Sexual dimorphism in metapodial and phalanges length ratios in the wood mouse

Relative length of metapodials and digits is sexually dimorphic in most primates and one rodent and one bird species studied so far. Recently, interest in digit ratios has increased because of their correlation with diverse physiological, psychological, and performance traits in humans. These correlations may reflect the effect of androgens during early ontogeny on digit development and their long-term organizational effects on extragenital organs. Inter- and intrasexual variation in digit ratios may be ultimately controlled by modulation of the expression of Hoxa and Hoxd genes. Since Hox genes are conserved in vertebrates, similar patterns of sex-related variation in length ratios may be expected across taxa. In fact, sexual dimorphism in length ratios has been documented for metapodials or digit bones in nonhuman vertebrates, but the specific pattern of sex-related variation varies considerably. However, no study has investigated sexual dimorphism in length ratios between all ray segments (metapodials plus phalanges) using osteometrical measures. In an outbred wild population of wood mice (Apodemus sylvaticus), we found extensive sex-related variation in ratios between osteometrical length of the phalanges, but not metatarsals, similar to that recorded on undissected digits of humans and laboratory mice. Most sexually dimorphic ratios involved the second digit. We found very weak evidence for directional asymmetry in length ratios. The present study shows that sex-related variation in length ratios between digit segments observed in mammals may actually depend on relative bone length. Hence, other species may be used to investigate the causal and semeiotic implications of variation in human digit ratios.