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

Abstract

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.

Sexual Dimorphism of Digit‐Length Ratio in a Viviparous Lizard: Influence of Age, but not Preservation State or Sex of Interuterine Twin

“…Furthermore, the direction of sexual dimorphism in digit ratio, where detected, appears independent of the direction of any sex difference in body size (Table 3; contra Rubolini et al, 2006). Radiographic evidence for two oviparous species of lizards supports the existence of sex differences in the skeletons of at least some feet (Van Damme et al, 2015), so as in mice (Zheng and Cohn, Means are 61 SE. Significant results are shown in bold.…”

“…Podarcis spp. ; Chang et al, 2006;Direnzo and Stynoski, 2012;Van Damme et al, 2015) as well as among 25 species of iguanians (Gomes and Kohlsdorf, 2011). Concerning 2D:3D on the hindlimbs, fewer species have been examined.…”

“…As yet, few studies have explored the possibility of links between digit ratios and locomotor performance in other lizard species, and evidence is weak. Among male painted dragons, 3D:4D was not associated with swimming endurance (Tobler et al, 2012), and in two species of Podarcis, average digit ratios (both sexes combined) was not associated with sprint speed (Van Damme et al, 2015). Nonetheless, in humans, a wealth of literature exists on the potential for 2D:4D to serve as a proxy for prenatal androgen exposure in ways that may help predict subsequent health and performance.…”

“…Lizards account for over 6,000 species of amniotes with a diverse range of life histories and morphologies, and are thus an important group in which to explore evolutionary and ecological patterns in digit ratios (Gomes and Kohlsdorf, 2011;Van Damme et al, 2015;Beaty et al, 2016). Here, we examined digit ratios in Woodworthia "Otago/Southland", the first viviparous lizard and the first gekkotan lizard to be examined.…”

Mixed Sex Effects on the Second‐to‐Fourth Digit Ratio of Túngara Frogs (Engystomops pustulosus) and Cane Toads (Rhinella marina)

“…From there, our understanding of the between‐sex variation in 2D:4D within major vertebrate taxa dwindles. The few studies that have investigated digit length ratios in reptiles have had inconsistent results (e.g., Rubolini et al, ; Direnzo and Stynoski, ; Van Damme et al, ) and the pattern across amphibians is also ambiguous. In two species of anurans, males exhibit a larger digit ratio than females (Chang, , Direnzo and Stynoski, ), while there is no difference between the sexes in a third species (Germano et al, ).…”