LOS ANGELES -- It's taken an army of mice (and clever Canadian researchers) to crack open an old sexual chestnut: For females, "Not tonight, dear, I have a headache" is not (always) a passive-aggressive rebuff to a mate's sexual invitation. It's a biological phenomenon with deep evolutionary roots.
Bodily pain puts a damper on sexual desire, new research has revealed. And pain reduction can help restore libido squelched by physical discomfort.
In the new study, the libido-busting effect of pain was not seen in male mice, who sought to mate with females whether or not the males (or potential female partners) were in pain. Pain made a female mouse significantly less receptive to mating, irrespective of where the hurt was.
For males, even a pain in the penis did not dampen the urge to have sex.
Over eons and across species, that gender-specific response to pain has likely served to reduce reproduction under circumstances less-than-ideal for a potential offspring's survival. A mother in pain, after all, is a mother whose full attention and physical strength may not be available to nurture her babies and protect them from predators.
Males were more apt to be off looking for another chance to spread their genetic material. So standing down for pain would serve no evolutionary purpose.
The study, published Tuesday in the Journal of Neuroscience, gleaned these insights by gauging responses of sexually compatible pairs of mice to injections of agents that induce inflammation in rodents -- the food additives carrageenan and zymosan.
In some pairs, the female got the hurt-shot; in other pairs, the male did. To rule out the possibility that mating was disrupted by specific discomforts, researchers injected the pain-inducing shots in various bodily sites. They chemically ensured that female mice would be sexually appealing to male mice. They left the pairs in a cage that let the female mouse stay and mate or withdraw to an adjoining room by herself.
Once they established that females in pain were far less likely to mate than those without pain, researchers explored what, if anything, helped. To some female mice, they administered the analgesic pregabalin, a drug for chronic nerve pain known by its commercial name, Lyrica. To others, they administered "pro-sexual" agents known to rev up sexual behavior in rodents.
The Lyrica was no aphrodisiac. It didn't promote more sexual activity among female mice that were not in pain.