Inclusion of females does not increase variability in rodent research studies

Underrepresentation of female subjects in animal research has gained attention in recent years, and new NIH guidelines aim to address this problem early, at the grant proposal stage. Many researchers believe that requirements regarding use of females will hamper research because of a need for increased sample sizes, and increased costs. Empirical research across multiple rodent species and traits demonstrates that females are not more variable than males, and that for most traits, female estrous cyclicity need not be considered. Statistical simulations, presented here, illustrate how factorial designs can reduce the need for additional research subjects, and cultural issues around the inclusion of male and female subjects in research are discussed.