Anuran 3D models reveal the relationship between surface area‐to‐volume ratio and climate

Abstract

AbstractAimThe relationship size–environment has been continuously debated for ectotherms. Surface‐to‐volume ratios are recurrently suggested as the cause of body size responses to climate, but most inferences on organismal size clines stem from single linear measurements of body length. Here, we illustrate how new photogrammetric techniques can be applied to characterize amphibian morphologies with 3D models. Using the natterjack toad (Epidalea calamita) as model organism, we compared the performance of climatic models in accounting for body size (measured as snout‐to vent length, SVL) and body geometry (surface area‐to‐volume ratios) variation across macroclimatic gradients.LocationPalaearctic.TaxonEpidalea calamita (former genus Bufo).MethodsWe used photogrammetry techniques to reconstruct 104 museum specimens of Epidalea calamita into 3D models and to collect geometric data (volume and surface area) to calculate surface area‐to‐volume ratios (SA:V). We then apply spatial and non‐spatial regression to examine the relationships between SA:V, SVL and a set of bioclimatic variables.ResultsIn accordance with the water conservation hypothesis, SA:V decreased towards more arid environments, a pattern that remained consistent after accounting for spatial autocorrelation effects. Only mean annual temperature was significant correlated with SVL. The contrast between both sets of models indicates that more traditional body size metrics (SVL) and body geometry (SA:V) do not operate the same constraints.Main conclusionsWe show that the variation in SA:V can be explained by the geographical variation in aridity. Evaporative water loss in wet‐skinned ectotherms is highly dependent on the surface area of the skin exposed to the air. In contrast, SVL models detected that body length was more associated with mean annual temperature. This finding stresses the importance of selecting different metrics of body geometry and body size when examining variation across climatic gradients.