Avian hind-limb digit length ratios measured from radiographs are sexually dimorphic

Sexual dimorphism in digit length ratios is well established in humans, and has been reported in other vertebrate species as well, including birds. The sign of sexual dimorphism in digit ratios may, however, vary both within and between vertebrate classes. It has been hypothesized that sex differences in digit ratios arise via differential prenatal exposure of the two sexes to steroids, which may affect the expression of the Hox genes controlling the osteometric development of digits and appendices. Among birds, the evidence for sex dimorphism in hind-limb digit ratios is conflicting, though all previous studies were based on measurements of undissected digits, implying that results could be confounded by sex-related variation in soft tissues. Here we report that digit ratios derived from radiographs of both feet of a large passerine bird, the hooded crow (Corvus corone), are sexually dimorphic, males showing larger 2D : 3D (effect size, r = 0.33) and 2D : 4D than females (effect size, r = 0.28). We also observed a good agreement (r = 0.45) between radiographic estimates of digit ratios and digit ratios calculated based on undissected digit measurements (thus including soft tissues). Importantly, we found that the patterns of sex and side differences were largely coherent between the two methods. Therefore, our findings show for the first time in avian species that sex differences in digit ratios have an osteometric basis, a fundamental prerequisite for a role of Hox genes in originating such dimorphism.