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.

Using Ecological Niche Models for Population and Range Estimates of a Threatened Snake Species (Crotalus oreganus) in Canada

“…Except for climate variables, all of the environmental features were in ESRI raster format and had the same geographic extent and pixel size (30 × 30 m-Table S1). Because rattlesnakes rely on behavioral thermoregulation to maintain their body temperature [79,80], solar radiation and weather variables are relevant for predicting their occurrence [36]. Therefore, we used minimum monthly temperatures and mean monthly precipitation from the active (non-hibernation) period for rattlesnakes in BC (1 April to 30 September, 1 km pixel size) from Chelsa bioclim data available at: http://chelsa-climate.org/bioclim/ accessed October 2020 [81,82].…”

“…Ruggedness can not only impact snake mobility, but it can also create a diversity of microclimates enabling snakes to locate suitable thermal surfaces, even though the overall averaged thermal index may be unsuitable for a particular area [36]. Thus, a second key factor in estimating the distribution and abundance of rattlesnakes is topography [84,85].…”

“…Since many topographic features are highly inter-correlated (e.g., slope, topographic position) and may be used in combination as habitat cues by rattlesnakes, we used a 'ruggedness' index [36]. This was calculated directly using the 'Terrain Ruggedness Index' in SAGA GIS [83].…”

“…Such information is needed urgently because reptiles are among the most rapidly declining taxa globally [29][30][31][32][33] and yet they play key roles in ecological food webs [27]. Snake distribution is strongly associated with spatial thermal homogeneity, and everevolving anthropogenically-altered lands may facilitate drastic alterations in the heterogeneity of thermal properties on the landscape [34][35][36]. Furthermore, many snake species have a unique combination of life history characteristics such as delayed sexual maturity, high adult survival and longevity with low fecundity [37].…”

Microhabitat and stopover site selection by juvenile Western Rattlesnakes (Crotalus oreganus)

“…In Canadian populations of Western Rattlesnakes (Crotalus oreganus), migrations between hibernating sites and foraging grounds are conducted each year, with the animals typically associated with grassland and open forested habitats in the summer (Gomez et al, 2015;Harvey and Larsen, 2020). Within these landscapes, adult microhabitat selection as determined through radiotelemetry is largely described by proximity to cover (Macartney, 1985;Bertram et al, 2001;Gomez, 2007), with adults showing a particular preference for structurally stable cover objects (i.e., shrubs and rocks).…”