Sexual dimorphism in digit length ratios in two lizard species

Abstract

Sexual dimorphism in digit length ratios has been reported for humans, a few other mammals, and two bird species. This dimorphism is thought to arise via an interaction between the prenatal exposure of the embryo to sex hormones and the Hox genes, which are highly conserved among vertebrates and control the development of both the appendices, including fingers and toes, and the urogenital system. In this study, we report on sexual dimorphism in 2D:3D, 2D:4D, and 3D:4D contralateral ratios of the forelimbs in two species of oviparous lizards, the common wall lizard (Podarcis muralis) and the tree skink (Mabuya planifrons), as measured on museum specimens. We found that male P. muralis had a larger 2D:4D ratio on both sides and larger 2D:3D ratio on the left side than females, whereas in M. planifrons, males had lower 2D:3D ratios than females on the left side. The two species show opposite patterns of sexual dimorphism in body size, males being larger than females in P. muralis, and the reverse in M. planifrons, suggesting that interspecific variation of sex differences in digit ratios could be associated with sex-specific growth trajectories. There was a limited evidence for directional asymmetry in digit ratios. Therefore, our findings provide the first evidence that digit ratios are sexually dimorphic in any reptile species and are consistent with the idea that the genetic link between limb development and the urogenital system had been established with the evolution of the earliest terrestrial tetrapods. Importantly, many lizard species with genetic sex determination, including the ones we studied, are oviparous and may represent valuable animal models for experimental tests of the association between prenatal exposure to androgens or estrogens and digit ratios. Anat Rec Part A 288A: 491-497, 2006.

Females Have Larger Ratio of Second‐to‐Fourth Digits Than Males in Four Species of Salamandridae, Caudata

“…Evidence for directional asymmetry in the digit ratio has also been shown in mice, where a lower digit ratio in right compared with left limbs was observed (Brown et al, 2002a). In other studies, differences between digit ratios in right and left limbs have been observed, but such evidence is limited (Chang et al, 2008;Rubolini et al, 2006).…”

“…In all studies on digit ratio in reptiles and amphibians, where differences between sexes were significant (Chang et al, 2008, Rubolini et al, 2006Chang, 2006;Penuela, 2011;Direnzo and Stynoski, 2012), males have a higher 2D:4D, except in the back limbs of Anolis humilis, where females have a higher 2D:4D (Direnzo and Stynoski, 2012). However, newt species have the same digit ratio pattern and chromosomal sexdetermination system as mammals (Schmid et al, 1979;Hillis and Green, 1990;Sessions, 2008).…”

“…We also calculated the correlation between the SVL and digit ratios; for this purpose we used R Spearman test, a nonparametric test. We calculated size of the effect as in other digit ratio studies (Bailey and Hurd, 2004;Rubolini et al, 2006;Direnzo and Stynoski, 2012) with within-and between-sex comparisons of digit ratios. For effect size calculation, we used equation (Rosenthal, 1991): r 5 Z/ͱN, where Z is the score of Mann-Whitney U test, N is the number of total observations, and r is the effect size.…”

Dimorfismo sexual en la proporción entre el segundo y cuarto digito (2D/4D) de Gonatodes albogularis (Gekkonidae) en Colombia

“…La única excepción aparece en babuinos de Guinea, en los cuales 2D/4D es mas grande en machos que en hembras (Roney et al, 2004). En aves diápsidas y reptiles, la proporción 2D/4D es generalmente mayor en machos que en hembras (Burley & Foster, 2004), lagartos de pared (Rubolini et al, 2006), y Lagartos Anolis verdes (Chang et al, 2008), siguen este patrón. Sin embargo, las lagartijas arbóreas como Mabuya planifrons no muestran dimorfismo sexual en 2D/4D (Rubolini et al, 2006).…”

“…En aves diápsidas y reptiles, la proporción 2D/4D es generalmente mayor en machos que en hembras (Burley & Foster, 2004), lagartos de pared (Rubolini et al, 2006), y Lagartos Anolis verdes (Chang et al, 2008), siguen este patrón. Sin embargo, las lagartijas arbóreas como Mabuya planifrons no muestran dimorfismo sexual en 2D/4D (Rubolini et al, 2006). Los diápsidos, reptiles y aves, tienen típicamente la proporción 2D/4D en machos más grande que en la hembra, aunque algunas excepciones pueden existir (Chang, 2008).…”