About 8 percent of domestic rams display preferences for other males as sexual partners. Scientists don't believe it's related 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 dimorphic in late gestation," Roselli said.
They would also like to know whether sexual preferences can be altered by manipulating the prenatal hormone environment, such as by using drugs to prevent the actions of androgen in the fetal sheep brain.
I predict that some day it will be possible to alter this structure in adult humans. Will more people at that point choose to switch from heterosexual to homosexual orientation or vice versa? There are more heterosexuals to make the switch. So that tilts the odds in favor of hetero to homo transitions. But on the other hand the stigma associated with homosexuality is still great enough to provide incentive to switch in the other direction.
If a test on fetuses for sexual orientation can be developed and if a treatment for altering fetal sexual orientation can also be developed then that would probably favor a net shift toward heterosexuality since most parents would choose to guarantee their children will be heterosexual. Also, even without such a test if it becomes possible to control the genetic and environmental factors that influence the development of the part(s) of the brain that determine sexual orientation then many parents will opt to, metaphorically speaking, tilt the playing field even more toward the odds of heterosexuality in their offspring. In other words, it seems reasonable to expect that most parents will avail themselves of medical treatments that will make sure their kids will turn out to be heterosexuals.
Whether the ability to alter sexual orientation at the fetal and adult stages will cause a net change in the balance of the population in a more homosexual or heterosexual direction is hard to predict. It seems likely that males and females will, on average, make different decisions. So the ratio of male to female homosexuality could either increase or decrease once sexual orientation becomes malleable. Also, the ratio will likely diverge between cultures and population groups as different groups make different choices on average.
Hello there little pup, I'm Big Gay Al. [Sparky looks at him] Have you been outcast? [Sparky pants an affirmative] Well, then I'm so glad you found my Big Gay Animal Sanctuary. We're all big gay friends here. Would you like to live with us? [Sparky pants an affirmative] Come on in little fellow, nobody will ever oppress you here.
|Share |||Randall Parker, 2004 March 09 11:53 AM Biological Mind|