Rats and mice could be stressed by scent of their male experimenters, which would skew the results of the study's finding.

A recent study found that the presence of male experimenters created a stress response in the rodents that did not occur when they were faced with human females, a McGill University news release reported.

"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 the news release.

The research team found the stress response was caused by "the scent of a man," the news release reported.

They determined this by placing cotton T-shirts that had been worn overnight by either a male or female next to the mice.

The researchers believe the response is caused by "chemosignals, or pheromones," the news release reported.  Men secrete these from their armpits at much higher concentrations than women do. The presence of these chemosignals alerts the rodents to the fact that a male animal is close by.

"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," Robert Sorge, a psychology professor at the University of Alabama, Birmingham. Sorge led the study as a postdoctoral fellow at McGill, said in the news release.

 "The problem 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," Mogil said.