Some 95 percent of all lab animals are mice and rats.

London - For more than a century laboratory rodents have been central to the advancement of science, being used in experiments to develop cancer drugs and all manner of medical wonders.

But a new study suggests millions of experiments may have been ruined because rodents are ‘stressed’ by the smell of researchers - with a man's scent far more distressing than that of a woman.

This is likely to mean mice are less concerned about revealing their presence when a woman is around, and are more likely to confront a female than they are a man.

The study, conducted by Canadian scientists at McGill University in Montreal, found that a researcher’s gender can have a significant impact on tests of pain.

Stress blocks pain, which means that when a male lab technician is the room, a rat or mouse will feel far less pain than when a woman is conducting the experiment.

The findings, published in the journal Nature Methods, are significant for any experiment looking at the side effects of drugs or any test involving monitoring pain. Some 95 percent of all lab animals are mice and rats.

Psychology professor Jeffrey Mogil, who conducted the study, said: ‘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 study found that pheromones, which are secreted by men at higher concentrations than women, had a distinct impression on lab rodents.

The chemical triggered 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.

The study involved 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 scientists themselves.

Study leader Professor Robert Sorge said: ‘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.’

Prof Mogil added: ‘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 behavioural testing.’ - Daily Mail