Cruel study finds the scent of a man stresses out rodents


According to a recent study, the smell of a man induces significant levels of fear and stress in mice and rats, which could confound experimental results.

Researchers in Canada stumbled across this discovery while conducting a study to determine if the presence of experimenters affects pain responses in rodents. In the experiment, mice and rats had chemicals that are known to cause pain and inflammation, injected into their ankles or paws before being placed inside small clear containers. Their facial expressions (e.g. grimacing in pain) and behaviour (e.g. flicking or licking their paws in pain) were then videotaped so that their pain responses could be measured. The researchers wanted to know if the presence of a person observing the mouse caused them stress which actually reduces their ability to feel pain – a phenomenon known as “stress-induced analgesia”.

The study found that the presence of a man, or his t-shirt, caused significantly more stress-induced analgesia than the presence of a woman.

To assess the extent of this stress effect, another experiment was conducted in which mice were exposed to human experimenters, t-shirts or cardboard cut-outs of people before being restrained in tubes for up to 30mins, forced to swim in water for 3 minutes and then decapitated to collect blood for stress level measurements. The results showed that the stress levels induced by male odours were equivalent to those induced by 15 minutes of restraint and 3 minutes of forced swim.

The researchers concluded “stress caused by male experimenters may represent a confound of much existing animal research”. In other words all previous and current research is potentially flawed because the animals may be responding to the scent of a man rather than the substance being tested. This cruel study provides yet another explanation for why animal experiments are already poorly predictive and adds to the evidence that animals find the laboratory environment stressful. Douglas Wahlsten, a professor of psychology at the University of Alberta, admitted “it’s the kind of result a lot of people wish wouldn’t happen.”


1. Male researchers stress out rodents. (2014). Nature, 28 April:

2. Olfactory exposure to males, including men, causes stress and related analgesia in rodents. (2014). Nature Methods, doi:10.1038/nmeth.2935. Original study can be found here: