Scientists use mice in experiments because over 95% of the mouse genome is similar to that of humans. But a new study, published in the journal Nature Methods, suggests male researchers stress mice and rats, which could lead to distorted findings.

The team, led by researchers at McGill University in Montreal, Canada, says growing concern over the accuracy of mouse studies has followed from the inability of some scientists to replicate study findings.

Aside from the fact that mice are small and easy to maintain in the lab, scientists employ the use of mice because they have a short breeding cycle, and they are the model organism most related to humans.

Additionally, human disease-causing mutations often cause similar diseases in mice, and the rodents have genes not present in other animal models.

Mouse models are used for a wide array of studies. Medical News Today recently reported on a mouse study - also from McGill University - that suggested pain reduces sex drive of women but not men. And another study suggested a chimeric mouse study could have prevented human deaths in a 1993 drug trial.

The stakes are high for mouse studies to yield accurate data, but this latest study throws such results into question.

To further investigate how scientists may impact on study results, the team measured stress effects on mice from both the presence of male and female researchers, as well as cotton T-shirts worn previously by both male and female experimenters.

Commenting on their study, Prof. Jeffrey Mogil, senior author from McGill, says:

"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."

Of mice and men: stress is in the smell

The researchers found that the presence of male - not female - experimenters elicited a stress response in both mice and rats that was equal to that caused by forced stressful environments, such as restraining the rodents in a tube for 15 minutes or forcing them to swim for 3 minutes.

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Mice and rats exhibited a significant stress response to the chemosignals secreted by male researchers, a response that could potentially skew findings, researchers say.

And this stress response of mice to men, say the researchers, is down to smell.

According to the team, all mammals share the same chemosignals, or pheromones, which human males tend to secrete at higher concentrations from the armpit, compared with women; these chemosignals alert rodents that male animals are nearby.

But pain is not the only sensation affected by male experimenters; other "behavioral assays sensitive to stress" were also affected, notes the team.

Study leader Prof. Robert Sorge says:

"Our findings suggest that one major reason for lack of replication of animal studies is the gender of the experimenter - a factor that's not currently stated in the methods section of published papers."

But Prof. Mogil says that by altering experimental procedures slightly, the problem could be solved.

"For example," he adds, "since the effect of males' presence diminishes over time, the male experimenter can stay in the room with the animals before starting testing. At the very least, published papers should state the gender of the experimenter who performed the behavioral testing."

Medical News Today recently reported on another mouse study, in which they discovered a brain pathway involved in emotional behaviors.