Sex identification of juvenile sand lizards, Lacerta agilis using digital images

Abstract

Sexing neonate animals is necessary for many evolutionary and ecological studies. Yet non-invasive sex identification of neonate reptiles is often problematic because these do not exhibit salient differences in colouration and body proportions. We examined digital images of the ventral body surface in 214 adult or subadult individuals (95 males + 119 females), 59 juveniles (29 + 30), and 156 hatchlings of the Eurasian sand lizard, Lacerta agilis. Two quantitative traits, the number of transverse rows of ventral scales and the width/length ratio of the anal plate, which are easily recordable from digital images and show no substantial correlation with body size, exhibit pronounced sexual dimorphism. A discriminant function derived from these two characters allows correct identification of the sex in 90% of juvenile individuals when males and females of older stages are used as reference samples. Also, we introduce a new qualitative trait, namely the presence/absence of the skin hyperaemia behind cloaca in hatchlings, and provide indirect evidence that this trait is likely to be strongly associated with sex.