LOS ANGELES — It’s taken an army of mice — and a group of clever Canadian researchers — to crack open an old sexual chestnut and get at the meat inside: For women, “Not tonight dear, I have a headache” is not a passive-aggressive rebuff to a mate’s sexual invitation — not always, at least. It’s a biological phenomenon with deep evolutionary roots.

Even for females who’ve never watched a 1950s movie or been schooled in the art of sexual gamesmanship, bodily pain puts a serious 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 were in pain. Pain made a female mouse significantly less receptive to mating irrespective of where the hurt was: in the cheek, tail, hind legs or genitals.

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 that are 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 less likely to be tending a brood and more likely to be off looking for another chance to spread their genetic material. So standing down for physical pain would have served no evolutionary purpose.