Does greater variation reside in the larger sex?

The question of whether males or females are the more variable sex is long-standing, and yet to be fully answered. We investigate the relationships between body mass and variation across species using a phylogenetically informed analysis of the body mass of 337 species representing six mammalian orders. Within each order, we found that the larger sex is typically the more variable sex, whether male or female, and the variation-size relationship is arguably often close to unity. Thus, size may be the main or even sole driver of variability in at least some orders. Deviations from the expected 1 : 1 relationship emerged, however, in regressions of male : female mass variance against male : female mean mass, for Chiroptera and Rodentia, which both presented hyperallometric relationships suggesting that drivers over and above size influence differences in variation between the sexes. In Chiroptera, most species have larger females. The y-intercept value for Artiodactyla and Primates were significantly greater than 0 suggesting greater male variation in species where the sexes are of commensurable size. Historic belief of exclusively greater male variability may have resulted from a focus on species with intense male-male competition and, thus, larger male body sizes. Our results suggest that it is often size, not sex per se, that influences within-sex variability, although additional sex-specific factors might be present in at least some orders.