Biomedical and clinical sciences are experiencing a renewed interest in the fact that males and females differ in many anatomic, physiological, and behavioural 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 estrous 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 >26,900 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. Sex-specific variability has eco-evolutionary ramifications, including sex-dependent responses to climate change, as well as statistical implications including power analysis considering sex difference in variance.

• We previously selected for large and small brain size in guppies. • Large-brained females outperformed small-brained females in a learning task. • Healy and Rowe challenged our interpretations of larger brains = better learning. • Here we argue why we think they are mistaken.

Inbreeding depression often affects male reproductive traits. Here, we measure inbreeding depression in sperm, body size, ornamental color patterns, and attractiveness in male guppies and ask whether different environmental conditions play a role in the severity of inbreeding depression. Male sperm traits and coloration were measured in inbred and outbred fish in 2 experiments under different environmental conditions (large mixed-sex greenhouse tanks and small individual laboratory tanks). Overall, we found that inbreeding leads to significant declines in sperm numbers. Moreover, males raised under greenhouse conditions showed a much stronger decline in sperm numbers with inbreeding compared with those raised under laboratory conditions. These differences might have been caused by a more competitive environment in the greenhouse. The decline in total sperm numbers with inbreeding was due to declining numbers of sperm bundles (spermiozeugmata), not to variation in numbers of spermatozoa within spermiozeugmata. Some, but not all ornamental traits including tail size and the areas of solid black as well as iridescent coloration, were significantly influenced by inbreeding, whereas the environment only affected differences in body size. Mating trials conducted on laboratory-raised fish revealed significantly greater attractiveness of outbred than inbred males. Female choice of outbred over inbred mates may yield direct benefits in the form of greater fertilization efficiency.