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Biology behind homosexuality in sheep, study confirms

elated to dominance or flock hierarchy; rather, their typical motor pattern for intercourse is merely directed at rams instead of ewes.

"They're one of the few species that have been systematically studied, so we're able to do very careful and controlled experiments on sheep," Roselli said. "We used rams that had consistently shown exclusive sexual preference for other rams when they were given a choice between rams and ewes."

The study examined 27 adult, 4-year-old sheep of mixed Western breeds reared at the U.S. Sheep Experiment Station. They included eight male sheep exhibiting a female mate preference female-oriented rams nine male-oriented rams and 10 ewes.

OHSU researchers discovered an irregularly shaped, densely packed cluster of nerve cells in the hypothalamus of the sheep brain, which they named the ovine sexually dimorphic nucleus or oSDN because it is a different size in rams than in ewes. The hypothalamus is the part of the brain that controls metabolic activities and reproductive functions.

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 the androgen hormone can facilitate typical male sexual behaviors. Aromatase expression was no different between male-oriented rams and ewes.

The study was the first to demonstrate an association between natural variations in sexual partner preferences and brain structure in nonhuman animals.

The Endocrinology study is part of a five-year, OHSU-led effort funded through 2008 by the National Center for Research Resources, a component of the National Institutes of Health. Scientists will work to further characterize the rams' behavior and study when during development these differences arise. "We do have some evidence the nucleus is sexually dimorph
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Contact: Jonathan Modie
modiej@ohsu.edu
503-494-8231
Oregon Health & Science University
8-Mar-2004


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