Despite what the Bible says, lust is inborn: our species couldn’t survive if we didn’t yearn to have sex with one another. But exactly how sexual desire develops is, even at this late date, poorly understood. We know more about the musculature of the left femur than we do about how our brains acquire and process the desire to engage sexually.
That’s why I was intrigued by a new study in the journal Psychological Science. Typically I don’t care about studies of rat behavior because, after all, the subjects are rats. But rats do share most of our DNA, and there’s no way to study human sexual development without getting really creepy. (More on Time.com: 5 Little-Known Truths About American Sex Lives)
The authors of the new study — psychologist Cynthia de Medeiros of the University of Toronto and four colleagues — followed a group of 88 rats after they were born. Rats live only about two or three years, so it was possible for the researchers to track their sexual development from birth to death. Here’s the news: boy rats who have more sisters are less reliable heterosexuals than boy rats who have fewer sisters. That’s not to say having a sister makes you gay, but the boy rats with lots of sisters were significantly less interested than other boy rats in mounting girl rats.
Previous rat studies have shown that how mothers treat their newborns can influence whether genes in those newborns become activated or stay muted. Rat pups that are neglected can literally end up with worse brain chemistry — less serotonin, fewer glucocortinoid receptors — than identical twin siblings that were nurtured (usually by licking, which is sort of gross, but again: they are rats). (More on Time.com: ‘It Gets Better’: Wisdom From Grown-Up Gays and Lesbians to Bullied Kids)
In the new study, the 88 rats were kept in Plexiglas cages and subjected to a life of extreme leisure: they were fed ad libitum and kept at a constant 75 degrees F (24 degrees C). Rats hit puberty when they are about 60 days old, so the researchers started watching them especially closely around that age. I’m sure it was embarrassing for the rats, but the researchers watched as the males walked up a ramp and then mounted (or didn’t) some lovely classmates. The researchers even kept track of whether the males ejaculated.
The results showed that rats who grew up for those first 60 days with sisters were less likely to mount than the ones who grew up with brothers. It didn’t matter whether the rats had been in the womb with their sisters — only that they grew up in early life with sisters. The study suggests that psychology, not hormones or chemicals, is at work. (More on Time.com: PHOTOS — Same Sex Overtures Across Species).
How to explain these findings? The authors offer a couple of theories. Maybe boy rats are less interested in girl rats because they are already familiar with girls? My favorite theory is that “males reared in a female-based litter may be less attractive to females because they secrete less or different odor cues.” That’s another way of saying that your sisters will make you use deodorant. Just be careful, guys: the ladies like the man smell.
More on Time.com: