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Ever felt sheepish about being gay? July 4, 2009

Posted by Geekgirl in anatomy, brain, Developmental.

You aren’t alone. Approximately 8% of domestic rams (male sheep) are gay. Well, let’s rephrase that. They have sexual relations with other male rams. Who knows, they could be transgendered.  This occurs naturally. Since they are mammals, they make a good model for studying the biological mechanisms involved in sexual orientation. (I know all the arguments about using or not using animals for research. I’m just the messenger, ok?)

What did these researchers find? For starters, it is typical for the cells in the hypothalamus to have larger nuclei in males than in females. Rams with smaller nuclei preferred male rams. Before birth, fetuses were exposed to either testosterone, to determine the effect on females, or an aromatase inhibitor to determine the effect on males. What is an aromatase inhibitor? It prevents the production of estrogen. The theory is, if the male rams are homosexual due to too much estrogen, treating them with an aromatase inhibitor should cause their cell nuclei to be larger, like the heterosexual male rams.

As you keep reading posts, you’ll start to see some themes and how some pieces of the puzzle keep reappearing. Notice the last sentence talks about an androgen receptor mechanism? Check out the earlier post about MtF transgendered folks having a longer than average androgen receptor gene. Notice that this article talks about the size of a cells nucleus being larger in males? Look back at the post about gender identity and anatomy versus the brain. Same phenomena happens in humans. Is a pattern emerging? That’s part of what research is all about.

Want to know the results? (Full text is not available for free)

J Neuroendocrinol. 2009 Mar;21(4):359-64.

Prenatal programming of sexual partner preference: the ram model.

Roselli CEStormshak F.

Department of Physiology and Pharmacology, Oregon Health and Sciences University, Portland, OR 97201-3098, USA. rosellic@ohsu.edu

In our laboratory, the domestic ram is used as an experimental model to study the early programming of neural mechanisms underlying same-sex partner preference.

This interest developed from the observation that approximately 8% of domestic rams are sexually attracted to other rams (male-oriented) in contrast to the majority of rams that are attracted to oestrous ewes (female-oriented).

One prominent feature of sexual differentiation in many species is the presence of a sexually dimorphic nucleus (SDN) in the preoptic/anterior hypothalamus that is larger in males than in females.

Lesion studies in rats and ferrets implicate the SDN in the expression of sexual preferences. We discovered an ovine SDN (oSDN) in the preoptic/anterior hypothalamus that is smaller in male- than in female-oriented rams and similar in size to the oSDN of ewes.

Neurones of the oSDN show abundant aromatase expression that is also reduced in male-oriented compared to female-oriented rams. This observation suggests that sexual partner preferences are neurologically hard-wired and could be influenced by hormones.

Aromatase-containing neurones constitute a nascent oSDN as early as day 60 of gestation, which becomes sexually dimorphic by day 135 of gestation when it is two-fold larger in males than in females. Exposure of fetal female lambs to exogenous testosterone from days 30-90 of gestation resulted in a masculinised oSDN. These data demonstrate that the oSDN develops prenatally and may influence adult sexual preferences.

Surprisingly, inhibition of aromatase activity in the brain of ram foetuses during the critical period did not interfere with defeminisation of adult sexual partner preference or oSDN volume. These results fail to support an essential role for neural aromatase in the sexual differentiation of sheep brain and behaviour.

Thus, we propose that oSDN morphology and male-typical partner preferences may instead be programmed through an androgen receptor mechanism not involving aromatisation.



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