Rodents are the workhorses of biomedical labs. So it’s important to know if they’re stressed out, which could affect results. Now we know that lab rodents may be regularly exposed to a big stressor: men.

Researchers noticed that mice showed a lower pain response—a sign of stress—when a human was present. So they put mice and rats into clear cubicles, where their faces were visible, and injected them with an irritant. The rodents expressed pain through grimacing when no humans were nearby. But when confronted with a male researcher, or even just his odoriferous T-shirt, the animals grimaced less.

Female observers did not get the same rodent reaction.

Turns out that smelling a human male made rodents produce higher levels of a stress hormone and increase their body temperature. That stress response also blunted their sensitivity to pain. The study is in the journal Nature Methods. [Robert E. Sorge et al, Olfactory exposure to males, including men, causes stress and related analgesia in rodents]

Researchers may have to come up with ways to ward off the stress effects of male researchers. Because to a rodent, men smell like hell.

—Sophie Bushwick

[The above text is a transcript of this podcast.]

[Scientific American is part of Nature Publishing Group.]