During the course of an experiment, students at McGill University in Montreal noticed something odd: Rodents didn't seem to be showing signs of pain if they were handled by male students.
The observation led to an experiment, which led to a finding that when mice are left alone with a man, they had an increase in the hormone corticosterone, which acts like a pain reliever.
The scientists tested the mice with women and men and women wearing T-shirts that had been worn overnight by men. What they found is that mice were reacting to the presence of certain pheromones produced by men.
"It's a primordial response," Jeffrey Mogil, one of the authors of the story, told Science magazine. "If you smell a solitary male nearby, chances are he's hunting or defending his territory." Pain means weakness, and you wouldn't want to show that.
The findings may have implications for other experiments that use rodents.
"The amount of stress felt by the rodents was 'massive,' said Jeffrey Mogil, a psychologist at McGill University and an author of the study, comparable with being 'in a very small tube so the mouse can't move for 15 minutes.'
"The findings, published in Nature Methods, could have far-reaching repercussions for research. Rodents account for more than 95 percent of all lab animals, according to the National Association of Biomedical Research. If a researcher's sex might affect results, it should be considered a confounding factor, Dr. Mogil said.
"And the effects may not be limited to living animals. 'Consider a study on liver physiology,' Dr. Mogil said. 'What if those liver cells came from a rat who, at the moment of sacrifice, had really high stress levels because it was killed by a man, or low because it was killed by a woman?' "
Discover magazine reports that it is not clear how these findings may affect future research.