Desert Tortoise Hibernation: Temperatures, Timing, and Environment

This research examined the onset, duration, and termination of hibernation in Desert Tortoises (Gopherus agassizii) over several years at multiple sites in the northeastern part of their geographic range, and recorded the temperatures experienced by tortoises during winter hibernation. The timing of hibernation by Desert Tortoises differed among sites and years. Environmental cues acting over the short-term did not appear to influence the timing of the hibernation period. Different individual tortoises entered hibernation over as many as 44 days in the fall and emerged from hibernation over as many as 49 days in the spring. This range of variation in the timing of hibernation indicates a weak influence at best of exogenous cues hypothesized to trigger and terminate hibernation. There do appear to be regional trends in hibernation behavior as hibernation tended to begin earlier and continue longer at sites that were higher in elevation and generally cooler. The emergence date was generally more similar among study sites than the date of onset. While the climate and the subsequent timing of hibernation differed among sites, the average temperatures experienced by tortoises while hibernating differed by only about five degrees from the coldest site to the warmest site.

Figure 1

Onset, duration, and termination of hibernation at the City Creek and Littlefield sites for the four winters from 1995 through 1998. The four winters at Littlefield are in the top half of the figure; City Creek is given in the bottom half. Years are sorted from bottom to top in each panel and listed as the year in which each hibernation period began. The distance between the onset and termination dates is the duration of hibernation. The median onset and termination dates for each site are shown as filled circles. The box surrounding the filled circle depicts the 25^th and 75^th quartile. The range of values for each measure is given by the dotted lines (“whiskers”) outside of each box, and possible outliers are given by the open circles outside the box.

Figure 2

Three examples of tortoise body temperatures prior to, during, and following hibernation. Data are presented as the daily minimum (filled circles) and maximum (unfilled circles) temperatures (C). Panel A is an example of a high buffering in the body temperature pattern. Panel B demonstrates a medium level of temperature buffering during hibernation. Panel C is an example of a low temperature buffering.

Figure 3

Onset, duration, and termination of hibernation at the City Creek, Littlefield, Lake Mead, and Bird Spring Valley sites for the winter of 1998–1999. See Fig. 1 for figure explanation.

Contributor Notes

Section editor: S. F. Fox.

Received: 14 Jul 2005

Accepted: 20 Dec 2006