Digit ratios in two lacertid lizards: sexual dimorphism and morphological and physiological correlates

摘要

Digit length ratio (primarily 2D:4D) has become increasingly popular as a possible biomarker of intrauterine steroid exposure in the human medical, social and psychological literature. Human males tend to have lower digit ratios than females, and individuals with low ratios tend to excel in physical performance, especially in endurance-related sports. Because early limb development is evolutionarily conservative, it has been speculated that these trends should also be visible in other tetrapod vertebrates. However, studies on non-human vertebrates are scant, and their results suggest that sexual dimorphism in digit ratios and the associations with physical performance are much more intricate and taxon-specific than presumed. In this study, we compared digit ratios of two Podarcis lizards among sexes, colour morphs and species. We also tested for associations with three performance characteristics that are of ecological relevance. Both species examined exhibit male-larger sexual dimorphism in digit ratio. 2D:4D, 3D:4D and 2D:3D ratios are tightly correlated within the manus and the pes, but less so between manus and pes. In the colour polymorphic species P. melisellensis, the yellow morph exhibits higher dimorphism than the orange and white morphs. Digit ratios did not correlate with individual performance for sprint speed or endurance, but within males of P. melisellensis, individuals with higher digit ratios correlated positively with head size and bite force. We conclude that digit ratios in lizards deserve attention, because they exhibit sexual dimorphism and correlate with ecologically relevant morphological and performance variables. As lizard species differ widely in mating systems, reproductive mode, habitat use and locomotor behaviour, they seem excellent model animals for studying patterns in digit length ratios.