Men can stress out rodents—and probably most other mammals, including pets—merely with a whiff of their armpit sweat, a new study says.
The far-reaching discovery, whose findings about male-induced stress may also apply to humans, comes from observing mice and rats that were found to be so unsettled by the masculine aroma that it numbed their sense of pain. Such pain suppression is a well-known animal response to danger. (Also see “Grasshopper Mice Immune to Bark Scorpion Stings.”)Mice can be particularly fearful of men. Photograph by Joel Sartore, National Geographic
Male body odor “induced a robust physiological stress response that results in stress-induced analgesia [pain relief],” according to the study, published April 28 in the journal Nature Methods.
The effect on lab rodents that was produced by the presence of male researchers—and by the presence of their T-shirts after being worn overnight—lasted 30 to 45 minutes, reported a team led by Jeffrey Mogil, head of the Pain Genetics Lab at McGill University in Montreal, Canada.
Mogil said the phenomenon, triggered by a cocktail of male-related pheromones released in sweat, is unlikely to be confined to mice and rats: “I would predict that we will eventually find that this is true in all mammals.”
The scientists also tested the effects of female researchers and their overnight T-shirts, but there was no stress-induced analgesia or similar stress effect. In fact, the female T-shirt odor was seen to have a calming effect on the mice, canceling out the effect of the male odor.
The brain of a frightened animal—a deer that knows it’s being hunted, for example—temporarily suppresses feelings of pain so it can put all its energies into escaping. The phenomenon is known as stress-induced analgesia.
Mogil and colleagues tested mice because they’d noticed during certain experiments that some mice didn’t show symptoms of pain. (See “Brain Region for Overcoming Fear, Anxiety Found.”)
When men or their used T-shirts were present, the mice exhibited signs of being scared, including the pain-numbing analgesia—assessed in part by using a method known as the Mouse Grimace Scale—higher stress hormone levels, increased body temperature, and increased pooping.
The study identified three pheromones, or “chemosignals,” released in male armpit sweat as the stress trigger in the rodents. “We expect in fact that the real olfactory stimulus isn’t any one of these chemicals, but a complex mixture of them,” Mogil said.
The reason for the animals’ fear, Mogil said, may be that some species are hardwired “to have an anxiety response when [they] smell an isolated male.
“Isolated males are up to no good, right?” he said. “They’re hunting or defending territory.” (Related: “Men Anger Opponents on Purpose.”)
“Of course, if there’s no danger and you don’t get attacked, then the stress habituates,” Mogil said. “It’s a very robust effect, but it’s not very long-lasting.”
The finding could have major implications for scientific research involving many different kinds of animal experiments: The results of those experiments could be skewed depending on the sex of the researchers carrying out the tests, Mogil said.
A person’s sex may not only influence the outcome of behavioral studies with live mice or rats, but also the results of other tests involving rodents. (Read “The Mystery of Risk” in National Geographic magazine.)
For example, rats used in a study of liver cells might have either very high or very low levels of stress hormones in their livers depending on the sex of the researcher who handled them.
Short of firing all male personnel in his lab, a solution would be to “sit in the room for 30 to 45 minutes with the mice, before you start the experiment,” Mogil said.
But he said scientists would be loath to adopt that approach: “That’s way too time-consuming and boring.”
Paul Flecknell, head veterinarian at Newcastle University’s Institute of Neuroscience in the United Kingdom, said the new study makes a convincing case for a male stress factor on rodents in the lab.
“You intuitively felt there was something going on, but nobody’s ever done such an extensive and rigorous study to show that this is a real effect,” he said.
“The really intriguing thing is that this effect really should apply to more than just pain,” he said, noting that if a male presence “changes the animal’s level of arousal, then it can change all sorts of responses.”
The ramifications of the new finding could go way beyond the animal lab. For instance, pets might react differently to a male vet than to a female vet. (Take National Geographic’s pet quiz.)
That’s quite possible, according to Flecknell, although he doesn’t think pets would react the same way as lab animals, even if they were pet rats or mice.
“We carry out a whole range of interactions with our pets that socializes them and gets them habituated to people,” he said. “That’s not to say that when they come across a strange individual that it isn’t going to cause a reaction, but the reaction may be smaller relative to the lab animal’s.”
As for interactions between humans, Mogil suspects the male body odor effect on people, if it exists, “is probably going to be subtler and shorter.
“Presumably, humans can convince themselves pretty quickly that there isn’t actually a danger,” he said. “So if we’re going to show this in humans, I think we’re going to have a very short time window.”