Scent of male experimenters stress out mice and rats in labs

In an attempt to find out why some scientists are unable to replicate research findings on rodents, a new study has concluded that it's all got to do with the gender of the experimenters.

An international team of pain researchers led by scientists at McGill University in Montreal found that the presence of male experimenters produced a stress response 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.

This stress-induced reaction made mice and rats of both sexes less sensitive to pain. Female experimenters produced no such effects.

Jeffrey Mogil, a psychology professor at McGill and senior author of the paper, said that 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.

The research team found that the effect of male experimenters on the rodents' stress levels was due to smell.

This was shown by placing cotton T shirts, worn the previous night by male or female experimenters, alongside the mice; the effects were identical to those caused by the presence of the experimenters, themselves

Further experiments proved that the effects were caused by chemosignals, or pheromones, that men secrete from the armpit at higher concentrations than women. These chemosignals signal to rodents the presence of nearby male animals. (All mammals share the same chemosignals).

These effects are not limited to pain. The researchers found that other behavioural assays sensitive to stress were affected by male but not female experimenters or T-shirts.

Robert Sorge, a psychology professor at the University of Alabama, said that the study suggests 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.

The good news, Mogil said, is that 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.

The study is published online April 28 in Nature Methods.