Environmental Variability Predicts Geographic Patterns of Body Size Variation in Gila Monsters ( Heloderma suspectum )

Aim Body size can influence nearly every aspect of organismal ecology, but drivers of intraspecific variation in body size are often poorly understood. We aimed to assess three hypotheses as potential explanations for intraspecific body size patterns in a desert ectotherm: that body size is driven by (i) potential foraging opportunities related to thermal constraints on activity; (ii) proxies for the abundance of resources (precipitation, primary productivity); and (iii) within and across year variability of resource availability. Location Southwestern United States, Northwestern Mexico. Methods We use a long‐lived lizard, Heloderma suspectum , to explore how environmental gradients inform body size patterns across a species range. We compile a dataset of measurements from 674 adult individuals spanning the entire species range and model body size as a function of 15 a priori selected environmental and mechanistic variables. Results We found body size was primarily influenced by spatial gradients in across‐year precipitation variability (+), within‐year resource seasonality (+), and within‐year average annual air temperature (−). Main Conclusions Our results suggest that within and among‐year patterns of resource availability are stronger drivers of body size than annual resource availability in the environment. Large body size may provide greater capacity for accumulating energy reserves, a potentially adaptive trait in environments where access to resources may be inconsistent across years and where resources may come in seasonal pulses.