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

Abstract

The existence of sex differences in digit-length ratio (especially between the second and fourth digits, 2D:4D) is well established for humans from fetal life onwards, and has been linked with later performance. In rodents, the ratio is affected prenatally by exposure to androgens and estrogens, with some research suggesting an influence from sex of the neighbouring intrauterine fetus. However, the ubiquity and ontogenetic development of sexual dimorphism in digit ratios is not well established among wild amniotes. We report the first digit ratios for a gekkotan lizard, representing a speciose lineage in which viviparity has evolved independently from mammals and other reptiles. For the gecko Woodworthia "Otago/Southland", in which up to two embryos develop in separate uteri, we found: (1) significant sexual dimorphism in adults in 2D:3D of the right hindlimb only (larger in males), but not in 2D:4D for any limb; (2) no dimorphism in ratios for young juveniles, with no influence of sex of the interuterine twin, and no relationship with sprint speed; (3) in preserved tissues of the same juveniles, no sexual dimorphism in ratios, but a change in relative lengths of some digits with preservation. The ontogenetic pattern might be explained by altered sex-steroid exposure at the transition to adulthood rather than during prenatal development. Our results support a phylogenetic hypothesis that sauropsids (birds and reptiles) differ from mammals in the direction of sex difference, if present. Experiments are needed to establish the roles of androgens and estrogens in establishing these sex differences in lizards. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc.

Sand lizards Lacerta agilis with higher digit ratios are more likely to autotomy

“…The use of various limbs digit ratios (2D:3D, 2D:4D and 3D:4D) in presented research poses a greater challenge in comparing the results (see in Introduction); this issue should be kept in mind for the purpose of designing future experiments. In addition to contributing to the advance of knowledge concerning digit ratios in Squamata, the necessity of which has been raised by numerous authors (Rubolini et al, 2006;Gomes and Kohlsdorf, 2011;Tobler et al, 2011;Van Damme et al, 2015;Woodhead et al, 2018), our results also provide new insights into the relationship between digit ratios and caudal autotomy. However, it should be emphasised that autotomy is a complex feature and tail damage is likely to be repeated throughout the life of a specimen, and the probability of keeping the tail intact decreases with age (Bustard and Hughes, 1966).…”

“…In the case of 2D:3D, detected differences between sexes were characterised by a high degree of variability among species. For example, in adult Otago/Southland geckoes Woodworthia, K E Y W O R D S caudal autotomy, Lacertidae, morphology, reptiles, sexual dimorphism, squamata significant sexual dimorphism in 2D:3D (with this ratio being higher in males) was found only in the right hindlimb (Woodhead et al, 2018), in Podarcis siculus only in the left hindlimb (Van Damme et al, 2015). In Podarcis muralis male had larger 2D:3D ratio on the left side than females, whereas in Mabuya planifrons males had lower 2D:3D ratios than females (Rubolini et al, 2006).…”

“…In comparison with studies on humans, significantly fewer published studies concern the examination of other vertebrates in either laboratory or field conditions (Lombardo et al ., 2008; Nelson and Shultz, 2010; Gomes and Kohlsdorf, 2011; Kaczmarski et al ., 2015; Kazimirski et al ., 2020). Digit ratio studies initially concerned the second and fourth digits, that is 2D:4D (Voracek and Loibl, 2009); subsequently, in further studies of some amphibians, sauropsids (birds and reptiles) or mammals, other ratios, such as 2D:3D (Romano et al ., 2005; Leoni et al ., 2008; Woodhead et al ., 2018), 2D:5D (Leoni et al ., 2008), 3D:4D (Tobler et al ., 2011; Tobler et al ., 2012), or all three variants (Rubolini et al ., 2006; Saino et al ., 2007; Ruuskanen et al ., 2011; Balogová et al ., 2015; Van Damme et al ., 2015; Kazimirski et al ., 2020) were also tested. Generally, females tend to have higher 2D:4D digit ratios than males in most mammals (Brown et al ., 2002; Leoni et al ., 2005; McIntyre et al ., 2009; Fuse and Sawada, 2019) as well as in Caudata/tailed amphibians (Kaczmarski et al ., 2015).…”

“…In the context of the latter, human individuals with lower 2D:4D ratios have a predisposition to achieve better results in endurance‐related sports (Manning and Hill, 2009). The relationship between digit ratio and physical performance has already been a subject of several research projects on vertebrates; for example, in the case of Squamata some attempts have been made to examine the relationship between digit ratio and sprint speed in Podarcis lizards as well as in Woodworthia gecko, but so far, no clear correlation was found (Van Damme et al ., 2015; Woodhead et al ., 2018). Furthermore, no association between digit ratio and endurance was detected in C .…”

Digit ratio in the common toad Bufo bufo: the effects of reduced fingers and of age dependency

“…However, it should be remembered that the lack of differences between sexes in the digit ratio may be a byproduct, or, in a case where individuals occupy different habitats, the result of natural selection (see Introduction, as well as [ 30 ]). Finally, digit ratio measurements may be susceptible to artificial variation resulting from the accuracy of the methods used or the condition of the preserved individuals [ 26 , 46 , 47 ]. [ 26 ] suggested that the differences in the results of the two studies on O. pumilio were related to differences in the measurement methodology adopted, i.e., hand calipers versus software analysis of digital photographs.…”