Rodents and other animals are often used in biomedical research as models for humans. Understanding how different sexes of mice react to a medication, for example, can help researchers predict adverse side effects in men and women. But most biomedical research has been done on exclusively male animals.
Rebecca Shansky is a neuroscience researcher at Northeastern University. She was studying how rats handle stressful experiences, when one of her graduate students noticed that the female rats responding differently.
“It’s really important that we that we understand how things work in both sexes,” says Shansky.
Stress-related illnesses such as post-traumatic stress disorder and major depressive disorder occur twice as frequently in women as they do in men. But for many years, researchers failed to include female animals in their studies out of fear that their changing hormones would muddy the data. Female mice go through an estrous cycle, which is like a faster version of the human menstrual cycle.
But male hormones vary too, Shansky says. Recent analyses have shown that data from female mice doesn’t vary any more than data from male mice.
In 2016, the National Institutes of Health issued a mandate that new grant proposals either incorporate animals of both sexes, or demonstrate a good reason not to. Other researchers have suggested conducting a study on males first, then repeating the study with females to see if the results are different.
But old perceptions of female mice as essentially deviant versions of male mice are still coloring how some studies are structured, says Shansky.
She proposes that researchers use equal numbers of male and female animals in their studies and then examine the data for potential sex-related differences. If there are any, researchers can conduct follow-up studies looking at differences in both dominance hierarchies in males and estrous cycles in females.