Habitat use is linked to components of fitness through the temperature-dependence of erformance in ratsnakes (elaphe obsoleta)

For ectothermic reptiles, habitat selection is mechanistically linked to fitness through the temperature-dependence of performance. Many reptiles occupy thermally heterogeneous environments and regulate their body temperature through selective use of habitats within their environments, making reptiles ideal subjects to understand the fitness consequences of habitat use. Our goal here was to investigate the link between habitat selection, thermoregulation, and fitness by comparing the expected distribution of performance for real ratsnakes that thermoregulate through selective use of habitat with the performance of hypothetical snakes that are assumed to use habitats randomly. Thermal data for real snakes were obtained using temperature-sensitive radio-transmitters implanted in free-living snakes, whereas thermal data for hypothetical snakes were obtained by sampling environmental temperatures that a randomly moving snake would encounter. Thermal data were then transformed into performance using an experimentally derived equation relating performance (swimming speed) to temperature. Habitat selection allowed snakes to avoid lethal temperatures and resulted in an average improvement of 18% in locomotor performance. A more exact measure of the fitness improvement accrued through habitat selection will have to await data relating body temperature to ultimate measures of fitness and a deeper understanding of the contribution of different performances to fitness.