A study of gay sheep appears to confirm the controversial suggestion that there is a biological basis for sexual preference.
Homosexuality is biological,
suggests gay sheep study
The work shows that rams that prefer male sexual partners had small but distinct differences in a part of the brain called the hypothalamus, when compared with rams that preferred to mate with ewes.
Kay Larkin and colleagues from Oregon Health and Science University found the difference was in a particular region of the hypothalamus - the preoptic nucleus. The region is generally almost twice as large in rams as in ewes. But in gay rams its size was almost identical to that in "straight" females.
The hypothalamus is known to control sex hormone release and many types of sexual behaviour. Several other parts of the hypothalamus showed consistent sex differences in size, but only this specific region showed differences that correlated with sexual preference.
The differences are almost identical to those identified by the neuroscientist Simon LeVay in his studies of the brains of gay men. His work has always been considered controversial, partly because the brains he studied were mostly from men who had died of AIDS. So it was not clear whether the differences were related to the disease or to sexual preferences.
Hormone converterBut the findings in sheep are an important confirmation of LeVay's work, says Jacques Balthazart from the University of Liege in Belgium.
Sheep are particularly interesting, he says, because besides humans, they are the only animal where the males may naturally express exclusively gay sexual preferences. As many as one in 10 rams can be gay.
Larkin's team also found that the hypothalamic region had a rich supply of the enzyme aromatase, which converts testosterone into oestrogen. It is in this form that the hormone interacts with the brain. This may help support one theory that sexual orientation, in part at least, may be related to the hormones present during fetal development, says Balthazart.
But Larkin suggests there may also be the influence of genes at work, at least in predisposing the animals to homosexuality. This is because selective breeding seems to have been responsible for the high proportion of gay sheep compared with other animals.
Larkin presented the research on Monday at the Society for Neuroscience meeting in Orlando Florida, US.
Helen Phillips, Orlando
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