The scent of a man influences the behavior of mice and rats during experiments, increasing their stress levels and dampening their pain responses. If lab rats are scared of men but not women, that's going to skew results... and spark off some huge problems for science.
For years, anecdotal evidence has shown that lab rodents react differently to male and female experimenters. In particular, when working with men, a mouse’s pain response is dulled -- an effect called stress-induced analgesia.
“Scientists whisper to each other at conferences that their rodent research subjects appear to be aware of their presence, and that this might affect the results of experiments, but this has never been directly demonstrated until now,” Jeffrey Mogil from McGill University, Montreal, says in a news release. When scientists are unable to replicate their results, that puts their reliability to question.
Mogil and colleagues decided to look at this diminished pain response systematically. They injected the rodents’ ankles with an inflammatory agent, and then measured their pain using the “mouse grimace scale,” which examines facial expressions. The rodents showed 36 percent decrease in pain response when men, rather than women, conduct the experiment. They also exhibit increased levels of the stress hormone corticosterone, as well as body temperature.
The stress men produced in mice and rats is equivalent to restraining them for 15 minutes in a tube or forcing them to swim for three minutes. This stress-induced reaction made mice and rats of both sexes less sensitive to pain.
With further tests, they found that the “shockingly stressful” effect of male experimenters was due to smell: an olfactory stimulation from a mixture of chemicals present in human sweat. The team was able to reproduce as strong of an effect just by placing cotton T-shirts worn the night before by men next to the rodents.
The same effect was achieved when the rodents were exposed to bedding material and pet beds from other male mammals, including guinea pigs, cats, dogs, and unfamiliar rodents. But female handlers produced no such effects -- in fact, their presence (or their t-shirts) counteracted the impact of men.
That’s because the effects were caused by chemosignals, or pheromones that men secrete from their armpits at higher concentrations than women. All mammals share the same chemosignals that indicate the presence of nearby males. “It’s a primordial response,” Mogil tells Science. “If you smell a solitary male nearby, chances are he’s hunting or defending his territory.” If you’re in pain, you’re showing weakness.
This is the first evidence that an experimenter’s gender could be throwing off behavioral studies. It’s serious, but it’s not as if scientists need to redo decades of animal research. “It’s a confounding factor, but not a fatal one,” Mogil tells Science. “We can change the bath water without throwing out the baby.”
The good news, he adds: “the problem is easily solved by simple changes to experimental procedures.” Since the effect of males’ presence diminishes over time, the experimenter can stay in the room with the animals before starting the tests. “At the very least, published papers should state the gender of the experimenter who performed the behavioral testing.” Or, Mogil tells the Verge jokingly, "fire all the men -- or have them chaperoned by a woman."
The work was published in Nature Methods this week.
Image: Alexander H. Tuttle
