Domestic rams were used as an animal model for this research because they display distinct, natural variations in sexual attraction, making them valuable in studying the biological basis for sexual partner preference. Previous studies documented that approximately 6 percent to 8 percent of domestic rams court and mate with other rams exclusively.
Sheep selected for this research were chosen after their sexual partner preference and mating habits were studied for two years. A total of 27 sheep were studied: nine rams that preferred to mate with males, eight rams that preferred to mate with females, and ten ewes. When researchers compared brains among the three groups, they recorded marked differences.
Research was focused on the preoptic hypothalamus, a region of the brain known to be involved in sexual behaviors and partner preferences; researchers identified a group of neurons there 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."
The sexually dimorphic nucleus was measured by examining the extent and level of expression of aromatase, a key enzyme in a hormonal pathway that is involved in the development and maintenance of masculine characteristics. Additional groups of neurons within the hypothalamus thought to be involved in other aspects of sexual behavior were analyzed for size differences. These groups of neurons were shown to be different in rams and ewes, and not different in the two groups of rams.
The goal of these studies is to further understand the biological basis of sexual behaviors. More specifically, researchers are trying to determine the role of brain anatomy and physiology in the expression and development of sexual behaviors and traits. The researchers believe the sheep animal model may also help provide answers about other brain-linked sexual functions.
"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 fetal 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 behaviors 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 behaviors as well."