Introduction Anthropogenic climate change poses significant threats to desert reptiles already living near their physiological limits. While behavioral thermoregulation can buffer thermal stress, its effectiveness and associated costs under extreme conditions remain poorly understood in Neotropical psammophilous species. Methods We evaluated thermoregulatory effectiveness ( E ) and thermal vulnerability in two sister species of sand-dwelling lizards ( Liolaemus scapularis and Liolaemus riojanus ) from the Argentine Monte desert. We integrated field body temperatures, operative temperatures ( Te ), preferred temperatures ( Tset ), and critical thermal maxima ( CTmax ) across dry and humid periods over four years. Results Both species exhibited high thermoregulatory effectiveness despite extremely poor environmental thermal quality; only 3–11% of available Te fell within Tset range. This behavioral buffering acts as a ‘behavioral shield’—consistent with the Bogert effect—that effectively protects lizards from lethal temperatures but imposes a severe opportunity cost through activity-time restriction. Paradoxically, E was significantly higher during the thermally harsher dry period, a pattern facilitated in L. scapularis by marked seasonal plasticity in Tset , representing a coordinated physiological-behavioral mechanism absent in Liolaemus riojanus . Despite this success, warming tolerance was consistently negative, forcing lizards to remain inactive for 18–50% of potential activity time. This restriction was most acute in the endangered L. scapularis during the humid season, when shaded microhabitats fail to provide a sufficient cooling margin. Discussion Our findings demonstrate that while behavioral plasticity can homogenize thermal responses to stress, such buffering masks an underlying physiological vulnerability. Under projected mid-century warming for arid South America, further compression of activity windows may exceed behavioral buffering capacity, threatening the long-term persistence of these desert specialists. Proactive conservation must prioritize microhabitat heterogeneity to preserve thermoregulatory refugia for these endemic reptile communities.
Salva et al. (Thu,) studied this question.