Researchers in China have reported a series of experiments indicating that sexual orientation — or at least sexual preference — in male mice is determined by the levels of serotonin in the rodents’ brain.
Mice first bred a little of boy rats whose brains were not receptive to serotonin then put them in a cage with females and other males. The male mice with brains not receptive to serotonin ignored all the other mice, male and female, at least in a sexual way. But when they put the not-receptive-to-serotonin boy rats in a cage with only other boy rats, “the modified males were far more likely to mount the male and emit a “mating call” normally given off when encountering females than unmodified males were,” according to reports on BBC News.
The researchers then bred a male mice that lacked the tryptonphan hydroxylase 2 gene, necessary for the production of serontonin and conducted the experiments again, getting similar results.