Sexual dimorphism in digits length and their ratios in Salamander species (Salamandridae)

Abstract

In recent decades, digit ratio has attracted considerable attention in biology. It has been suggested that the digit ratio may be used as a biomarker of early developmental effects. In particular, the second-to-fourth digit ratio has been linked to the effects of sex hormones and their receptor genes. Also, this ratio is a morphological feature that is important for tetrapod locomotory performances in different microhabitats. Here, we focus on examining sex differences in digit ratios (2D:3D, 2D: 4D and 3D: 4D) in 15 species of the family Salamandridae with different patterns of aquatic to terrestrial locomotion. The main questions of this study are: (1) whether sexual dimorphism exists in digit length and their ratios (2) whether size, phylogeny or ecology has an influence on digit length and its ratios. We assume that different selective pressures related to aquatic and terrestrial locomotion could override the prenatal developmental cues. The results suggest a different degree of dimorphism in analyzed digits and digits ratios in analyzed salamanders, with strong effects of ecology, i.e. the proportion of time spent using each locomotion type, swimming or walking, implying that function has a large impact on limb growth. The results of our study are in line with others which can help to evaluate whether the developmental or functional processes have the most impact on limb evolution.