Homosexual behavior in sheep
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Homosexual behavior in sheep is common and well documented in domestic sheep (Ovis aries)[1][2] and at least four species of wild sheep (members of the genus Ovis).[3]: 405–409 About 8% to 10% of domestic rams (male sheep) refuse to mate with ewes (female sheep) but do readily mate with other rams,[4][1][5] making them the only known mammal besides humans to exhibit exclusively same-sex sexual behavior.[5] Around 18% to 30% of all domestic rams demonstrate at least some homosexual behavior.[6][7] Male domestic sheep's homosexual behavior and preference has received much academic discussion.[11]
Several observations indicate that male–male sexual preference in rams is sexually motivated. Rams routinely perform the same courtship behaviors (including foreleg kicks, nudges, vocalizations, anogenital sniffs and flehmen) prior to mounting other males as observed when other rams court and mount estrous females. Furthermore, pelvic thrusting and ejaculation often accompany same-sex mounts by rams.[10] Additionally, a small number of females that were accompanied by a male fetus in utero (i.e. as fraternal twins) are freemartins (female animals with intersex characteristics like being behaviorally masculine and lacking functioning ovaries).[16]
Research on potential social factors
A number of studies have reported differences in brain structure and function between male-oriented and female-oriented rams, suggesting that sexual partner preferences are neurologically hard-wired.[10] Long-term studies of homosexual behavior in domesticated sheep led by Charles Roselli have found that 6-8% of rams have a homosexual preference throughout their life. Dissection of ram brains also found a similar smaller (feminized) structure in homosexually oriented rams compared to heterosexually oriented rams in the equivalent brain region to the human sexually dimorphic nucleus (SDN), the ovine sexually dimorphic nucleus (oSDN).[1]: 107–110
The size of the sheep oSDN has also been demonstrated to be formed in utero, rather than postnatally, underscoring the role of prenatal hormones in masculinization of the brain for sexual attraction.[17][18] Their 2003 study at Oregon Health and Science University stated that homosexuality in male sheep 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 heterosexual male sheep.[4] 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, a form of estrogen which is believed to 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."[4] As noted before, given the potential unaggressiveness of the male population in question, the differing aromatase levels may also have been evidence of aggression levels, not sexuality. It should also be noted that the results of this particular study have not been confirmed by other studies.[citation needed]
View source data.
Average volumes of the equivalent cell group in sheep (oSDN) for heterosexual and homosexual rams and for ewes.[1] Sex differences are formed under the influence of prenatal hormones in utero, rather than acquired after birth.[17]
Heterosexually oriented rams
Homosexually oriented rams
Ewes (females)
Sheep are one of the few animals where the molecular basis of the diversity of male sexual preferences has been examined.[4] However, this research has been controversial, and much publicity has been produced by a study at the Oregon Health and Science University that investigated the mechanisms that produce homosexuality in rams. Organizations such as PETA campaigned against the study, accusing scientists of trying to cure homosexuality in the sheep.[19] OHSU and the involved scientists vehemently denied such accusations.[19]
Studies have failed to identify any compelling social factors that can predict or explain the variations in sexual partner preferences of domestic rams.[10] Homosexual orientation and same-sex mounting in rams is not related to dominance, social rank or competitive ability. Indeed, male-oriented rams are not more or less dominant than female-oriented rams.[2][10] Homosexual orientation in rams is also not affected by rearing conditions, i.e., rearing males in all-male groups, rearing male and female lambs together, early exposure of adolescent males to females and early social experiences with females do not promote or prevent homosexual orientation in rams.[2][10] Male-oriented partner preference also does not appear to be an artifact caused by captivity or human management of sheep.[10]
