Rattlesnake migrations and the implications of thermal landscapes

Jessica A Harvey,Karl W Larsen

doi:10.1186/s40462-020-00202-0

Abstract

BackgroundThe 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.ResultsWe 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.ConclusionIn 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.

Highlights

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
Thirty of 35 telemetered snakes were tracked through their entire annual migration, resulting in 13 to 27 location data points per individual
Overall, the results of our study indicate that the annual migrations of these northern snakes are dictated at least in part by thermal attributes of landscapes, at a relatively large-scale, for snakes that utilize forested habitat

Summary

Introduction

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. Migration is differentiated from dispersal movements by way of bidirectional movement, movement of greater duration than ranging, and (2020) 8:21 movement along relatively straight-line paths [20] Such responses may be apparent when they involve clear, contrasting shifts in habitat use, such as birds migrating between temperate and tropical regions. Northern species, provide rare examples of terrestrial ectotherms conducting annual migrations; these migrations can include considerable movements relative to the mobility of the animal and the brevity of the active season (e.g., [36, 47, 73]). The potential linkages between these movements and thermal landscape patterns has not been wellstudied [72] By testing for this mechanism, we gain a better understanding of why particular movements (including seasonal migrations) occur. This, in turn, will facilitate more refined habitat mapping and consideration of how other land uses could potentially influence migrations

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