The basis for homosexual behavior has frequently—and often hotly—been debated. Is it nature? Nurture? A combination of both? Researchers from the Universidad Veracruzana, Mexico, have thrown their hat into the ring. They have been able to show that conditioned homosexual preference in male rats can be induced by oxytocin and the psychoactive drug quinpirole.

The drug quinpirole is known to have the same effect on the brain as the neurotransmitter dopamine, which plays a major role in reward-motivated behavior. The process of having sex reinforces sexual preference between males and females, as the brain releases huge levels of dopamine during the act, giving the rat a happy hit and conditioning the animal's preference for its mate. This is backed up almost immediately after ejaculation, when the male's brain is flooded with the hormone oxytocin. This is thought to act to crystallize their social attachment to their mate by increasing their trust, reward, and inducing a state of calm.

When sexually naive male rats were exposed to either the hormone oxytocin and/or quinpirole, and then made to cohabit with other sexually active males, they developed a social preference for the other males, even when the drugs were no longer present in their system. Interestingly, their preference wasn’t just limited to that social situation. When given a choice days later between a male and a sexually receptive female, the treated rats displayed sexual preference not for the females, but again for the males.

So how do you tell if a rat is socially, or sexually, attracted to another rat of the same sex? Well, the researchers used certain cues to ascertain their proclivity, including how much time the treated rats spent with the other males, how much body contact they had, and how many times they sniffed their genitals. In addition to these friendly gestures, the treated males also displayed some more sexy signals, such as “non-contact erections” and “female-like solicitations.”

Amazingly, the effects of the hormones and drugs were not just limited to behavioral responses, but also changed the physiology of the rats' brains. It has long been thought that a region of the hypothalamus in the brain, the sexually dimorphic nucleus of the medial preoptic area (SDN-POA), is related to sexual preference. For all species of mammals so far investigated, it has been shown to be sexually dimorphic, with the male SDN 5-7 times larger than the females. This is thought to be related to the amount of testosterone experienced during the first postnatal days. During the experiment, the researchers found that those rats treated with oxytocin saw their SDN shrink.

All, however, is not so simple. Whilst the SDN did shrink with exposure to oxytocin, it happened regardless of their partner preference, and so the size of the SDN did not predict a same-sex partner inclination. This goes against other studies, one of which suggested that the size of the SDN can be related to sexual preference in male sheep and that homosexual behavior might be related to differences in brain anatomy.

But let's not get ahead of ourselves. Locking two men in a room and feeding them oxytocin and quinpirole isn't going to turn them gay, but the study does suggest that it is possible for apparently heterosexual male rats to develop a conditioned homosexual social and sexual leaning, given the right conditions.