Proximate causes of altitudinal differences in body size in an agamid lizard

Body size is directly linked to key life history traits such as growth, fecundity, and survivorship. Identifying the causes of body size variation is a critical task in ecological and evolutionary research. Body size variation along altitudinal gradients has received considerable attention; however, the underlying mechanisms are poorly understood. Here, we compared the growth rate and age structure of toad-headed lizards (Phrynocephalus vlangalii) from two populations found at different elevations in the Qinghai-Tibetan Plateau. We used mark-recapture and skeletochronological analysis to identify the potential proximate causes of altitudinal variation in body size. Lizards from the high-elevation site had higher growth rates and attained slightly larger adult body sizes than lizards from the low-elevation site. However, newborns produced by high-elevation females were smaller than those by low-elevation females. Von Bertalanffy growth estimates predicted high-elevation individuals would reach sexual maturity at an earlier age and have a lower mean age than low-elevation individuals. Relatively lower mean age for the high-elevation population was confirmed using the skeletochronological analysis. These results support the prediction that a larger adult body size of high-elevation P. vlangalii results from higher growth rates, associated with higher resource availability.