Adult body size = f (initial size + growth rate × age): explaining the proximate cause of Bergman’s cline in a toad along altitudinal gradients

Animals that exhibit indeterminate growth obey such a functional relationship: adult body size = f (initial size + growth rate × age). Using this framework, we investigated how and why body sizes of a toad species ( Bufo andrewsi ) covaried across six altitudes (760–2,100 m) in western China. Towards high altitudes, toads tended to produce large eggs, attain large sizes at metamorphism and have great average age, but grow slowly. This indicated that the former three variables contributed more to the observed altitudinal increase in body size than did the last one. The altitudinal variation in these life-history traits should be adaptive to increased climate harshness and decreased predation risks at higher altitudes. We suggest that the relative significance of responses of these size-related parameters to local environments may provide critical cues to explaining considerable variability in geographic size pattern among ectothermic vertebrates.