Body-Size Variation across Environmental Gradients in an Ectothermic Organism: An Intraspecific Approach to Ecogeographic Patterns

Bergmann's rule states that endothermic species from cool areas tend to have larger body sizes than those from warm areas. Allen's rule maintains that endothermic species from cool environments should have shorter extremities than their counterparts from warm areas. Although a number of studies have supported both rules, the validity of these trends remains controversial, especially for ectothermic organisms. Here, we studied intraspecific morphological variation in Four-Eyed Frogs (Pleuroaema thaul) to determine if these rules are valid in an ecogeographic context. Specifically, we analyzed how morphology varies across nine traits in males of P. thaul collected from nine localities in Chile. A discriminant analysis of all measured traits indicated differentiation between localities, and the first two canonical variables explained similar to 80% of the total variation in morphometric features. Additionally, these two canonical variables were correlated with precipitation and monthly minimum temperature. To determine the validity of Allen's rule, a stepwise discriminant analysis was performed incorporating three morphological variables measured on the extremities of the individuals. This analysis indicated no variation in the traits between populations. The first canonical variable of the discriminant analysis explained 90% of the total variation in extremity morphology and was not correlated with environmental variables. We did, however, detect a correlation between the canonical variables of the stepwise discriminant analysis, and both minimum temperature and precipitation. Overall, our results indicate that the populations of P. thrall in Chile comply with Bergmann's rule.