Lab Animals Stressed Out by Men, Study Finds

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.

Fear Factor

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.”

Skewed Results?

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.”

Convincing Case

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.”

James Owen is a journalist and author based in Stockholm, Sweden. After cutting his teeth on the news and features desks of several UK newspapers, he struck out as a freelance writer, specializing in life sciences and natural history. His fish biography 'Trout' (Reaktion Books) was published in 2012.
  • kurian

    interesting finding wonder why the female scent had a calming effect?

  • Amber Halsell

    Better solution would be to END ANIMAL TESTING

  • Auroness

    Larger mammals are affected as well. When I work at a sheep farm, the animals showed no stress when I was in the pen. When the men entered the pen, the sheep would start moving around and rams would be more aggressive. On a stud farm, the owners specifically hired female groomers and handlers, because the horse were calmer.

  • bonnie

    ^ to the ‘end animal testing’

    maybe this is the quick evolution of lab rats, the numb sensing rats survived years of torture as a species

  • Richard Birr

    In the course of my interests and studies I have found that the interactions of living matter in it’s classifications maybe more linked to what life could be like than it is now.
    Given all life including those that may harm one another do have a desire to continue with their lives. From tiny things and larger things like viruses and on through the invertebrates etc. to us Humans could it be that the sharing of desire for more life to come in a rational way bring about harmony with the rocks and waters of the shared planet we call Earth.

  • Jen L

    Odd….I have the same reaction too!

  • Yvonne

    Yes I am on board end animal testing then the fear will end

  • Kamasue

    Brilliant idea! Stop animal testing and test on the sick children that some of these tests are trying to help. Get over it people, they’re just rodents!

  • Kel

    I’ll be the same effect would be found with teenagers. Fear of dad keeps mine in line.

  • Kel

    “I’ll bet …” Not “I’ll be”

  • AnnieLaurie Burke

    Not sure if lab animals would have the same reaction to women, but I sympathize with the women who have had the same reaction as the mousies!

  • Ro

    I have read this is how the human race/man survived. Animals avoided them because of their ordor.

  • Janet Wolfe

    While working with lab rats in attempting to get them to run a maze to get food, I found that just the individual sitting in the room with the rat made a difference. As long as the same person ran the test every day, the rats were more “at ease” and ran the maze much more easily. A different person would cause the rats to take much more time and make more mistakes.
    When I was holding a rat to demonstrate how to hold her and take a specimen, the rat grabbed hold of my thumb because there were different people in the room and she became scared. Both before and after that incident, she was very easy to handle, which I did every day for a period of time.
    So even different people can have a different affect on the animal’s reaction.
    By the way, when the rat – I never tried it with the mice – gets comfortable up against your shoulder, it will purr. So you can pet it just as you would your cat or kitten! Of course, that can’t last very long – there is work to be done.

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