Thermal Ecology and Behavioral Activity in Hibernating Timber Rattlesnakes (Crotalus horridus)

Abstract

Hibernation is an important behavioral event in temperate region reptiles for escaping periods of harsh winter environmental temperatures. Snakes are arguably the most thoroughly studied reptilian hibernator; and at high latitudes, snakes may spend a large proportion of the year underground in a state of reduced metabolic activity. Although generally associated with periods of inactivity, some evidence does support limited activity during the winter, both above or below ground. However, observations of such events are difficult due to their typical subterranean refugia. This study examines occurrence of both terrestrial and subterranean activity during winter in the timber rattlesnake, <italic>Crotalus horridus</italic>. Hourly body temperatures (T_bs) and small-scale movements of snakes throughout a ~5 month hibernation period in Middle Tennessee were monitored with the purpose of identifying mid-winter activity and potential shuttling behavior in and out of hibernation. Environmental temperatures (air, soil, and den cavities) and snake operative temperatures (using biophysical models) were recorded to help estimate time periods when snakes were at the surface. Snakes returned to the denning areas for ingress (mean SE) on 11 Octoberr 3 days, and emerged for egress on 7 April 4.7 days. Visual observations of snakes basking on the surface were rare (0 of 4 snakes in winter 2011 - 2012; 4 of 13 snakes in winter 2012 - 2013), however, hourly sampling of snakes with surgically implanted temperature loggers revealed that 50% (2011 - 2012) and 69% (2012 - 2013) of the snakes emerged to the forest floor surface to bask two or more times (2011 - 2012: 12 total basking events; 2012 - 2013: 48 total basking events). Mean snake T_bs during winter were 11.9 1.5C (2011 - 2012) and 11.0 1.2C (2012 - 2013). A total of 48,223 T_bs were collected over two hibernating periods, with T_bs ranging from 1.1 - 33.7C. Snakes made on average 6.1 1.2 movement bouts throughout winter, accumulating a total distance of 146.4 35.5 m. All individuals made small (< 10 m) movements to different micro-hibernation sites throughout winter. If mild winters become more common due to climate change, suitable surface temperatures for snakes emerging from dens during mid-winter warming periods may increase. The energetic costs associated with increasing T_b (via basking) during hibernation have little effect on the total energy budget required to survive winter and therefore the functional significance of mid-winter basking remains to be determined.