Animal sexual behaviour

Animal sexual behavior takes many different forms, even within the same species. Researchers have drawn parallels between this and masturbation, heterosexuality, homosexuality, bisexuality, intersexuality and transgender behavior in humans.

The study of animal sexuality is a rapidly developing field. It used to be believed that only humans and a handful of species performed sexual acts other than for procreation, and that animals sexuality was instinctive and a simple response to the "right" stimulation (sight, scent).

Current understanding is that many species believed monogamous have now been proven to be promiscuous or opportunistic in nature, a wide range of species appear to both masturbate and to use objects as tools to help them do so, in many species animals try to give and get sexual stimulation with others where procreation is not the aim, and homosexual behavior has now been documented in over 450 species.

Types of activity

Masturbation

File:Dog and toy.png

Dog (canis familiaris) masturbating against a stuffed toy. (Source: movie still)

It appears that many animals, both male and female, masturbate, both when partners are available and otherwise.

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, and thus the use of devices to prevent such behavior is strongly discouraged and can be harmful to the stallion." [1] 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 (McDonnell et al, 1991). 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)." [2]

File:M90.org kangeroo film still.png

Still from TV show, accidentally capturing animal masturbation by a kangaroo

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. [3]

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.

File:Cow and tractor.png

Inanimate farm equipment such as tractors or hay, is sometimes employed as a tool for animal masturbation

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.

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.

Oral sex

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.

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

Homosexual behavior

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. It appears to be widespread among insects, birds and mammals, particularly the apes. Many male penguins that apparently mate for life have been observed in homosexual pairs and refuse to pair with females when given the chance [4].

One report on sheep cited below states:

"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..."

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

File:Racoon and dog.png

A raccoon attempting coitus with a dog. Photographs of cross species sex involving wild species are rare, since many species do not share their territory with competitor species, or are hard to observe.

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. 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.

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.

File:Yak and mare.jpg

A yak (large domestic animal common in Asia) seeks sex from a female horse

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 orangutangs 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") [5]

Sexual fetishes

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.

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'. On the other hand, human fetishism also can very often 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 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.

Sex between adults v. sexually immature individuals

It has also been recorded that certain species of mole will impregnate newborns of their own species, the biological advantage to this is that when those moles mature and become fertile, they will become pregnant with the sperm of the mole that had mated with them at a very young age. 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 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

File:Necromouse12hd.jpg

A living mouse having sex with a dead mouse.

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.

Mating Systems

In sociobiology and behavioural ecology, the term mating system is used to describe the ways in which animal societies are structured in relation to sexual behaviour. The mating system specifies what males mate with what females under what circumstances.

The following are some of the mating systems generally recognised in 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 within the social group mates with any female.

Monogamy

Main article: List of animals displaying homosexual behavior

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 offsrping 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.

The discovery that social monogamy does not imply sexual monogamy or genetic monogamy has contributed to ongoing debates about the expectation of lifetime sexual monogamy in human beings. David Barash (a University of Washington zoologist and professor of psychology) and Judith Lipton(a psychiatrist) document in their book The Myth of Monogamy that:

"Until recently,lacking the appropriate genetic techniques, we had little choice but to define monogamy by the social relationships involved; only with the explosion in DNA fingerprinting technology have we started to examine the genetic connections, those most important to evolution. Thus, according to the highly respected book by David Lack, Ecological Adaptations for Breeding in Birds, fully 92 percent of bird species are monogamous. Socially, this figure is still accurate; sexually, it is way off. 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)

Barash and Lipton's main argument in The Myth of Monogamy is that, because monogamy is so rare among animals it's likely not natural in humans.

"Having looked, although briefly, at the diversity of human mateships, what can we conclude? For one thing, it seems undeniable that human beings have evolved as mildly polygynous creatures whose 'natural' mating system probably involved one man mated, when possible, to more than one woman. It is also clear that even in societies that institutionalized some form of polygyny, monogamy was nonetheless frequent, although, for men at least, this typically meant making the best of a bad situation. ...Certainly there is no evidence, either from biology, primatology, or anthropology, that monogamy is somehow 'natural' or 'normal' for human beings." (Barash & Lipton, 2001, page 153)

"Human beings are enormously flexible creatures, at least socially. They are capable of living a variety of lives depending on the demands and expectations of the society in which they live. To some extent, then, human inclinations may be able to fit whatever matrimonial pattern happens to exist in the society they happen to experience. But, on the other hand, mild polygyny is likely so much a part of the primitive human condition that it is reflected not only in our anatomy, physiology, and development--not to mention the anthropological record--but also in our behavioral tendencies. If this is true, then the marriage bed may be a procrustean bed as well, insofar as it denies the possibility of nonexclusive sexual relationships. Deprived of both socially approved polygyny and EPCs [extra-pair copulations], perhaps it is not surprising that many people, throughout history and around the globe, have chafed at lifetime monogamy and often circumvented it." (Barash & Lipton, 2001, page 153-154)

The expectation of lifelong sexual monogamy is a cultural invention that may be unrealistic and should not be imposed on people. But Barash and Lipton do not suggest the unnaturalness of sexual monogamy makes it impossible or undesirable. They write:

"This is not to say that monogamy--even happy, fulfilled monogamy--is impossible, because, in fact, it is altogether within the realm of human possibility. But since it is not natural, it is not easy. Similarly, this is not to say that monogamy isn't desirable, because there is very little connetion, if any, between what is natural or easy and what is good." (Barash & Lipton, 2001, page 191)

So, the discovery that social monogamy is often accompanied by sexual non-monogamy in animals adds to the arguments that sexual monogamy is not natural in human beings, and this may help explain why large numbers of human beings throughout history and around the globe have not been sexually monogamous. Yet, saying that sexual monogamy is not natural in human beings in no way implies sexual monogamy is impossible or undesirable. It is simply difficult to achieve because of our evolutionary heritage.

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

Psychogenic abortion

In some species, notably those with harem male structures, only one of few males in a group of females will mate. 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.

Notes on specific species

Bonobos

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

Birds

File:Gay penguins NY Zoo.jpg

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

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

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

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

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

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

Lizards

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

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

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

Sheep

An October 2003, study by Dr. Charles E. Roselli et al. (Oregon Health and 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.

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

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

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

Spotted Hyena

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

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

Bottlenose Dolphins

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

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

Other evidence of interspecies sexual activity

Main article: Chuman

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 clearly diverged from apes, interspecies mating none the less occured regularly enough to change certain genes in the new gene pool:

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

The research suggests that:

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

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

Source: ny times (May 2006).

References

• Bruce Bagemihl: Biological Exuberance: Animal Homosexuality and Natural Diversity. St. Martin's Press, 1999. ISBN 0312192398
• McDonnell SM, Henry M, Bristol F (1991) Spontaneous erection and masturbation in equids. Proceedings Vth International Equine Reproduction Symposium. JReprod Fert Suppl 44: 664-665.

Deaner M. O., Khera A. V. & Platt M. L. Curr. Biol. published online here (2005).

• Houser, Ward Animal Homosexuality. Encyclopedia of Homosexuality. Dynes, Wayne R. (ed.), Garland Publishing, 1990. pp. 60-62.

See also

• List of animals displaying homosexual behavior
• Biology and sexual orientation
• Choice and sexual orientation
• Animal behaviour
• Ethology
• Zoology
• Pair bonding
• Homosexuality
• Bisexuality
• Freemartin (a female bovine with a masculinized brain which acts sexually as a male)

External links

• 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"