Environmental steepness, tolerance gradient, and ecogeographical rules in glassfrogs (Anura: Centrolenidae)

Spatial variation in biological traits reflects evolutionary and biogeographical processes of the history of clades, and patterns of body size and range size can be suitable to recover such processes. In the present study, we test for latitudinal and altitudinal gradients in both body and range sizes in an entire family of tropical anurans, Centrolenidae. We partition the species latitudinal, and altitudinal distributions into an indirect measure of tolerance, and then test its effect on the body size gradient. We use an assemblage-based approach to correlate the traits with altitudinal and latitudinal axes, taking into account both phylogenetic and spatial autocorrelation in data. Centrolenids lack any gradient in range size but show a positive cline of both body size and adaptive body enlargement with altitude. This pattern is also positively correlated with an altitudinal gradient of cold tolerance, thus lending support to the heat balance hypothesis as an explanation of the body size cline. By using an entire Neotropical clade of anurans, we add further support for Bergmann's rule in ectotherms, warn for a likely effect of environmental steepness in fashioning the gradient, and offer evidence for an historical scenario (the OligoceneEocene Andean uplift) as its likely trigger.(c) 2013 The Linnean Society of London