On the importance of body-size proxies when scaling sexual dimorphism: A case study in Mustelidae

Sexual dimorphism in a focal trait can arise from sex-specific selection acting directly on this trait. Alternatively, it may result from sex-specific selection acting on body size, to which the focal trait is genetically correlated. Quantifying the allometric relationship between trait size and body size is therefore essential for distinguishing between these different causes of sexual dimorphism. However, body size is often estimated by proxy measurements, and the choice of these proxies can influence the results of the analysis. We illustrate these issues by estimating sexual dimorphism in three craniodental traits in four mustelid species, the Eurasian otter, the European badger, the beech marten and the stoat. Depending on the trait, 16% to 100% of intersexual differences in trait size can be attributed to the allometric effects of body size dimorphism. This suggests that a large part of the sexual dimorphism in the focal traits is often caused by sex-specific selection on body size. For canine size, however, we find a consistent sexual dimorphism independent of body size, suggesting that this trait is under sexual selection. Using two different body size proxies produces estimates of size-independent sexual dimorphism that differ by up to 69%. We discuss the criteria for choosing body-size proxies in such analyses.