It takes a lot to deter a male from wanting sex. A new study has found that male mice keep trying to copulate even when they are in pain, whereas females engage in less sex. But when given drugs that target pleasure centers in the human brain, the females again became interested. The findings could shed light on the nature of libido across various animal species.
To assess how pain influences sexual desire, researchers first identified pairs of mice that wanted to have sex. “What we found early on was not all mice will mate with each other,” says clinical psychologist Melissa Farmer, who led the study while earning her Ph.D. at McGill University in Montreal, Canada. The team set up the rodents on a series of “dates,” during which a male and female were paired together for 30 minutes. Couples that copulated for most of the session were deemed compatible and moved into a cage with separate rooms. A small doorway allowed a female mouse to freely cross over from her chamber, but the male—which is larger—could not.
The scientists then induced pain in males or females by applying a small dose of inflammatory compounds to the cheek, tail, foot, or genitals. The sensation would primarily be soreness, like a bad sunburn, says Farmer, who now works at Northwestern University’s Feinberg School of Medicine in Chicago, Illinois.
Female mice that were in pain, whether genital or nongenital, spent 50% less time with their male partners, implying a decrease in sexual motivation. Even when they did visit their paramours, females wouldn’t allow males to mount them with the same frequency, the team reports online today in The Journal of Neuroscience.
The researchers then set up the reverse scenario in which males dictated the encounters. They placed both mice in an “open-field” cage with no barriers. Males desired just as much sex even if they were in pain, and the researchers observed no significant difference in mounting attempts or other sexual behaviors.
Pain-relieving drugs increased females’ willingness to mate, and two sex-drive enhancers—apomorphine and melanotan II—did the same. Both drugs tweak neural circuits tied to pleasure and arousal.
Women exercise more caution when selecting a mate, which many psychologists attribute to cultural expectations, such as when female modesty is pushed by some religions. In other words, social norms might impose female repression of sexual desire. In contrast, Farmer and her colleagues conclude, somewhat controversially, that women might simply be more biologically prone to sexual repression in certain contexts.
Jill Becker, a behavioral neuroscientist at the University of Michigan, Ann Arbor, supports the idea. “It’s valid given the amount of female investment [in reproduction] and the way that these systems must have evolved to ensure reproductive success.”
Yet other researchers hesitate to build such a major bridge between mice and man. “Claiming an evolutionary basis to sexual behavior based on one study trivializes the rich complexity of human behavior,” says Barry Komisaruk, a behavioral neuroscientist at Rutgers University, Newark, in New Jersey. “It implies inevitability to a woman's behavior—that she has no voluntary control. That’s too much to extrapolate from one mouse study.”