Exposure of mice and rats to male experimenters or their clothing increases these rodents’ stress levels and influences their behavior during experiments, reports a study published online this week in Nature Methods. This study shows that an experimenter’s gender can have a confounding effect on behavioral studies and should be taken into account.
There has been anecdotal evidence that mice react differently in the presence of male versus female experimenters. In particular, it has been noted that a mouse’s pain response is blunted in the presence of a male, rather than a female, experimenter.
To investigate these observations, Jeffrey Mogil and colleagues used the mouse grimace scale, a sensitive measure of pain in rodents, to compare mouse response to pain in the presence of male or female experimenters. They report a marked reduction in pain sensation, known as stress-induced analgesia, when a man conducted the experiment. Further tests showed that this effect was due to olfactory stimulation from a mixture of chemicals present in human sweat and could be reproduced by exposing the mice to a T-shirt that had previously been worn by a man. The mice also showed increases in corticosterone levels-a stress hormone-and body temperature, indicative of higher stress levels. Mogil and colleagues confirmed the effect of the male olfactory stimuli in a second behavioral experiment in which they saw that the presence of a male-worn T-shirt increased mice’s anxiety in an open-field test.