Rattlesnake migrations and the implications of thermal landscapes

Abstract

Background: The importance of thermal resources to terrestrial ectotherms has been well documented but less often considered in larger-scale analyses of habitat use and selection, such as those routinely conducted using standard habitat features such as vegetation and physical structure. Selection of habitat based on thermal attributes may be of particular importance for ectothermic species, especially in colder climates. In Canada, Western Rattlesnakes (Crotalus oreganus) reach their northern limits, with limited time to conduct annual migratory movements between hibernacula and summer habitat. We radio-tracked 35 male snakes departing from 10 different hibernacula. We examined coarse-scale differences in migratory movements across the region, and then compared the route of each snake with thermal landscapes and ruggedness GIS maps generated for different periods of the animals' active season.

Results: We observed dichotomous habitat use (grasslands versus upland forests) throughout most of the species' northern range, reflected in different migratory movements of male snakes emanating from different hibernacula. Snakes utilizing higher-elevation forests moved further during the course of their annual migrations, and these snakes were more likely to use warmer areas of the landscape.

Conclusion: In addition to thermal benefits, advantages gained from selective migratory patterns may include prey availability and outbreeding. Testing these alternative hypotheses was beyond the scope of this study, and to collect the data to do so will require overcoming certain challenges. Still, insight into migratory differences between rattlesnake populations and the causal mechanism(s) of migrations will improve our ability to assess the implications of landscape change, management, and efficacy of conservation planning. Our findings suggest that such assessments may need to be tailored to individual dens and the migration strategies of their inhabitants. Additionally, local and landscape-scale migration patterns, as detected in this study, will have repercussions for snakes under climate-induced shifts in ecosystem boundaries and thermal regimes.

Keywords: Crotalus oreganus; Ectotherm; Habitat selection; Incident solar radiation; Migration strategies; Random walk; Thermal ecology; Thermoregulation.

Fig. 1

Typical and atypical habitat associations for the Western Rattlesnake in British Columbia. Open grassland habitats generally occur at elevations of 300 to 800 m in the Bunchgrass and Ponderosa Pine biogeoclimatic zones. Forested habitats generally occur at elevations of 500 to 1200 m in the Interior Douglas-fir biogeoclimatic zone [57]. Hibernacula typically occur at elevations of 500 to 625 m on south-facing, rocky slopes. Photos by lead author

Fig. 2

Step-by-step process by which assessment of the thermal properties of migration routes was conducted for rattlesnakes in British Columbia, Canada. The entire process outlined was separately carried out on the entire Outward migration pathway, as well as separately on the Initial and Late stages of the same migration pathway. Definitions and additional details are provided in the Methods section of the text

Fig. 3

Examples of Western Rattlesnake migrations from the hibernaculum to destination habitat. In the top illustration, the study hibernaculum is represented by a star (☆), and different snake movements are represented by different-coloured triangles (Δ) and connecting lines. In the top image, the thermal landscape is represented using modelled average incident solar ration as a proxy for temperature. Telemetered snakes in this example all used warmer and forested areas of the thermal landscape (shown as darker green in the bottom image)

Fig. 4

Average percentile scores of migration path values within values derived from 100 random-walk simulations, for each category of migration, compared to migration path ruggedness for telemetered Western Rattlesnakes in British Columbia. Trend lines (shown) were fit for the pooled group of snakes during the entire Outgoing migration (R² = 0.13), the Initial migration (R² = 0.09) and the Late stage of migration (R² = 0.13). Forest snakes are represented by solid markers; Open-Habitat snakes are represented by open markers