Climate warming drives a temperate-zone lizard to its upper thermal limits, restricting activity, and increasing energetic costs

Abstract Ectotherms are considered vulnerable to climate change as many operate at close to their thermal maxima. Exposure to higher temperatures could reduce activity by forcing animals to shelter in thermal refugia to avoid exceeding lethal limits. While rising temperatures should reduce activity in tropical species, the situation is less clear for temperate-zone species where activity can be constrained by both low and high temperatures. Here, we measure the effects of natural variation in environmental temperatures on activity in a temperate grassland lizard and show that it is operating at its upper thermal limit in summer even when sheltering in thermal refuges. As air temperatures increased above 32°C, lizard activity declined markedly as individuals sought refuge in cool microhabitats while still incurring substantial metabolic costs. We estimate that warming over the last two decades has required these lizards to increase their energy intake by over 20% to offset metabolic losses caused by rising temperatures. Our results show that recent increases in temperature are sufficient to exceed the thermal and metabolic limits of temperate-zone lizards. Extended periods of high temperatures could place natural populations of ectotherms under significantly increased environmental stress and contribute to population declines and extinction.