"[It is] something that people have been whispering about at meetings for years," the study's lead author Jeffrey Mogil told Nature. "But no one had bothered to look at this systematically."
Mogil and his research colleagues at McGill University in Montreal, Canada, did look at the hypothesis systematically and found that rats and mice given a painful shot in the ankle had a 40 percent smaller pain response when a male researcher remained in the room, as opposed to female lab worker.
The same results were recorded when the negatively stimulated mice were exposed to a tee shirt worn by a man the day before.
The male-induced stress caused elevated levels of the stress hormone corticosterone in the blood of the test mice and rats, which effectively quashed the animals' pain response.
Further testing revealed that it's not just male humans that have rodents worked up and worried. The presence of male guinea pigs, rats, cats and dogs all caused similar amounts of stress.
“What this boils down to is that olfactory exposure to male stimuli is stressful for mice -- and just shockingly stressful, compared to other known stressors,” says Mogil.
Researchers say the study, which was published this week in the journal Nature Methods, should change the way scientists should approach the use of lab rats and mice.
"We need to think about animals as more like human subjects than as controllable reagents," said Joseph Garner, a biologist who studies mouse behavior at Stanford.