Biomedical and clinical sciences are experiencing a renewed interest in the fact that males and females differ in many anatomic, physiological, and behavioral traits. Sex differences in trait variability, however, are yet to receive similar recognition. In medical science, mammalian females are assumed to have higher trait variability due to estrus cycles (the "estrus-mediated variability hypothesis"); historically in biomedical research, females have been excluded for this reason. Contrastingly, evolutionary theory and associated data support the "greater male variability hypothesis". Here, we test these competing hypotheses in 218 traits measured in over 27,000 mice, using meta-analysis methods. Neither hypothesis could universally explain patterns in trait variability. Sex-bias in variability was trait-dependent. While greater male variability was found in morphological traits, females were much more variable in immunological traits. We discuss the importance of sex-specific trait variability in relation to their eco-evolutionary ramifications (e.g., sex-dependent responses and adaptations to climate change) and statistical implications (e.g., power analysis considering sex difference in variance).
