Mice and rats, long the experiment subjects of choice by scientists throughout the world, apparently get stressed around men, according to new research from McGill University in Montreal, and that stress, in turn, leaves the rodents less receptive to stimuli and, arguably, less reliable for tests.

Findings published online April 28 in the journal Nature Methods explained the presence of male experimenters produced a stress reaction in mice and rats equivalent to that caused by restraining the rodents for 15 minutes in a tube or forcing them to swim for three minutes.

The stress-induced response left both mice and rats of both sexes less sensitive to pain.

"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, a psychology professor at McGill and senior author of the paper, said in a news release.

The research team, which included pain experts from Haverford College in Pennsylvania and the Karolinska Institutet in Sweden, as well as a chemosensory expert from Université de Montreal, determined the effect of male experimenters on the rodents' stress levels was linked to scent.

Cotton T shirts, worn the previous night by male or female experimenters, were placed alongside lab mice, inducing effects identical to those caused by the presence of the experimenters themselves.

Further testing proved the stress was triggered by chemosignals, or pheromones, that men secrete from the armpit at higher concentrations than women.

The chemosignals in question, shared by all mammals, tipped off the rodents that male animals were nearby, the research team said.

Meanwhile, the physical effects, which weren't limited to pain, were not produced when the critters were in the presence of females or the shirts they wore.

"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 sections of published papers," said Robert Sorge, a psychology professor at the University of Alabama, Birmingham, who led the study as a postdoctoral fellow at McGill.

The problem, Mogil said, "is easily solved by simple changes to experimental procedures. For example, 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."