According to this press release, researchers at McGill University in Montreal have discovered that the presence of a male researcher (or even his worn t-shirt) stresses laboratory mice and rats, the equivalent to making them swim for 3 minutes or restraining them in a tube for 15 minutes. The research has just been published in the journal Nature Methods (1), but its not available open access so I haven’t been able to read it yet.
A knock on effect of stress is a phenomenon called stress-induced analgesia – in other words, pain relief. Researchers injected the rodents ankles with a chemical which induces an inflammatory reaction, which should cause pain, and then measured the level of pain they were experiencing using the ‘grimace scale’ (2). This measures pain using things like the position of the animal’s ears and whiskers and how open their eyes are.
The researchers found that when the mice could smell a male researcher (or even bedding belonging to male guinea pigs, cats, dogs, and unfamiliar rodents) they had lower scores on the ‘grimace scale’, that is, they were experiencing pain relief. But the presence of female researchers produced no such response. The researchers also noted that the amount of poo the animals produced was also affected, with more produced when the male researchers were present.
I’m keen to read the paper and read the nitty gritty (how many different types of laboratory mice and rats they used, what ages, etc) but the results are very intriguing and could go some way to explain why it is difficult to reproduce some experiments from lab to lab. As the researchers say, at the very least this data suggests that the gender of the people handling the animals should be stated in the materials and methods sections of papers.
1. Sorge RE, et al (2014). Olfactory exposure to males, including men, causes stress and related analgesia in rodents. Nature Methods doi:10.1038/nmeth.2935
2. Langford DJ, et al (2010). Coding of facial expressions of pain in the laboratory mouse. Nature Methods. doi:10.1038/nmeth.1455