A future warmer climate is generally predicted to threaten reptiles. But many high-altitude reptiles that rely on external heat to survive current cold climate may benefit from future warmth. This prediction can be tested by simulating activity time and energetics of reptiles with a biophysical model. We modelled the potential response of a high-altitude snake, Trimeresurus gracilis, to a 3 °C increase in air temperature, using the biophysical/behavioural NicheMapper™ model. Using animal and environmental properties, we first analysed the effects of shade level (i.e. the reduction of solar radiation) on activity time and energetics of T. gracilis. We then computed activity time, energetics and potential habitats in its current range of Taroko National Park, Taiwan (>1800 m in altitude), with a spatial climate, vegetation and topography data set. In the current climate, increased shade levels were predicted to decrease activity time, maintenance energy requirements and discretionary energy (i.e. energy left after maintenance costs are met). In heavy shade (i.e. 81-100% reduction of solar radiation) at 3000 and 3500 m, T. gracilis was predicted to have impaired digestive capacity and hence to be unable to meet basic maintenance costs for survival. In a warmer climate, the predicted digestive capacity, energetics and activity time of T. gracilis increase. But in heavy shade at 3500 m, the digestive capacity was predicted to remain impaired for supporting basic energy requirements for survival. In the current climate, the predicted suitable habitat area (i.e. where T. gracilis obtains positive discretionary energy) in coniferous and coniferous-broadleaf forests decreased with increasing elevation. At elevations higher than 3000 m, most suitable habitats were in open areas like grasslands where T. gracilis was also predicted to have more discretionary energy than in heavily forested habitats. Our field sightings agreed with model predictions. A warmer climate was generally predicted to increase the activity time, energetics and habitat area, given the current vegetation pattern. Overall, we demonstrate that metabolism, digestive function and vegetative shade levels are critical elements affecting T. gracilis' energetics and habitats with climate change. Trimeresurus gracilis is likely to benefit energetically from future warmth.
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