The differences are similar to those seen in some homosexual humans, but probably only go a small way to explaining the causes of different sexual preferences, the team at Oregon Health & Science University said.

"We are not trying to explain human sexuality by this study," Charles Roselli, a professor of physiology and pharmacology who led the study, said in a telephone interview. "Whether this is a big component of what contributes in humans, that's still debatable."

Working with a team at the US Department of Agriculture's Sheep Experiment Station in Dubois, Idaho, Roselli's team studied 27 sheep--10 ewes, nine rams that mated only with other rams and eight rams that mated only with females.

The "gay" sheep are strongly homosexual, Roselli said.

"They don't pair-bond," he said. "But they are exclusive. They don't court or mate with females. They only court and mate with males."

First the scientists watched the sheep to be sure of their behavior--something that cannot be done with humans. Then they took apart their brains.

"There had been reports in humans that a certain area of the hypothalamus, the preoptic area ... was usually larger in males than females," Roselli said. This area was also found to be larger in heterosexual humans than in homosexual men.

But the researchers had used the brains of men who had died of AIDS in their study, which meant the disease or drugs used to treat it could have had an effect on the brain.

"With an animal model you can be more selective and do more controlled studies," Roselli said.

The sheep had similar differences in their brains, the researchers told a meeting in Orlando, Florida, of the Society for Neuroscience.

"In a sense we confirmed what's been found in humans," Roselli said.

The brain cells in this area also made greater amounts of an enzyme called aromatase in the heterosexual rams. Aromatase is involved in the action of testosterone, the so-called male hormone.

This does not mean the gay rams had less testosterone in their brains, Roselli stressed. "It is not necessarily the activational effect of the hormone," he said. Other types of neurons are probably active--they just have not been identified yet.

No differences in testosterone relating to sexuality have been found either in the sheep or in humans, he said.

"It's not that gay men have lower levels of testosterone," he said. "And it's not the case with these sheep."

Roselli believes that exposure to hormones while still in the mother's womb may affect the brain and cause differences in sexual preference, and more experiments will aim to show whether this is true.

( November 5, 2002)