Animal sexual behavior takes many different forms, even within the same species. Researchers have observed monogamy, promiscuity, sex between species, sexual arousal from objects or places, sex apparently via duress or coercion, copulation with dead animals, homosexual, heterosexual and bisexual sexual behaviour, and situational sexual behaviour and a range of other practices among animals other than humans. Related studies have noted diversity in sexed bodies and gendered behaviour, such as intersex and transgender animals. The study of animal sexuality (and primate sexuality especially) is a rapidly developing field. It used to be believed that only humans and a handful of species performed sexual acts other than for procreation, and that animals' sexuality was instinctive and a simple response to the "right" stimulation (sight, scent). Current understanding is that many species believed monogamous have now been proven to be promiscuous or opportunistic in nature, a wide range of species appear both to masturbate and to use objects as tools to help them do so, in many species animals try to give and get sexual stimulation with others where procreation is not the aim, and homosexual behavior has now been observed among 1,500 species, and in 500 of those it is well documented.
Notes on specific species
Bonobos
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.[citation needed]
Birds
Some black swans of Australia form sexually active male-male mated pairs and steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs. More of their cygnets survive to adulthood than those of different-sex pairs possibly due to their superior ability to defend large portions of land.
In early February 2004 the New York Times reported that a male pair of chinstrap penguins named Roy and Silo in the Central Park Zoo in New York City were partnered and had successfully hatched a female chick from an egg. Other penguins in New York have also been reported to be forming same-sex pairs.
Zoos in Japan and Germany have also documented male penguin couples. The couples have been shown to build nests together and use a stone to replace an egg in the nest. Researchers at Rikkyo University in Tokyo, found twenty such pairs at sixteen major aquariums and zoos in Japan. Bremerhaven Zoo in Germany attempted to break up the male couples by importing female penguins from Sweden and separating the male couples; they were unsuccessful. The zoo director stated the relationships were too strong between the couples.
Recently, a mated pair of swans in Boston were found to both be female. They too had attempted to raise eggs together.
Studies have shown that ten to fifteen percent 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.
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 & Science University) states that homosexuality in male sheep (found in eight percent of rams) is associated with a region in the rams' brains which the authors call the "ovine Sexually Dimorphic Nucleus" (oSDN) which is two times smaller than the corresponding region in other male sheep.
However, some view this study to be flawed in that the determination of homosexuality within the sheep, (sample population of twenty-seven for the study), was to have animals who were unable to mount female ewes placed in a cage with two stanchioned males and two unstanchioned females (that is, the males could not move or struggle while the females could). Given the aggressive nature of the sheep copulation, the uneven treatment of males and females, many see this as simply evidence that the sheep in question were unable to be aggressive enough to mount females. Some say that the results were situational sexuality, unlike the bonds seen in human homosexuality. However the physical brain anatomy of the rams that preferred males were different.
The scientists found that, "The oSDN in rams that preferred females was significantly larger and contained more neurons than in male-oriented rams and ewes. In addition, the oSDN of the female-oriented rams expressed higher levels of aromatase, a substance that converts testosterone to estradiol so that the androgen hormone can facilitate typical male sexual behaviors. Aromatase expression was no different between male-oriented rams and ewes."
"The dense cluster of neurons that comprise the oSDN express cytochrome P450 aromatase. Aromatase mRNA levels in the oSDN were significantly greater in female-oriented rams than in ewes, whereas male-oriented rams exhibited intermediate levels of expression." These results suggest that "...naturally occurring variations in sexual partner preferences may be related to differences in brain anatomy and its capacity for estrogen synthesis." Read the abstract of the study. As noted previously, given the potential unagressiveness of the male population in question, the differing aromatase levels may also have been evidence of aggression levels, not sexuality. The results of this study have not been confirmed by others.
Spotted Hyena
The female Spotted Hyena has a unique urinary-genital system, closely resembling the penis of the male, called a pseudopenis. The family structure is matriarchal and dominance relationships with strong sexual elements are routinely observed between related females.
They are notable for using visible sexual arousal as a sign of submission and not dominance, in males as well as females (females have a sizable erectile clitoris), to the extent that biologist Robert Sapolsky speculates that in order to facilitate this, their sympathetic and parasympathetic nervous systems may be partially reversed in respect to their reproductive organs.
Bottlenose 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 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.
Seahorses
Seahorses, long upheld as monogamous and mating for life, are identified as "promiscuous, flightly, and more than a little bit gay" according to research published in 2007.
Scientists at 15 aquariums studied 90 seahorses of 3 species. Of 3168 sexual encounters, 37% were same sex acts. Flirting was common (up to 25 potential partners a day of both genders); only one species (the British Spiny Seahorse) included faithful representatives, and for these 5 of 17 were faithful, 12 were not. Bisexuality was widespread and considered "both a great surprise and a shock", with big bellied seahorses of both genders not showing partner preference. 1986 contacts were male-female, 836 were female-female and 346 were male-male.
Lions
Male lions often lead their social groups jointly with one or more of their brothers. To ensure loyalty, the male co-leaders will "strengthen the bonds by often having sex with each other."
