Differences in the brains of gay sheep when compared to heterosexual sheep have been identified by researchers from the Oregon Health & Science University (OHSU) in collaboration with the U.S. Sheep Experiment Station in Dubois, Idaho.

Researchers, hoping to expand on existing human studies that have found differences in the brains of heterosexual men, homosexual men and women, found the first evidence that the same differences exist in animals.

The sheep were selected for the study after more than two years of observation to determine their sexual orientation and mating habits. Of the 27 sheep chosen for the study, nine were males (or rams) that only mate with other males, eight were males that only mate with females and ten were female sheep or ewes.

When focusing on the preoptic hypothalamus, a part of the brain linked to sexual behaviours, researchers found significant differences between these three groups of sheep.

A group of neurons were also identified in that part of the brain called the sexually dimorphic nucleus.
"Interestingly, this bundle of neurons is smaller in ewes and in rams with same-sex preferences than it is in rams that prefer ewes," said Kay Larkin, Ph.D., a postdoctoral fellow in physiology and pharmacology in the OHSU School of Medicine, and lead author of the paper. "We also determined that the volume of the sexually dimorphic area is approximately the same in rams that prefer rams as it is in ewes."

Other groups of neurons within the hypothalamus possibly linked to sexual behaviour were also analysed by researchers for size differences. These groups of neurons were shown to be different in male and female sheep, but not different in the two groups of rams.

"Future studies will address functional aspects of the observed differences in the hypothalamus and test the hypothesis that differences in brain anatomy and sexual partner preference arise as a consequence of hormone exposure during foetal development," said Charles Roselli, Ph.D., professor of physiology and pharmacology in the OHSU School of Medicine and senior author of the paper. "While we realize that sexuality is more complex in humans than reproductive behaviours in sheep, this model will help illuminate the basic principles that apply to all mammals, and may be helpful in understanding the biology of human behaviours as well."

Domestic rams were used in this study because of their variance in sexual orientation; approximately six percent to eight percent of domestic rams are sexually exclusive with other rams, according to previous research estimates.

Findings were presented at the annual meeting of the Society for Neuroscience in Orlando, FL, on November 4, 2002.