Validation of the post-hoc method to estimate snout-vent length in the order Caudata

Amphibians are the most endangered class of vertebrates, with a high rate of decline recorded since the 20th century. Even activities related to the study of these animals, for instance, handling them to collect individual biometric parameters, can have negative effects on amphibian health. A post hoc method for estimating snout-vent length from dorsal photographs has been developed to reduce handling time and stress for individuals, thereby improving precision and repeatability of measurements. However, to date, this methodology has been tested on only approximately 1% of known salamanders, thereby limiting its broad applicability. Here, we tested this method on a diverse sample of Caudata comprising 25 species across 5 families, characterized by diverse morphologies. The correlation between predicted SVL (estimated from dorsal photographs) and observed SVL (measured directly from ventral photographs) values was assessed using Linear Mixed Models. The results showed a significant correlation between observed and predicted SVL, with an average and constant discrepancy of approximately 1.6 mm. When considering the increase of SVL, there was a slight tendency to underestimate SVLe in newts, plethodontids, and proteids. Estimation errors slightly increased with the SVL. The error increased in larger newts, while decreased in larger plethodontids. Our study highlighted the reliability and applicability of adopting this methodology for data collection in all Caudata species.