Homosexuality is biological, suggests gay sheep study 10:51 05 November 02 NewScientist.com news service A study of gay sheep appears to confirm the controversial suggestion that there is a biological basis for sexual preference. 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.