Other scientists have offered fascinating but unpersuasive explanations, most of them focusing on some kind of compensatory benefit, in the same way that the gene responsible for sickle cell anemia also protects against malaria. That argument caused a stir when he and a colleague proposed it six years ago, but with no research done to test it, it remains just another theory. If a prime motivation of all species is to pass genes on to future generations, and gay men are estimated to produce 80 percent fewer offspring than straight men, why would a gay gene not have been wiped out by the forces of natural selection? This evolutionary disadvantage is what led former Amherst College biologist Paul Ewald to argue that homosexuality might be caused by a virus - a pathogen most likely working in utero. There is, however, a towering question that Sanders's study will probably not be able to answer. (Again, it's conceivable that these differences could be showing effect rather than cause.) Researchers found that a clump of neurons similar to the one LeVay identified in human brains was also smaller in gay rams than straight ones. About 8 percent of domestic rams are exclusively interested in sex with other rams. This research once again connecting the hypothalamus to sexual orientation comes on the heels of work with sheep. Instead, their hypothalamus lit up when they smelled the male-sweat compound, which was the same way straight women had responded. Brain scans showed that when straight men smelled the female urine compound, their hypothalamus lit up.
The first compound came from women's urine, the second from male sweat. In May, Swedish researchers reported finding important differences in how the brains of straight men and gay men responded to two compounds suspected of being pheromones - those scent-related chemicals that are key to sexual arousal in animals.