Metabolic Flexibility and High Food Availability Buffer Energy Expenditure across Climates in a Widespread Lizard

The proposed direct relationship between time available for activity and the resulting energetics of populations has been a central hypothesis for many studies in thermal and physiological biology. However, climates differ beyond thermal constraints on activity, and variation in primary productivity and food availability could decouple the expected relationship between activity time and energetics. Additionally, phenotypic flexibility in metabolic rates may permit populations in relatively restrictive climates to match or exceed annual energy expenditure measured in populations with climates that permit greater annual activity. Here, we tested the hypothesis that higher food availability and increased rates of metabolic expenditure permit coastal populations of Western Fence Lizards (Sceloporus occidentalis) to maintain an equivalent energy budget to inland populations that achieve greater annual activity. We integrated oxygen consumption rates measured in the laboratory with biophysical models that predict amounts of daily activity during the activity season, length of the activity season, and annual activity, and energy expenditure within each population. We found that lizards along the cool Salish Sea coast of Washington State had higher metabolic rates than lizards in the warmest inland climate. Additionally, our model revealed that despite having less available activity time than lizards at the warmest inland site, coastal lizards achieved higher annual energy expenditure. Thus, this study shows how food availability and metabolic flexibility may decouple the expected relationship between energy expenditure and activity time across climates.