Navigating thermal landscapes: habitat use and thermoregulatory trade-offs in a northern viper.

The thermal landscape is a key driver of habitat selection by ectotherms, which must optimize body temperatures with other life history requirements such as prey acquisition, predator avoidance, mate searching, and reproduction. For snakes in temperate regions, the thermal consequences of varying habitat use will have profound effects. The detailed study of the summer migration of northern western rattlesnakes (Crotalus oreganus) away from their winter hibernacula has revealed striking differences between individuals and populations in habitat use that ranges from low-elevation grasslands to higher elevation forests. We investigated the implications of this different summer habitat 'choice' on the thermoregulation, thermoregulatory behaviour, and body condition of individuals in southern British Columbia, Canada. Using data from 30 telemetered male snakes carrying implanted thermologgers at multiple study sites, we determined that snakes that remained in lower elevation grassland (open) habitats faced overall fewer thermoregulatory constraints (higher average body temperature, more accurate thermoregulation) than individuals that migrated relatively greater distances into forests. However, despite this cost of forest habitat use, snakes that migrated into forests exhibited significantly better end-of-season body condition and gained proportionally more weight over the active season than snakes remaining in open habitats. Our findings reveal a complex interplay of costs and benefits that individual snakes experience according to the habitat in which they base themselves during the short northern summer.