Animal sexual behaviour
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 sexual behaviour, heterosexual, bisexual sexual behaviour, situational sexual behaviour, and a range of other practices.
When animal sexual behaviour is reproductively-driven, it is often termed mating or copulation; for most non-human mammals, mating and copulation occur at the point of estrus (the most fertile period of time in the female's reproductive cycle), which increases the chances of successful impregnation. However, the study of animal sexuality (especially that of primates) is a rapidly developing field. While it used to be believed that only humans and a handful of other species performed sexual acts other than for reproduction, 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 to be monogamous are promiscuous or opportunistic in nature; a 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 sexual reproduction is not the focus, and homosexual behaviour has been observed among 1,500 species, with 500 of those being well-documented.
Some animal sexual behaviour involves competition, sometimes fighting, between multiple males. In these circumstances, females often select males for mating only if they appear strong and able to protect themselves. The male that wins a fight may also have the chance to mate with a larger number of females and will therefore pass on his genes to their offspring.
- 1 Mating systems
- 2 Interpretation bias
- 3 Genetics and sex
- 4 Sex for pleasure
- 5 Types of activity
- 6 Neurochemistry and hormones
- 7 Specific species
- 8 Other evidence of interspecies sexual activity
- 9 Role in discussion of human sexuality
- 10 See also
- 11 References
- 12 Bibliography
- 13 Further reading
- 14 External links
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.
A defining trait of any mating system is the extent of tournament or pair-bonding behaviour. The varying degrees of these opposing tendencies allows for a wide spectrum of behaviours, some of which play a crucial role in defining the social structure of a species, and some that are only observed during mating seasons.
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 main social structures 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.
Tournament and pair-bonding behaviour
A tournament species is one in which "mating tends to be highly polygamous and involves high levels of male-male aggression and competition." Tournament behaviour often correlates with high levels of sexual dimorphism, examples of species including chimpanzees and baboons.
A pair-bonding species is one in which the individuals tend to form long-lasting monogamous pairs. In contrast with tournament species, there are lower levels of male aggression, competition and little sexual dimorphism. Species include marmoset monkeys and pigeons.
A monogamous mating system is one in which individuals form long-lasting pairs and cooperate in raising offspring. These pairs may last for a lifetime, such as in pigeons, or it may occasionally change from one mating season to another, such as in emperor penguins. 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 [are] 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.
A polygamous mating-system is any one in which individuals carry sexual relationships with multiple partners during their lives. Most polygamous species present high levels of tournament behaviour, with a notable exception being bonobos.
Polygamy takes two main forms – polygyny (a male mating with multiple females) and polyandry (a female mating with multiple males). 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.
Polygynous mating structures are estimated to occur in up to 90% of mammal species.
In a small number of species, individuals can display either polygamous or monogamous behaviour depending on environmental conditions. An example is the social wasp Apoica flavissima.
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.
- amongst wild horses and baboons, the male will "systematically harass" pregnant females until they miscarry.
- 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 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.
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.
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.
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.) In earlier periods, bias tended to support what would now be described as conservative sexual mores; today, liberal social or sexual views are often projected upon animal subjects of research. Popular discussions of bonobos are a frequently cited example. Current research frequently expresses 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.
An example of overlooking behaviour relates to descriptions of giraffe mating:
When nine out of ten pairings occur between males, "[e]very male that sniffed a female was reported as sex, while anal intercourse with orgasm between males was only [categorized as] 'revolving around' dominance, competition or greetings.
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 is not 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
|This section needs additional citations for verification. (June 2014)|
It is a common misconception 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 that do. This is sometimes formulated "animals mate only for reproduction".
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.—
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.—
Types of activity
||It has been suggested that Animal co-opted sexual behavior be merged into this section. (Discuss) Proposed since July 2014.|
Autoeroticism or masturbation
It appears that many animals, both male and female, masturbate, both when partners are available and otherwise. For example, it has been observed in dogs, male deer, and male monkeys.
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
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 [ie, male ferret] she pines and becomes ill. If a smooth pebble is introduced into the hutch, she will masturbate upon it, thus preserving her normal health for one season. But if this artificial substitute is given to her a second season, she will not, as formerly, be content with it." [...] Blumenbach observed a bear act somewhat similarly on seeing other bears coupling, and hyenas, according to Ploss and Bartels, have been seen practicing mutual masturbation by licking each other's genitals.
In his 1999 book, Biological exuberance, Bruce Bagemihl 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.
..perhaps the most creative form of animal masturbation is that of the male bottlenose dolphin, which has been observed to wrap a live, wriggling eel around its penis.
Among elephants, female same-sex behaviours have been documented only in captivity where they are known to masturbate one another with their trunks.
Animals of several species are documented as engaging in both autofellatio and oral sex. Although easily confused by laypeople, autofellatio and oral sex are separate, sexually oriented behaviors, .
In the greater short-nosed fruit bat, copulation by males is dorsoventral and the females lick the shaft or the base of the male's penis, but not the glans which has already penetrated the vagina. While the females do this, the penis is not withdrawn and research has shown a positive relationship between length of the time that the penis is licked and the duration of copulation. Post copulation genital grooming has also been observed.
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.
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 [male] rams exhibit sexual preferences [that is, even when given a choice] for male partners (male-oriented rams) in contrast to most rams, which prefer female partners (female-oriented rams). We identified a cell group within the medial preoptic area/anterior hypothalamus of age-matched adult sheep that was significantly larger in adult rams than in ewes...
- Male bighorn sheep are divisible into two kinds: the typical males among whom homosexual 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, 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.
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 of mating is more difficult, 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.
Male seals have been observed forcibly copulating with penguins.
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.
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 and sexual fetishes
Some claims have been made that animals engage in prostitution. A small number of pairbonded females within a group of penguins helped themselves to some nesting material after copulating with a nonpartner male. Some chimpanzees were said to be trading food for sex, even though they were never observed to do so directly. Instead long-term relationships were forged with groups who shared food, and within those groups mating occurred over time.
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, male animals may 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 rhesus macaques 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 [male] 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.
Sex in a forceful or apparently coercive context has 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.—
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.
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 and necrophilia
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 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.
Neurochemistry and hormones
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.
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.
Oxytocin and rat sexual behaviour
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.
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.
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.
Oxytocin and primate sexual behaviour
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.
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.
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.
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.
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.
Exposure to Depo-Provera 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:
[The] 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.
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 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.
North American 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.
Raccoons usually mate in a period triggered by increasing daylight between late January and mid-March.
Mating in fossas includes a copulatory tie, which may be enforced by the male's spiny penis.
The mating system of pinnipeds varies from extreme polygyny to serial monogamy.
- Male walruses reach sexual maturity as early as seven years, but do not typically mate until fully developed at around 15 years of age.
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.
As with most tetrapods, canine copulation involves the male mounting the female from behind, a position informally referred to as "doggy style".
- 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.
- Fennec foxes are social animals that mate for life, with each pair or family controlling their own territory.
- The mating season for raccoon dogs begins from early February to late April, depending on location. Raccoon dogs are monogamous animals, with pair formations usually occurring in autumn.
- The golden jackal's courtship rituals are remarkably long, during which the breeding pair remains almost constantly together.
- The gray wolf is generally monogamous, with mated pairs usually remaining together for life, unless one of the pair dies.
- Bobcats generally begin breeding by their second summer, though females may start as early as their first year.
- Jaguar females reach sexual maturity at about two years of age, and males at three or four. Jaguars are believed to mate throughout the year in the wild, although births may increase when prey is plentiful.
- Mating among clouded leopards usually occurs during December and March.
- Mating among tigers can occur all year round, but is generally more common between November and April. Depending on the region, leopards may mate all year round.
- The breeding season in Canada lynx lasts only for a month, ranging from March to May, depending on the local climate.
- The mating season for Eurasian lynx lasts from January to April.
- Oestrus in sand cats lasts from five to six days, and is accompanied by calling and increased scent marking.
- Female jungle cats are sexually mature at the age of 11 months; estrus appears to last from January through to mid-April.
- Ocelots typically breed only once every other year, although the female may mate again shortly after losing a litter.
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.
Scientists at 15 aquaria 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. Bisexual behaviour 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.
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.
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.
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.
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. Research analysis of human and animal genes in 2006 by Peterson et al. found evidence that after humans had diverged from other apes, interspecies mating nonetheless occurred regularly enough to change certain genes in the new gene pool. Nicholas Wade, commenting in The New York Times, wrote:
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 suggested that:
If 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."
David Brown, writing in the Washington Post, commented, "If this theory proves correct, it will mean modern people are descended from something akin to chimp-human hybrids."
However in 2012, this evidence was called into question:
There have been conflicting arguments as to what happened in the human-chimpanzee speciation event. [..] We conclude that [..] there is no strong reason to involve complicated factors in explaining the autosomal data.
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.
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.
- Pre-copulatory isolation mechanisms in animals
- Anthropophilia in animals
- Biology and sexual orientation
- Choice and sexual orientation
- Cuckoldry in fish
- 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
- Kent, Michael (2000). Advanced biology. Oxford University Press. pp. 250–253. ISBN 978-0-19-914195-1.
- Thorpe, Showick and Thorpe, Edgar (2009). General Studies Manual. Pearson Education India. p. page 17 (xvii). ISBN 9788131721339.
- "1,500 animal species practice homosexuality". News-medical.net. 23 October 2006. Retrieved 19 February 2007.[unreliable source?]
- Wickler, Wolfgang; Lorenz; Konrad and Kacher, Hermann (1974). "The sexual code : the social behaviour of animals and me".
- Robert Sapolsky (2005). "Biology and Human Behavior: The Neurological Origins of Individuality, 2nd edition". The Teaching Company. Retrieved 2010-11-10.
- "Pigeon". Encyclopædia Britannica. Encyclopædia Britannica. Retrieved 26 August 2014.
- Lily Whiteman (13 February 2013). "Animal Attraction: The Many Forms of Monogamy in the Animal Kingdom". National Science Foundation. National Science Foundation. Retrieved 26 August 2014.
- Ågren, G.; Zhou, Q.; Zhong, W. (1989). "Ecology and social behaviour of Mongolian gerbils, Meriones unguiculatus, at Xilinhot, Inner Mongolia, China". Animal Behaviour 37: 11. doi:10.1016/0003-3472(89)90002-X.
- Barash, D. P. (1981). "Mate guarding and gallivanting by male hoary marmots (Marmota caligata)". Behavioral Ecology and Sociobiology 9 (3): 187–193. doi:10.1007/BF00302936. JSTOR 4599431.
- Birkhead, T.R. & Møller, A.P.; Møller (1995). "Extra-pair copulations and extra-pair paternity in birds". Animal Behaviour 49 (3): 843–848. doi:10.1016/0003-3472(95)80217-7.
- Birkhead, T.R. & Møller, A.P. (1996) "Monogamy and sperm competition in birds". In J. M. Black (Ed.), Partnerships in Birds: The Study of Monogamy, pp. 323–343, Oxford: Oxford University Press ISBN 0-19-854860-5.
- Foltz, D. W. (1981). "Genetic Evidence for Long-Term Monogamy in a Small Rodent, Peromyscus polionotus". The American Naturalist 117 (5): 665–675. doi:10.1086/283751. JSTOR 2460752.
- Gursky, S. (2000). "Sociality in the spectral tarsier, Tarsius spectrum". American Journal of Primatology 51 (1): 89–101. doi:10.1002/(SICI)1098-2345(200005)51:1<89::AID-AJP7>3.0.CO;2-7. PMID 10811442.
- Hasselquist, D. (2001). "Social mating systems and extrapair fertilizations in passerine birds". Behavioral Ecology 12 (4): 457–466. doi:10.1093/beheco/12.4.457.
- Hubrecht, R. C. (1985). "Home-range size and use and territorial behavior in the common marmoset, Callithrix jacchus jacchus, at the tapacura field station, recife, Brazil". International Journal of Primatology 6 (5): 533–550. doi:10.1007/BF02735575.
- Mason, W.A. (1966). "Social organization of the South American monkey, Callicebus moloch: a preliminary report". Tulane Studies in Zoology 13: 23–28.
- McKinney, F.; Mineau, P.; Derrickson, S. R. (1983). "Forced Copulation in Waterfowl". Behaviour 86 (3): 250. doi:10.1163/156853983X00390. JSTOR 4534287.
- Reichard, U. (2010). "Extra-pair Copulations in a Monogamous Gibbon (Hylobates lar)". Ethology 100 (2): 99–99. doi:10.1111/j.1439-0310.1995.tb00319.x.
- Reichard, U.H. (2002). "Monogamy—A variable relationship" (PDF). Max Planck Research 3: 62–67. Retrieved 24 April 2013.[dead link]
- Richardson, P.R.K. (1987). "Aardwolf mating system: overt cuckoldry in an apparently monogamous mammal". South African Journal of Science 83: 405–412.
- Welsh, D. & Sedinger, J.S.; Sedinger (1990). "Extra-Pair copulations in Black Brant". The Condor 92 (1): 242–244. doi:10.2307/1368407. JSTOR 1368407.
- Westneat, D. F.; Stewart, I. R. K. (2003). "EXTRA-PAIRPATERNITY INBIRDS: Causes, Correlates, and Conflict". Annual Review of Ecology, Evolution, and Systematics 34: 365. doi:10.1146/annurev.ecolsys.34.011802.132439.
- Owens, I. P. F.; Hartley, I. R. (1998). "Sexual dimorphism in birds: Why are there so many different forms of dimorphism?". Proceedings of the Royal Society B: Biological Sciences 265 (1394): 397–407. doi:10.1098/rspb.1998.0308. JSTOR 50849. PMC 1688905.
- Solomon, N. G.; Keane, B.; Knoch, L. R.; Hogan, P. J. (2004). "Multiple paternity in socially monogamous prairie voles (Microtus ochrogaster)". Canadian Journal of Zoology 82 (10): 1667. doi:10.1139/z04-142.
- Reichard, U.H. (2003). Monogamy: Past and present. In U.H. Reichard and C. Boesch (Eds.), Monogamy: Mating strategies and partnerships in birds, humans, and other mammals, pp. 3–25, Cambridge: Cambridge University Press, ISBN 0521819733.
- Barash, D.P. & Lipton, J.E. (2001). The Myth of Monogamy. New York, NY: W.H. Freeman and Company, ISBN 0805071369.
- Angier, Natalie (21 August 1990). "Mating for Life? It's Not for the Birds of the Bees". The New York Times.
- Morell, V. (1998). "EVOLUTION OF SEX:A New Look at Monogamy". Science 281 (5385): 1982–1983. doi:10.1126/science.281.5385.1982. PMID 9767050.
- Aloise King, Edith D.; Banks, Peter B.; Brooks, Robert C. (1 January 2013). "Sexual conflict in mammals: consequences for mating systems and life history". Mammal Review 43 (1): 47–58. doi:10.1111/j.1365-2907.2011.00200.x.
- 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.
- Haartman, L. V. (1951). "Successive Polygamy". Behaviour 3: 256. doi:10.1163/156853951X00296.
- Silverin, B. (1979). "Effects of long-acting testosterone administration on testes in free living pied flycatchers Ficedula hypoleuca". Endokrinologie 74 (2): 141–146. PMID 583410.
- Marraro, CH; Nursall JR (1983). "The reproductive periodicity and behaviour of Ophioblennius atlanticus at Barbados". J Zool 61 (2): 317–325. doi:10.1139/z83-042.
- For example, masturbation, trial mounting, and other behaviours are regularly seen in male animals out of season
- 2006 Danish Animal Ethics Council report: "The mucous membrane in the female animal’s vagina and the animal’s 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)[dead link]
- Katz, L. S.; McDonald, T. J. (1992). "Sexual behavior of farm animals". Theriogenology 38 (2): 239–253. doi:10.1016/0093-691X(92)90233-H. PMID 16727133.
- Young, W. C. (1947). "Animal Endocrines in Relation to Sexual Behavior". Annals of the New York Academy of Sciences 47 (5): 609–616. doi:10.1111/j.1749-6632.1947.tb31725.x.
- Danish Animal Ethics Council report[dead link] Udtalelse om menneskers seksuelle omgang med dyr published November 2006. Council members included two academics, two farmers/smallholders, and two veterinary surgeons, as well as a third veterinary surgeon acting as secretary.
- Danish: "Selv om det evolutionsmæssige formål med at parre sig kan siges at være reproduktion, er det ikke selve det, at dyrene får afkom, der i første omgang får dem til at parre sig. Det er til gengæld sandsynligt, at de parrer sig, fordi de er motiverede for selve parringsakten, og at denne er forbundet med en positiv oplevelse. Det er derfor rimeligt at antage, at der er en eller anden form for behag eller tilfredsstillelse forbundet med akten. Denne antagelse bekræftes af adfærden hos handyr, der for mange arters vedkommende er parate til at arbejde for at få adgang til hundyr, især hvis hundyret er i brunst, og handyr der i avlsøjemed er vant til at få tappet sæd – de viser stor ivrighed, når det udstyr, de forbinder med sædopsamlingen, tages frem." Report November 2006 (PDF)[dead link]
- 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)[dead link]
- 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.
- 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.
- Journal of the American Veterinary Medical Association. American Veterinary Medical Association. 1931.
- Müller, Georg Alfred (1897). Diseases of the dog and their treatment. W.H. Hoskins. pp. 183–.
- Leonard Lee Rue III (2004). The Deer of North America. Globe Pequot Press. ISBN 978-1-59228-465-8. Retrieved 8 September 2013.
- Leonard Lee Rue, III (2001). The Deer Hunter's Illustrated Dictionary: Full Explanations of More Than 600 Terms and Phrases Used by Deer Hunters Past and Present. Globe Pequot Press. ISBN 978-1-58574-349-0.
- A. F. Dixson (26 January 2012). Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes, and Humans. Oxford University Press. ISBN 978-0-19-954464-6.
- Jean-Baptiste Leca; Michael A. Huffman; Paul L. Vasey (19 January 2012). The Monkeys of Stormy Mountain: 60 Years of Primatological Research on the Japanese Macaques of Arashiyama. Cambridge University Press. ISBN 978-0-521-76185-7. Retrieved 8 September 2013.
- Bedford-Guaus, Sylvia. "Breeding Soundness Examination of the Stallion". www.petplace.com. Retrieved 4 June 2010.
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.
- 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.
- McDonnell, S. M.; A. L., AL (2005). Squires, E., ed. "Aversive conditioning of periodic spontaneous erection adversely affects sexual behavior and semen in stallions". 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. [..The effects of aversive conditioning] are consistent with suppressed sexual arousal and reduced breeding efficiency. Semen volume and total number of sperm per ejaculate were significantly less
- 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 4 June 2010.
- McDonnell, S. M.; Diehl, N. K.; Garcia, M. C.; Kenney, R. M. (1989). "Gonadotropin Releasing Hormone (GnRH) Affects Precopulatory Behavior in Testosterone-Treated Geldings". Physiology & Behavior 45 (1): 145–148. doi:10.1016/0031-9384(89)90177-7. PMID 2657816.
- Bagemihl, pp. 71, 209–210
- Linden, David J. (2011). Pleasure: How Our Brains Make Junk Food, Exercise, Marijuana, Generosity, and Gambling Feel So Good. p. 98. ISBN 9781851688241.
- Bagemihl, B. (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity. St. Martin's Press. pp. 427–30. ISBN 1-4668-0927-2.
- These Bears Are Having Lots Of Oral Sex, And Scientists Think They Know Why (The Huffington Post) By: Grenoble, Ryan.
- Hideshi Ogawa (2006). Wily Monkeys: Social Intelligence of Tibetan Macaques. Kyoto University Press. pp. 4–. ISBN 978-1-920901-97-4.
- Fox, M. W. (1972). "The Social Significance of Genital Licking in the Wolf, Canis lupus". Journal of Mammalogy: 637–640. JSTOR 1379064.
- G. R. Pafumi (15 January 2010). Is Our Vision of God Obsolete?. Xlibris Corporation. ISBN 978-1-4500-0396-4.
- Tan, M.; Jones, G.; Zhu, G.; Ye, J.; Hong, T.; Zhou, S.; Zhang, S.; Zhang, L. (2009). Hosken, David, ed. "Fellatio by Fruit Bats Prolongs Copulation Time". PLoS ONE 4 (10): e7595. doi:10.1371/journal.pone.0007595. PMC 2762080. PMID 19862320.
- Tan, Min; Gareth Jones; Guangjian Zhu; Jianping Ye; Tiyu Hong; Shanyi Zhou; Shuyi Zhang; Libiao Zhang (28 October 2009). Hosken, David, ed. "Fellatio by Fruit Bats Prolongs Copulation Time". PLoS ONE 4 (10): e7595. Bibcode:2009PLoSO...4.7595T. doi:10.1371/journal.pone.0007595. PMC 2762080. PMID 19862320. Retrieved 28 October 2009.
- "Oslo gay animal show draws crowds". BBC News. 19 October 2006. Archived from the original on 29 October 2006. Retrieved 19 October 2006.
- "Central Park Zoo's gay penguins ignite debate" by Dinitia Smith, San Francisco Chronicle, 7 February 2004
- Roselli, C. E.; Larkin, K.; Resko, J. A.; Stellflug, J. N.; Stormshak, F. (2003). "The Volume of a Sexually Dimorphic Nucleus in the Ovine Medial Preoptic Area/Anterior Hypothalamus Varies with Sexual Partner Preference". Endocrinology 145 (2): 478–483. doi:10.1210/en.2003-1098. PMID 14525915.
- In Brief: Rams Will Be Rams. washingtonpost.com (4 July 2004). Retrieved on 15 February 2011.[dead link]
- STANFORD Magazine: May/June 2004 > Feature Story > On the Originality of Species. Stanfordalumni.org (2 July 2003). Retrieved on 15 February 2011.
- 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 (inactive 2014-10-26).
- 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
- Paoli, T.; Palagi, E.; Tacconi, G.; Tarli, S. B. (2006). "Perineal swelling, intermenstrual cycle, and female sexual behavior in bonobos (Pan paniscus)". American Journal of Primatology 68 (4): 333–347. doi:10.1002/ajp.20228. PMID 16534808.
- Kirkpatrick, RC; Lévi-Strauss, C (2000). "The Evolution of Human Homosexual Behavior". Current anthropology 41 (3): 385–413. doi:10.1086/300145. PMID 10768881.
- Walker, Matt (2 May 2008) Science/Nature 'Sex pest' seal attacks penguin. BBC News. Retrieved on 15 February 2011.
- 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)
- Early humans had sex with chimps. Agençe France-Presse. 18 May 2006.
- Harris, Heather S., et al. "Lesions and behavior associated with forced copulation of juvenile Pacific harbor seals (Phoca vitulina richardsi) by southern sea otters (Enhydra lutris nereis)." Aquatic Mammals 36.4 (2010): 331-341.
- "Wild find: Half grizzly, half polar bear: Hunter bags what expert 'never thought would happen' in wild". MSNBC.MSN.com. 11 May 2006. Retrieved 14 May 2006.
- Heavy Petting. Utilitarian.net. Retrieved on 2011-12-22.
- "Penguins are turning to prostitution". BBC. 26 February 1998. Retrieved 12 June 2008.
- Connor, Steve (8 April 2009). "Sex for meat – how chimps seduce their mates". The Independent (London).
- Tumiel, Cindy (19 October 2004) Chimp village. Express-News.
- Biology News: Monkeys pay for sexy pics. Bioedonline.org (1 February 2005). Retrieved on 15 February 2011.[dead link]
- Deaner M. O., Khera A. V. and Platt M. L. (2005). "Monkeys pay per view: adaptive valuation of social images by rhesus macaques". Current Biology 15 (6): 543–548. doi:10.1016/j.cub.2005.01.044. PMID 15797023.
- Gray, Denis D. (27 November 2006) 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 22 December 2011.
- Angier, N. (18 February 1992) Dolphin courtship: brutal, cunning and complex, New York Times.
- Making love can be a real sacrifice. smh.com.au (11 August 2003). Retrieved on 15 February 2011.
- Bailey, R.O.; Seymour, N. R. and Stewart, G.R. (1978). "Rape behaviour in blue-winged teal". Auk 95: 188–90. doi:10.2307/4085514. JSTOR 4085514.
- Barash, D. P. (1977). "Sociobiology of Rape in Mallards (Anas platyrhynchos): Responses of the Mated Male". Science 197 (4305): 788–789. doi:10.1126/science.197.4305.788. PMID 17790773.
- Emlen, S. T.; Wrege, P. H. (2010). "Forced Copulations and Intra-specific Parasitism: Two Costs of Social Living in the White-fronted Bee-eater". Ethology 71: 2. doi:10.1111/j.1439-0310.1986.tb00566.x.
- "Not tonight, not ever. I’ve got a headache. Don’t come near me", The Times, 25 June 2007 p. 25
- Siebert, Charles. (2006-10-08) ''An Elephant Crackup?'', Charles Siebert, New York Times Magazine, October 8, 2006. Nytimes.com. Retrieved on 2011-12-22.
- "Have elephants begun raping rhinos in the wild?". The Straight Dope. 25 July 2008. Retrieved 5 March 2013.
- Doncarlos, Michael W., Petersen, Jay S., Tilson, Ronald L.; Petersen; Tilson (1986). "Captive biology of an asocial mustelid; Mustela erminea". Zoo Biology 5 (4): 363–370. doi:10.1002/zoo.1430050407.
- Kruuk, H. (1972) The Spotted Hyena, University of Chicago Press, ISBN 0226455084, p. 232
- Jeffrey Moussaieff Masson (21 October 2009). When Elephants Weep: The Emotional Lives of Animals. Random House Publishing Group. ISBN 978-0-307-57420-6. Retrieved 28 May 2013.
- Dawkins, Richard (2004). "Chimpanzees". The Ancestor's Tale. Houghton Mifflin. ISBN 1-155-16265-X.
- 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.
- 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.
- Moeliker, C.W. (2001). "The first case of homosexual necrophilia in the mallard Anas platyrhynchos (Aves: Anatidae)". Deinsea 8: 243–247. ISSN 0923-9308.
- DEINSEA states on its website[dead link]: "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[dead link] as a summary and also the paper is available[dead link] in PDF format.
See also: MacLeod, Donald (8 March 2005). "Necrophilia among ducks ruffles research feathers". London: The Guardian. Archived from the original on 10 April 2006. Retrieved 5 April 2006.
- MacLeod, Donald (8 March 2005). "Necrophilia among ducks ruffles research feathers". London: The Guardian. Archived from the original on 10 April 2006. Retrieved 5 April 2006.
- "Hormone Involved in Reproduction". oxytocin.org.
- Kennedy, Marilyn (2 November 1997). "Maternal Instincts". Chicago Tribune.
- Bear, Mark F. (2007). Neuroscience, exploring the brain. Baltimore, MD: Lippincott Williams and Wilkins. pp. 544–545. ISBN 0781760038.
- Kennett, J. E.; McKee, D. T. (2012). "Oxytocin: An Emerging Regulator of Prolactin Secretion in the Female Rat". Journal of Neuroendocrinology 24 (3): 403–412. doi:10.1111/j.1365-2826.2011.02263.x. PMC 3288386. PMID 22129099.
- Smith, A. S.; Ågmo, A.; Birnie, A. K.; French, J. A. (2010). "Manipulation of the oxytocin system alters social behavior and attraction in pair-bonding primates, Callithrix penicillata". Hormones and Behavior 57 (2): 255–262. doi:10.1016/j.yhbeh.2009.12.004. PMC 2824532. PMID 20025881.
- Lambert, K. (2011). The Lab Rat Chronicles. New York: penguin group. pp. 151–172.
- Azar, B. (March 2011). "Oxytocin's other side". Science Watch 42 (3): 40.
- Aubert, Y.; Gustison, M. L.; Gardner, L. A.; Bohl, M. A.; Lange, J. R.; Allers, K. A.; Sommer, B.; Datson, N. A.; Abbott, D. H. (2012). "Flibanserin and 8-OH-DPAT Implicate Serotonin in Association between Female Marmoset Monkey Sexual Behavior and Changes in Pair-Bond Quality". The Journal of Sexual Medicine 9 (3): 694–707. doi:10.1111/j.1743-6109.2011.02616.x. PMID 22304661.
- Gil, M.; Bhatt, R.; Picotte, K. B.; Hull, E. M. (2011). "Oxytocin in the medial preoptic area facilitates male sexual behavior in the rat". Hormones and Behavior 59 (4): 435–443. doi:10.1016/j.yhbeh.2010.12.012. PMC 3081415. PMID 21195714.
- Scott, Graham (2004). Essential Animal Behavior. Wiley-Blackwell. pp. 166–197. ISBN 0632057998.
- Agrati, D.; Fernández-Guasti, A.; Ferreño, M.; Ferreira, A. (2011). "Coexpression of sexual behavior and maternal aggression: The ambivalence of sexually active mother rats toward male intruders". Behavioral Neuroscience 125 (3): 446–451. doi:10.1037/a0023085. PMID 21517149.
- McHenry, J. A.; Bell, G. A.; Parrish, B. P.; Hull, E. M. (2012). "Dopamine D1 receptors and phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein-32 in the medial preoptic area are involved in experience-induced enhancement of male sexual behavior in rats". Behavioral Neuroscience 126 (4): 523–529. doi:10.1037/a0028707. PMC 3409344. PMID 22708956.
- Tiger, Lionel (1992). The Pursuit of Pleasure. Transaction Publishers. pp. 66 ff. ISBN 0-7658-0696-7.
- Smith, Janet E. (2011). "Contraception: Why Not? (revised)". Archived from the original on 5 June 2011. Retrieved 8 May 2011.
- Homosexual Activity Among Animals Stirs Debate, National Geographic (22 July 2004) Retrieved 6 November 2007
- Zihlman, A. L.; Hunter, W. S. (1972). "A biomechanical interpretation of the pelvis of Australopithecus". Folia primatologica; international journal of primatology 18 (1): 1–19. doi:10.1159/000155465. PMID 4658666.
- Central Park Zoo's gay penguins ignite debate. Sfgate.com (7 February 2004). Retrieved on 22 December 2011.
- Unwin, Brian (22 January 2008). "'Tougher laws' to protect friendly dolphins". London: The Telegraph.
- Miniature Horse Homosexuality bisexuality – Equine Sexual preference[dead link]. Mini-horse.org. Retrieved on 15 February 2011.
- Fay, F.H. (1985). "Odobenus rosmarus". Mammalian Species 238 (238): 1–7. doi:10.2307/3503810. JSTOR 3503810.
- 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.
- Sapolsky (1998), Why Zebras don't get Ulcers, W.H. Freeman and Co., ISBN 0-7167-3210-6, pp. 127–129.
- Harris & Yalden 2008, p. 417
- Mellen, J. D. (1993). "A comparative analysis of scent marking, social and reproductive behavior in 20 species of small cats (Felis)". American Zoologist 33 (2): 151–166. doi:10.1093/icb/33.2.151.
- Geptner, V. G., Sludskii, A. A. (1972). Mlekopitaiuščie Sovetskogo Soiuza. Vysšaia Škola, Moskva (in Russian); English translation: Heptner, V.G., Sludskii, A.A., Komarov, A., Komorov, N.; Hoffmann, R.S. (1992). Mammals of the Soviet Union. Vol III: Carnivores (Feloidea). Smithsonian Institution and the National Science Foundation, Washington DC. pp. 356–397.
- Sunquist, M., Sunquist, F. (2002) Wild Cats of the World. University Of Chicago Press, Chicago
- Simon De Bruxelles Promiscuous and bisexual — the 'faithful' seahorse has a secret sex life. Timesonline.co.uk. 31 January 2007. Retrieved on 22 December 2011.
- Smith, Dinitia (7 February 2004). "Love That Dare Not Squeak Its Name". New York Times (New York).
- Shapiro, Beth (15 August 2005) Gay Rams, Lesbian Swans, & Queer Penguins: The Summer Of Love. 365Gay.com. New York.
- Smith, Dinitia (7 February 2004) Central Park Zoo's gay penguins ignite debate. New York Times via Sfgate.com. Retrieved on 4 April 2012.
- Bagemihl, pp. 479–481
- 'Gay penguins' rear adopted chick, BBC, 3 June 2009
- Newman, Leslie. "Fighting to mate: flatworm penis fencing". PBS.
- "Chan Lee Peng, August 2008". Scienceray.com. Retrieved 5 March 2013.
- Sexual Appetite and Animal behaviour – Sex and the single spider 400 (8742). The Economist Intelligence Unit. 14 July 2011. ISSN 0013-0613.
- Patterson, Nick; Daniel J. Richter; Sante Gnerre; Eric S. Lander; David Reich (June 29, 2006). "Genetic evidence for complex speciation of humans and chimpanzees". Nature 441 (7097): 1103–1108. doi:10.1038/nature04789. ISSN 0028-0836. PMID 16710306.
- Wade, Nicholas (18 May 2006) Two Splits Between Human and Chimp Lines Suggested, New York Times.
- Brown, David (May 18, 2006). "Human Ancestors May Have Interbred With Chimpanzees". Washington Post. Retrieved 27 September 2013.
- Yamamichi, M; Gojobori J; Innan H. (Jan 2012). "An autosomal analysis gives no genetic evidence for complex speciation of humans and chimpanzees". Mol Biol Evol 29 (1): 145–56. doi:10.1093/molbev/msr172. PMID 21903679.
- Curtis, Gary N. "Appeal to Nature". Fallacyfiles.org. Retrieved 12 December 2011.
- Bagemihl, Bruce (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity. St. Martin's Press. ISBN 0-312-19239-8.
- Schaller, G.B. (1972). The Serengeti Lion. University of Chicago Press. ISBN 0226736601.
- Ronald D. Nadler (1980) Reproductive physiology and behaviour of gorillas., in R. V. Short and Barbara J. Weir (eds) The Great Apes of Africa, Cambridge, 1980. Journals of Reproduction and Fertility Ltd.
- Richard Estes (1991). The Behavior Guide to African Mammals: Including Hoofed Mammals, Carnivores, Primates. University of California Press. ISBN 978-0-520-08085-0.
- John Vandenbergh (28 August 1983). Pheromones and Reproduction in Mammals. Elsevier. ISBN 978-0-323-15651-6.
- Media related to Animal sex at Wikimedia Commons
- Media related to Interspecies sex at Wikimedia Commons
- Media related to Mammal sex at Wikimedia Commons
- Media related to Lepidoptera sex at Wikimedia Commons
- 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"?