Bergmann's rule in amphibians: Combining demographic and ecological parameters to explain body size variation among populations in the common toad bufo bufo

Abstract

Large-scale patterns of body size variation are described by well-known generalizations such as Bergmann's rule; the generality and underlying causes of these patterns have been much debated. Intraspecific extension of this rule was tested in various ectotherms, and evidence was found for both Bergmann and converse Bergmann clines. In this study, we explored spatial patterns of variation in a widespread amphibian, the Common toad (Bufo bufo), along a 2240 km latitudinal gradient across Europe. We tested for covariation of adult body size, age and growth parameters with latitude, altitude, length of activity period and mean temperature during this period using both original and literature data. We selected 13 European populations, representing a latitudinal range from 43 to 63°N and altitudinal range from 15 to 1850 m a.s.l. The length of activity period (12-33 weeks) and Tmean (6.6-15.6°C) significantly decreased as latitude and altitude of these populations increased. Mean body size decreased as latitude increased (not with altitude), and increased with Tmean (not with length of activity period). Mean and minimal adult age increased with latitude and altitude, longevity increased with altitude only. Age increased as length of activity period decreased (not with Tmean). The growth coefficient (0.32-0.92 in males, 0.18-0.74 in females, available for six populations) decreased as altitude increased, and increased as both length of activity period and Tmean increased; latitudinal trend was non-significant. Our analysis shows that B. bufo clearly exhibited a converse Bergmann cline along latitudinal gradient, but not along altitudinal gradient; the main effect of elevation was on age. The effects of ecological conditions also differed: body size increased with Tmean, while age parameters were related to the length of activity period. This study highlights that, to identify causal factors underlying general ecogeographical rules, we have to take into account different phases of the life cycle, co-variation among life history traits and ecological factors acting on each of these traits. In amphibians with complex life cycles, lack of appropriate demographical or ecological data may affect our understanding of the variety of observed body size patterns.