Baby snakes: maternal investment and neonate sexing in smooth snakes

Observed sex ratios of adult smooth snakes Coronella austriaca vary widely across studies, raising the question whether this species shows skewed sex ratios already at birth, and whether this depends on the condition of the mother. A total of 15 gravid females were caught and kept in captivity to give birth. We measured and weighed these females before and after parturition, as well as each of their offspring. DNA was collected from each individual. Neonate sex was identified using multiple methods, allowing us to compare the accuracy of the conventional approach (total-to-tail length ratio) and hemipenal eversion (or 'popping') with the results of molecular analysis. Based on the 107 offspring, we found that hemipenal eversion did not lead to the same sex identification as DNA analysis in 3.6% of the cases, while the conventional approach disagreed in 8.3% of the cases. Given the high level of expertise and risk involved with the eversion technique, we recommend using the conventional method to identify the sex of neonates, augmented with DNA analyses for intermediate length ratios, between 5.32 and 5.68. Females lost on average 44.0% of their weight at parturition, with heavier females giving birth to more or heavier offspring. Individual neonate weight was positively related to the weight of their mothers, but negatively with the number of siblings. While neonate sex ratios varied strongly among mothers, the overall male to female ratio in this study was 1:0.91. The skewed sex ratios observed in the field (female-biased in summer: 1:3.44, male-biased in spring and autumn: 1:0.57) are therefore most likely due to sex-specific behaviour and resulting detection probabilities changing over the seasons, and potentially by sex-specific survival rates. Keywords: Coronella austriaca, CTNNB1, hemipenal eversion, offspring, sex ratio, WAC