Digit ratio and length asymmetry in calves' limbs

This study considered possible sexual dimorphism in the relative lengths of the third, fourth and fifth digits (digit ratio), in calves. Furthermore, a different length of the bone structures of the third (3D) and of the fourth (4D) digits has been examined as an evolutionary adaptation to locomotion on soft ground. The length of the digital bones of the right fore-limb of 33 females and 15 male calves was measured in vivo using a portable X-ray machine. The vestigial structure of the 3D and 4D, from metacarpus to the third phalanx, and the rudimental metacarpus of the fifth digit (5D) were considered in a mixed model, as well as some ratios between different parts of 3D or 4D and 5D (3D:5D and 4D:5D). A covariate for the mean finger length was considered for digit ratios to control for possible biases due to shape allometry. Shorter first phalanx and trotter were found in 3D than in 4D, and the reverse for the third phalanx. The 5D was significantly shorter in females, as well as the second phalanges of 3D and 4D. Significant sex differences were found for the first phalanges of 3D:4D, that were shorter in males than in females, as 2D:4D and 3D:4D ratios found in most mammals. The asymmetry between 3D and 4D could mean a functional adaptation for locomotion. Sex differences in 3D:4D were found, but not involving all the digit parts, as in some mammal species like field vole or red squirrel. This study is the first investigation of digit ratio in Ungulates, whose limbs differ from the limbs of most mammals, maintaining five digits. The presence of a significant sex differences only in the first phalanx of 3D:4D could be due to the peculiar nature of the bovine digits, constituting the cannon bone, and to the hormone patterns acting on this digit during development, but further research is required around this topic.

Automated summary

This study focused on the possible sexual dimorphism in the relative lengths of the third, fourth, and fifth digits in calves, as well as the asymmetry in the bone structures of the third and fourth digits as an adaptation to locomotion. Significant sex differences were found in the length and ratios of different digit parts, particularly in the first phalanx of 3D, suggesting a functional adaptation for locomotion.