Abstract
Environmental temperatures directly affect physiological rates in ectotherms by constraining the possible body temperatures they can achieve, with physiological processes slowing as temperatures decrease and accelerating as temperatures increase. As environmental constraints increase, as they do northward along the latitudinal thermal gradient, organisms must adapt to compensate for the slower physiological processes or decreased opportunity time. Evolving faster general metabolic rates is one adaptive response posited by the metabolic cold adaptation (MCA) hypothesis. Here we test the MCA hypothesis by examining metabolism of prairie lizard populations across the latitudinal thermal gradient. Our results show that populations from cooler environments have higher standard metabolic rates (SMRs), but these are explained by associated larger body sizes. However, metabolic rates of fed, postprandial individuals (MR(Fed)) and metabolic energy allocated to digestion (MR(Δ)) were highest in the population from the coldest environment after accounting for the effect of body size. Our results suggest cold-adapted populations compensate for lower temperatures and shorter activity periods by increasing metabolic rates associated with physiological processes and thus support the MCA hypothesis. When examining energy expenditure, metabolic rates of individuals in a postprandial state (MR(Fed)) may be more ecologically relevant than those in a postabsorptive state (SMR) and give a better picture of energy use in ectotherm populations.