Rodents Fear Men (Women, Not So Much)

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Men stress out rodents, according to a new study, which found that even the smell of a man could elicit fear in mice and rats.

The study, published in the latest issue of Nature Methods, demonstrates how the hardwiring of some animals may cause them to react differently toward men or women. It has important applications for laboratory studies involving rodents, since the sex of the experimenter could affect research outcomes.

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Jeffrey Mogil of McGill University’s Department of Psychology and colleagues used what is known as “the mouse grimace scale” to compare mouse responses to pain in the presence of male or female experimenters. Reading about this study may make some grimace, themselves.

The researchers induced pain in mice via injections of an inflammatory agent. They then compared facial grimacing of the mice in the presence of either a male or a female experimenter.

Mogil and his team noted a marked reduction in pain sensation, known as “stress-induced analgesia,” when a man conducted the experiment. In keeping with that finding, the mice in the presence of men also showed increases in body temperature and corticosterone levels. Corticosterone is a stress hormone.

The same thing happened when the female experimenters donned T-shirts that previously had been worn by men, strongly suggesting that the odor of the men is what triggered the stress.

The rodents left behind their own smelly evidence.

“Supporting the assertion that exposure to male odor is stressful is the significant increase in fecal boli deposited by mice in the 30-minute testing period in which they were exposed to male, but not female, worn t-shirts,” the researchers wrote.

It could be that testosterone or male pheromones trigger fear in rodents, but the exact reasons remain a mystery.

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Pet rodents doted on by male caretakers seem to display no such fear, so it may be possible that life experiences can overcome any mice or rat predispositions.

Laboratory animals, however, obviously don’t receive that kind of pet pampering, so the study could have far-reaching implications for future research involving mice, rats and other rodents.

As Mogil and his team conclude, “Our findings strongly suggest that standard laboratory practice should account for experimenter sex when investigating any phenomenon possibly affected by stress.”

Image: Alexander H. Tuttle

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