Biology:Homosexual behavior in sheep

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Short description: Sexual behavior among sheep that is interpreted as homosexual


Flock of sheep

Homosexual behavior in sheep has been well documented and studied. The domestic sheep (Ovis aries) is the only species of mammal except for humans which exhibits exclusive homosexual behavior.[1][2][3] "About 10% of rams (males), refuse to mate with ewes (females) but do readily mate with other rams."[2] Thirty percent of all rams demonstrate at least some homosexual behavior.[4] One report on sheep found that 8% of rams exhibited homosexual preferences—that is, even when given a choice, they chose male over female partners.[5] This documented homosexual preference has garnered much discussion.[1][5][6][7][8][9] Such rams prefer to court and mount other rams only, even in the presence of estrous ewes.[1] Moreover, around 18–22% of rams are bisexual.[7]

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.[9] The Merck Manual of Veterinary Medicine lists homosexuality as a "normal behavior" in up to 30% of all rams in its section on behavioral problems.[4] 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 lack functioning ovaries).[10][11][12][13]

Biological causes

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.[9] Long-term studies of homosexual behavior in domesticated sheep led by Charles Roselli have found that 6-8% of rams have a homosexual preference through 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).[2]: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.[14][15] 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.[5] 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."[5] As noted before, given the potential unagressiveness 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]

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Average volumes of the equivalent cell group in sheep (oSDN) for heterosexual and homosexual rams and for ewes.[2] Sex differences are formed under the influence of prenatal hormones in utero, rather than acquired after birth.[14]
  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.[17] 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.[18] OHSU and the involved scientists vehemently denied such accusations.[18]

Research on social factors

Studies have failed to identify any compelling social factors that can predict or explain the variations in sexual partner preferences of domestic rams.[9] 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.[3][9] 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.[3][9] Male-oriented partner preference also does not appear to be an artifact caused by captivity or human management of sheep.[9]

Homosexual behavior in related species

Further information: Biology:Homosexual behavior in animals

Homosexual courtship and sexual activity routinely occur among rams of wild sheep species, such as bighorn sheep (Ovis canadensis), thinhorn sheep (Ovis dalli), mouflons and urials (Ovis orientalis).[19]:456–461 Usually a higher ranking older male courts a younger male using a sequence of stylized movements. To initiate homosexual courtship, a courting male approaches the other male with his head and neck lowered and extended far forward in what is called the 'low-stretch' posture. He may combine this with the 'twist,' in which the courting male sharply rotates his head and points his muzzle toward the other male, often while flicking his tongue and making grumbling sounds. The courting male also often performs a 'foreleg kick', in which he snaps his front leg up against the other male's belly or between his hind legs. He also occasionally sniffs and nuzzles the other male's genital area and may perform the flehmen response. Thinhorn rams additionally lick the penis of the male they are courting. In response, the male being courted may rub his cheeks and forehead on the courting male's face, nibble and lick him, rub his horns on the courting male's neck, chest, or shoulders, and develop an erection. Males of another wild sheep species, the Asiatic mouflons, perform similar courtship behaviors towards fellow males.[19]:456–461

Sexual activity between wild males typically involves mounting and anal intercourse. In Thinhorn sheep, genital licking also occurs. During mounting, the larger male usually mounts the smaller male by rearing up on his hind legs and placing his front legs on his partner's flanks. The mounting male usually has an erect penis and accomplishes full anal penetration while performing pelvic thrusts that may lead to ejaculation. The mounted male arches his back to facilitate the copulation. Homosexual courtship and sexual activity can also take place in groups composed of three to ten wild rams clustered together in a circle. These non-aggressive groups are called 'huddles' and involve rams rubbing, licking, nuzzling, horning, and mounting each other. Female Mountain sheep also engage in occasional courtship activities with one another and in sexual activities such as licking each other's genitals and mounting.[19]:456–461

See also

References

  1. ↑ 1.0 1.1 1.2 Poiani, Aldo; Dixson, A. F. (2010). Animal Homosexuality: A Biosocial Perspective. Cambridge, England: Cambridge University Press. p. 179. ISBN 9781139490382. https://books.google.com/books?id=EftT_1bsPOAC&pg=PA179. "This makes O. aries (ram) only the second mammal known, apart from humans, capable of displaying exclusive homosexuality." 
  2. ↑ 2.0 2.1 2.2 2.3 Levay, Simon (2017). Gay, Straight, and The Reason Why: The Science of Sexual Orientation (2nd ed.). Cambridge, Massachusetts: Oxford University Press. pp. 38, 119. ISBN 978-0-19-029737-4. https://books.google.com/books?id=-J6kDAAAQBAJ. 
  3. ↑ 3.0 3.1 3.2 Roselli CE; Reddy RC; Kaufman KR (2011). "The development of male-oriented behavior in rams". Frontiers in Neuroendocrinology (Amsterdam: Elsevier) 32 (2): 164–9. doi:10.1016/j.yfrne.2010.12.007. PMID 21215767. 
  4. ↑ 4.0 4.1 Scrivener, C. J. (2005). "Social Behavior and Behavioral Problems". Merck Manual of Veterinary Medicine (9th ed.). Whitehouse Station, New Jersey, USA: Merck & Co.. p. 1312. https://archive.org/details/merckveterinarym09edunse/page/1312/mode/1up. 
  5. ↑ 5.0 5.1 5.2 5.3 "The volume of a sexually dimorphic nucleus in the ovine medial preoptic area/anterior hypothalamus varies with sexual partner preference". Endocrinology (Washington D.C.: Endocrine Society) 145 (2): 478–83. February 2004. doi:10.1210/en.2003-1098. PMID 14525915. 
  6. ↑ "The neurobiology of sexual partner preferences in rams". Hormones and Behavior (Amsterdam: Elsevier) 55 (5): 611–20. May 2009. doi:10.1016/j.yhbeh.2009.03.013. PMID 19446078. 
  7. ↑ 7.0 7.1 "Prenatal programming of sexual partner preference: the ram model". Journal of Neuroendocrinology (Hoboken, New Jersey, USA: Wiley-Blackwell) 21 (4): 359–64. March 2009. doi:10.1111/j.1365-2826.2009.01828.x. PMID 19207819. 
  8. ↑ "Luteinizing hormone and testosterone response of sexually active and inactive rams". Journal of Animal Science (Champaign, Illinois: American Society of Animal Science) 70 (7): 2086–93. July 1992. doi:10.2527/1992.7072086x. PMID 1644682. https://www.animalsciencepublications.org/publications/jas/articles/70/7/2086. Retrieved 2015-07-07. 
  9. ↑ 9.0 9.1 9.2 9.3 9.4 9.5 9.6 "Sexual partner preference, hypothalamic morphology and aromatase in rams". Physiology & Behavior (Amsterdam: Elsevier) 83 (2): 233–45. November 2004. doi:10.1016/j.physbeh.2004.08.017. PMID 15488542. 
  10. ↑ Padula, A.M. (2005). "The freemartin syndrome: an update.". Animal Reproduction Science 87 (1/2): 93–109. doi:10.1016/j.anireprosci.2004.09.008. PMID 15885443. http://www.journals.elsevierhealth.com/periodicals/anirep/article/S0378-4320(04)00231-3/abstract. 
  11. ↑ Parkinson, T. J. (2001). "Inter-relationships among gonadotrophins, reproductive steroids and inhibin in freemartin ewes.". Reproduction 122 (3): 397–409. doi:10.1530/rep.0.1220397. PMID 11597305. 
  12. ↑ Szatkowska, I.; Switonski, M. (1996). "Evidence on hereditary occurrence of placental anastomoses in heterosexual twins in sheep". Hereditas 124 (2): 107–110. doi:10.1111/j.1601-5223.1996.t01-1-00107.x. PMID 8782431. 
  13. ↑ Smith, K. C. (2003). "Morphological, histological and histochemical studies of the gonads of ovine freemartin". Vet. Rec. 152 (6): 164–169. doi:10.1136/vr.152.6.164. PMID 12622286. 
  14. ↑ 14.0 14.1 "Neurobiology of gender identity and sexual orientation". Journal of Neuroendocrinology (Hoboken, New Jersey, USA: Wiley-Blackwell) 30 (7): e12562. July 2018. doi:10.1111/jne.12562. PMID 29211317. 
  15. ↑ "Sexual Orientation, Controversy, and Science". Psychological Science in the Public Interest (Thousand Oaks, California: Sage Publishing) 17 (2): 45–101. September 2016. doi:10.1177/1529100616637616. PMID 27113562. 
  16. ↑ Zhou, Jiang-Ning; Hofman, Michel A.; Gooren, Louis J. G.; Swaab, Dick F. (November 1995). "A sex difference in the human brain and its relation to transsexuality". Nature (London: Springer Nature) 378 (6552): 68–70. doi:10.1038/378068a0. PMID 7477289. 
  17. ↑ Roselli CE; Larkin K; Resko JA; Stellflug JN; Stormshak F (2004). "The volume of a sexually dimorphic nucleus in the ovine medial preoptic area/anterior hypothalamus varies with sexual partner preference". Endocrinology (Washington D.C.: Endocrine Society) 145 (2): 478–83. doi:10.1210/en.2003-1098. PMID 14525915. 
  18. ↑ 18.0 18.1 Schwartz, John (25 January 2007). "Of Gay Sheep, Modern Science and Bad Publicity". The New York Times (New York City). https://www.nytimes.com/2007/01/25/science/25sheep.html. 
  19. ↑ 19.0 19.1 19.2 Bagemihl, Bruce (1999). Biological Exuberance: Animal Homosexuality and Natural Diversity (Stone Wall Inn ed.). New York City: St. Martin's Press. ISBN 9780312253776. https://archive.org/details/biologicalexuber00bage/page/164/mode/1up. 



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