Co-occurring agamid lizards vary in thermal vulnerability despite similar climatic exposure

Global warming increasingly exposes ectotherms to temperatures near or beyond their physiological limits, threatening survival and persistence. Vulnerability of species to rising temperatures is shaped by multiple factors, including the thermal environments they experience, their physiological tolerances, and the temperature dependence of key performance traits. Here, we investigated the thermal ecology and climate vulnerability of two co-occurring tropical agamid lizards from semi-arid Southern India: the arboreal generalist Calotes versicolor and the saxicolous specialist Psammophilus dorsalis. For both species, we measured microenvironmental temperatures using physical copper models in open and shaded microhabitats, quantified field-active body temperatures (Tb), measured preferred temperature range (Tpref), critical thermal limits (CTmin and CTmax) and determined thermal performance curves for sprint speed and bite force. Thermal vulnerability indices (thermoregulatory accuracy, habitat thermal quality based on physical models, effectiveness of thermoregulation, warming tolerance, and thermal safety margin) were calculated and we projected future performance declines under IPCC Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP5-8.5). Although both species experienced similar microhabitat temperatures, they differed in thermoregulatory strategies and vulnerability. The widely distributed C. versicolor had a wider Tpref range, higher CTmax, lower CTmin, broader thermal tolerance range, greater thermoregulatory effectiveness in open microhabitats, lower deviations from preferred temperatures, and more favorable habitat thermal quality. In contrast, P. dorsalis displayed narrower Tpref range, lower CTmax, higher CTmin, narrower thermal tolerance and reduced thermoregulatory accuracy and effectiveness, operating closer to its upper thermal limits and experiencing narrower safety margins. Sprint speed had higher thermal optima and narrower performance breadths than bite force in both species, rendering locomotion more sensitive to warming. Projections indicated minimal impacts on both sprint speed and bite force, even under high-emission scenarios. Nevertheless, narrow warming tolerances, thermal safety margins, and the saxicolous specialization of P. dorsalis suggest potential vulnerability through other ecological and physiological axes. Our findings reveal that closely related co-occurring lizards can occupy different thermal niches and face unequal climate risks despite shared ambient conditions. Species differences in microhabitat specialization, rather than exposure alone, emerges as a key driver of thermal vulnerability, underscoring its critical role in shaping resilience to ongoing climate warming.