Summary
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.
Methodology
Study area
We collected data from 1997 to 2000 in a 10 km x 3 km area at the Queen’s University Biological Station, 100 km south of Ottawa, Canada (44º33'N; 76º19'W). Habitat was predominantly mature, second-growth deciduous forest. Lakes, wetlands, and rocky outcrops provided natural gaps and edge habitats in the forest. Artificial gaps and edges were created by small hayfields (Blouin-Demers and Weatherhead, 2001a).
Radio-telemetry
We captured snakes at communal hibernacula during spring emergence and, opportunistically, throughout the active season (Blouin-Demers et al., 2000a). From all snakes captured, we selected a subset for implantation of temperature-sensitive radio transmitters (Model SI-2T, Holohil Systems Inc., Carp, Ontario, 8.6 g, 20 months’ battery life at 20 °C). Snakes selected to be implanted had to be large enough to bear the transmitter (maximum ratio of transmitter mass:body mass = 0.025:1). We used isoflurane, delivered via a precision vaporizer, to anesthetize the snakes and then sterile surgical techniques to implant the radio transmitters in the body cavity and the antennae under the epidermis (Blouin-Demers et al., 2000b; Weatherhead and Blouin-Demers, 2004). From May 1997 to November 1999, we followed 53 ratsnakes for periods ranging from 1 to 30 months (mean ± SE = 13.1 ± 1.1 months). Twenty-three snakes were followed in multiple years and therefore we have data for 79 “snake-years”.
Radio transmitters had pulse rates proportional to temperature. Calibration curves were supplied by the manufacturer for each transmitter (pulse rates from 0 °C to 40 °C in 5 °C increments). We used polynomial regressions (including all terms up to degree 4) and eight calibration points for each radio transmitter to derive an equation to predict temperature based on pulse rate. All calibration equations provided a very high degree of fit (all R2 ≥ 0.99).