World Policy Institute: St. Martin’s Press.
60. Walker, Matt (2008-05-02) Science/Nature | 'Sex pest' seal attacks penguin. BBC News. Retrieved on 2011-02-15.
61. 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 (e.g., 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)
62. Early humans had sex with chimps. Agençe France-Presse. 18 May 2006.
63. "Wild find: Half grizzly, half polar bear: Hunter bags what expert 'never thought would happen' in wild". MSNBC.MSN.com. May 11, 2006. Retrieved 2006-05-14.
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65. "Penguins are turning to prostitution". BBC. 1998-02-26. Retrieved 2008-06-12.
66. Connor, Steve (2009-04-08). "Sex for meat – how chimps seduce their mates". The Independent. London.
67. Cindy Tumiel Chimp village. Express-News (2004-10-19)
68. Biology News: Monkeys pay for sexy pics. Bioedonline.org (2005-02-01). Retrieved on 2011-02-15.
69. Deaner M. O., Khera A. V. and Platt M. L. (2005). "Monkeys pay per view: adaptive valuation of social images by rhesus macaques" (PDF). Current Biology. 15 (6): 543–548. doi:10.1016/j.cub.2005.01.044. PMID 15797023.
70. Gray, Denis D. (2006-11-27) Porn sparks panda baby boom in China: Research — and blue movies — attributed to record-high birth rate in 2006. Associated Press (via MSNBC). Retrieved on 2011-12-22.
71. Angier, N. (1992-02-18) Dolphin courtship: brutal, cunning and complex, New York Times.
72. Making love can be a real sacrifice. smh.com.au (2003-08-11). Retrieved on 2011-02-15.
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75. Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1111/j.1439-0310.1986.tb00566.x, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1111/j.1439-0310.1986.tb00566.x instead.
76. "Not tonight, not ever. I’ve got a headache. Don’t come near me", The Times, 25 June 2007 p. 25
77. Siebert, Charles. (2006-10-08) ''An Elephant Crackup?'', Charles Siebert, New York Times Magazine, October 8, 2006. Nytimes.com. Retrieved on 2011-12-22.
78. "Have elephants begun raping rhinos in the wild?". The Straight Dope. 2008-07-25. Retrieved 2013-03-05.
79. Doncarlos, Michael W., Petersen, Jay S., Tilson, Ronald L. (1986). "Captive biology of an asocial mustelid; Mustela erminea". Zoo Biology. 5 (4): 363–370. doi:10.1002/zoo.1430050407.{{cite journal}}: CS1 maint: multiple names: authors list (link)
80. Kruuk, H. (1972) The Spotted Hyena, University of Chicago Press, ISBN 0226455084, p. 232
81. Dawkins, Richard (2004). "Chimpanzees". The Ancestor's Tale. Houghton Mifflin. ISBN 1-155-16265-X.
82. Bloom, Richard W. and Dess, Nancy Kimberly (2003). Evolutionary psychology and violence: a primer for policymakers and public policy advocates. Greenwood Publishing Group. pp. 87–. ISBN 978-0-275-97467-1.{{cite book}}: CS1 maint: multiple names: authors list (link)
83. Sommer, Volker and Vasey, Paul L. (2006). Homosexual behaviour in animals: an evolutionary perspective. Cambridge University Press. pp. 290–. ISBN 978-0-521-86446-6.{{cite book}}: CS1 maint: multiple names: authors list (link)
84. Moeliker, C.W. (2001). "The first case of homosexual necrophilia in the mallard Anas platyrhynchos (Aves: Anatidae)". Deinsea. 8: 243–247. ISSN 0932-9308. {{cite journal}}: Check |issn= value (help)
85. DEINSEA states on its website: "DEINSEA publishes original papers and short communications dealing with zoology, paleontology and (urban) ecology. Contributions that are entirely or partly based on material from the collection of the Natural History Museum Rotterdam (coll. NMR) and/or that are the result of research by NMR staff, have priority." The Mallard paper and photographs are available online as a summary and also the paper is available in PDF format.
    See also: MacLeod, Donald (2005-03-08). "Necrophilia among ducks ruffles research feathers". London: The Guardian. Archived from the original on 10 April 2006. Retrieved 2006-04-05. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
86. MacLeod, Donald (2005-03-08). "Necrophilia among ducks ruffles research feathers". London: The Guardian. Archived from the original on 10 April 2006. Retrieved 2006-04-05. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
87. ^ "Hormone Involved in Reproduction". oxytocin.org.
88. Kennedy, Marilyn (1997-11-02). "Maternal Instincts". Chicago Tribune.
89. ^ Bear, Mark F. (2007). Neuroscience, exploring the brain. Baltimore, MD: Lippincott Williams and Wilkins. pp. 544–545. ISBN 0781760038.
90. Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 22129099, please use {{cite journal}} with |pmid=22129099 instead.
91. Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 20025881, please use {{cite journal}} with |pmid=20025881 instead.
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95. Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 21195714, please use {{cite journal}} with |pmid=21195714 instead.
96. Scott, Graham (2004). Essential Animal Behavior. Wiley-Blackwell. pp. 166–197. ISBN 0632057998.
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98. Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 22708956, please use {{cite journal}} with |pmid=22708956 instead.
99. Homosexual Activity Among Animals Stirs Debate, National Geographic (2004-07-22) Retrieved November 6, 2007
100. Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 4658666, please use {{cite journal}} with |pmid=4658666 instead.
101. Central Park Zoo's gay penguins ignite debate. Sfgate.com (2004-02-07). Retrieved on 2011-12-22.
102. ^ Unwin, Brian (2008-01-22). "'Tougher laws' to protect friendly dolphins". London: The Telegraph.
103. Miniature Horse Homosexuality bisexuality – Equine Sexual preference. Mini-horse.org. Retrieved on 2011-02-15.
104. 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
105. Srivastav, Suvira (15–31 December 2001). "Lion, Without Lioness". TerraGreen: News to Save the Earth. Terragreen. Retrieved 2007-09-02.
106. Schaller, p. 29.
107. Schaller, p. 174.
108. Asdell, Sydney A. (1993) . Patterns of mammalian reproduction. Ithaca: Cornell University Press. ISBN 978-0-8014-1753-5. {{cite book}}: External link in |title= (help)
109. Schaller, p. 142.
110. Dan Rice (3 October 2008). The Complete Book of Dog Breeding. Barron's Educational Series. pp. 51–. ISBN 978-0-7641-3887-4. Retrieved 7 February 2013.
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112. George A. Feldhamer; Bruce C. Thompson; Joseph A. Chapman (21 October 2003). Wild Mammals of North America: Biology, Management, and Conservation. JHU Press. ISBN 978-0-8018-7416-1. Retrieved 28 March 2013.
113. Harris & Yalden 2008, pp. 418–19 harvnb error: no target: CITEREFHarrisYalden2008 (help)
114. north american river otter. Conservenature.org. Retrieved on 2013-01-09.
115. Ware, George W. (2001). Reviews of Environmental Contamination and Toxicology. Springer. ISBN 0-387-95137-7.
116. East, Marion L.; Hofer, Heribert and Wickler, Wolfgang (1993). "The erect 'penis' is a flag of submission in a female-dominated society: greetings in Serengeti spotted hyenas". Behavioral Ecology and Sociobiology. 33 (6): 355–370. doi:10.1007/BF00170251.{{cite journal}}: CS1 maint: multiple names: authors list (link)
117. Sapolsky (1998), Why Zebras don't get Ulcers, W.H. Freeman and Co., ISBN 0-7167-3210-6, pp. 127–129.
118. ^ Kruuk, Hans (1972). "The Spotted Hyena: A Study of Predation and Social Behaviour" (Document). University of California Press. {{cite document}}: Invalid |ref=harv (help); Unknown parameter |isbn= ignored (help)
119. ^ Szykman. M., Van Horn, R. C., Engh, A.L. Boydston, E. E. & Holekamp, K. E. (2007) Courtship and mating in free-living spotted hyenas. Behaviour. 144: 815–846.
120. How Hyenas Avoid Incest by LiveScience Staff, posted: 15 August 2007 01:00 pm ET.
121. It’s a dog’s life – aggressive male hyenas fail to impress the girls from innovations report, May 14, 2003.
122. Estes, Richard (1998). "The safari companion: a guide to watching African mammals, including hoofed mammals, carnivores, and primates" (Document). Chelsea Green Publishing. {{cite document}}: Invalid |ref=harv (help); Unknown parameter |isbn= ignored (help)
123. ^ "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.
124. ^ 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. {{cite web}}: Cite has empty unknown parameter: |coauthors= (help)
125. Selim, Jocelyn (2006-04-28). "Top Dogs: Steroid hormones give hyenas a head start". Discover Magazine. Retrieved 2007-09-11.
126. ^ Forger, Nancy G. (6 December 1998,). "Sexual Dimorphism of Perineal Muscles and Motoneurons in Spotted Hyenas". The Journal of Comparative Neurology. 375 (2, Pages 333 – 343): 333. doi:10.1002/(SICI)1096-9861(19961111)375:2<333::AID-CNE11>3.0.CO;2-W. PMID 8915834. Retrieved 2007-09-11. {{cite journal}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: extra punctuation (link)
127. "Wilson, Sexing the Hyena - "The males mount each other"". University of Chicago Press. Retrieved 2007-09-11. {{cite journal}}: Cite journal requires |journal= (help)
128. ^ Estes, Richard (1992). "The behavior guide to African mammals: including hoofed mammals, carnivores, primates" (Document). University of California Press. {{cite document}}: Cite has empty unknown parameter: |url= (help); Invalid |ref=harv (help); Unknown parameter |isbn= ignored (help)
129. ^ Heptner & Naumov 1998, pp. 154–155 harvnb error: no target: CITEREFHeptnerNaumov1998 (help)
130. Estes 1992, p. 403
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132. Smith, Douglas W. (2006) Decade of the Wolf: Returning the Wild to Yellowstone, Lyons Press, ISBN 1-59228-886-3
133. ^ L. David Mech; Luigi Boitani (23 November 2003). Wolves: Behavior, Ecology, and Conservation. University of Chicago Press. pp. 44–. ISBN 978-0-226-51696-7. Retrieved 10 December 2012.
134. L. David Mech (1 July 2003). The Wolves of Minnesota: Howl in the Heartland. Voyageur Press. pp. 77–. ISBN 978-0-89658-509-6. Retrieved 10 December 2012.
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137. ^ Simon De Bruxelles Promiscuous and bisexual — the 'faithful' seahorse has a secret sex life. Timesonline.co.uk. 31 January 2007. Retrieved on 2011-12-22.
138. Smith, Dinitia (2004-02-07). "Love That Dare Not Squeak Its Name". New York Times. New York.
139. Beth Shapiro Gay Rams, Lesbian Swans, & Queer Penguins: The Summer Of Love. 365Gay.com. New York. August 15, 2005
140. Dinitia Smith Central Park Zoo's gay penguins ignite debate. New York Times via Sfgate.com (2004-02-07). Retrieved on 2012-04-04.
141. Bagemihl, pp. 479–481
142. 'Gay penguins' rear adopted chick, BBC, 2009-06-03
143. Newman, Leslie. "Fighting to mate: flatworm penis fencing". PBS.
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146. Wade, Nicholas (2006-05-18) Two Splits Between Human and Chimp Lines Suggested, New York Times.
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• Carey, Bjorn (April 26, 2006). "The Painful Realities of Hyena Sex". LiveScience. Retrieved November 7, 2012.

Bibliography

• Bagemihl, Bruce (1999) Biological Exuberance: Animal Homosexuality and Natural Diversity. St. Martin's Press, ISBN 0-312-19239-8

• Szykman, M.; Van Horn, R. C.; et al. (2007). "Courtship and mating in free-living spotted hyenas" (pdf). Behaviour. 144 (7). doi:10.1163/156853907781476418. Retrieved 2012-07-11. {{cite journal}}: Invalid |ref=harv (help)

External links

• Gay flamingos celebrate fifth anniversary with their children
• Gay penguins resist split attempts
• National Geographic

• San Francisco Zoo has run a "sex tour" covering animal sexuality, on Valentine's Day
• A wild, and gay, kingdom World Science
• Is it relevant to look at the animal kingdom to determine if human same-sex behaviour is "natural"?

• Articles with links needing disambiguation from August 2012
• Articles with links needing disambiguation from September 2012
• Animal sexuality
• Ethology
• Reproduction in animals
• Mating