Autumn season lizard metabolism is explained by temperature and body mass but does not support cold adaptation across the latitudinal thermal gradient

Abstract Compensatory evolution is often observed in traits of ectotherms that span large latitudinal ranges because populations have had to adapt to local environmental conditions to survive. Indeed, many species of ectotherms grow faster and larger in colder environments compared with warmer environments, despite their reliance on environmental temperature for physiological rates. While this pattern has been observed for decades, the question persists: how do ectotherms evolve the greater physiological efficiency to support more growth? Shifts in metabolic rates are often proposed as part of the answer because metabolic rate is a proxy for an organism's energy expenditure. Accordingly, the metabolic cold adaptation (MCA) hypothesis posits that populations inhabiting higher latitudes and altitudes will have increased metabolic rates to compensate for colder temperatures and shorter activity periods constraining energy use. Here, we test the MCA hypothesis by measuring metabolic thermal performance curves in the prairie lizard, Sceloporus consobrinus . We examined four populations exhibiting the trend of greater growth and body size in cooler environments across the latitudinal thermal gradient. We found temperature and body mass to have positive and cumulative effects on the standard metabolic rates of individuals across all populations, but our results do not support MCA. Metabolic thermal performance curves were similar among populations. We measured standard metabolic rates during the autumn season and recognize that seasonal differences in metabolic rates often occur, with warmer seasons resulting in greater divergence among populations along thermal gradients. As such, measuring metabolic rates across all seasonal periods may still reveal increased overall rates of energy expenditure in populations inhabiting colder environments.