Publication Brief for Resource Managers
Release
June 2007
Contact
Dr. Kenneth E. Nussear
Phone
702-564-4515
Email and web page
knussear@usgs.gov
http://www.werc.usgs.gov/lasvegas/nussear.asp
Address
Las Vegas Field Station
160 N. Stephanie
Henderson, NV 89074


Desert Tortoise Hibernation: Temperatures, Timing, and Environment

The federally listed Mojave population of the desert tortoise receives much attention from scientists and land managers alike. The cryptic behavior of desert tortoises and propensity to live insubterranean burrows can occlude our ability to fully understand their activity patterns and to evaluate their population status. Little has been published on their hibernation patterns, and yet land management decisions are made to allow more invasive activities while tortoises are thought to be in hibernation. In a recent study published in the journal Copeia, scientists at the USGS and University of Nevada, Reno, explored the timing of hibernation in tortoises and the possible mechanisms that influence the timing of hibernation in this species.

The study examined the onset, duration, and termination of hibernation indesert tortoises over several years at multiple sites in thenortheastern part of their geographic range, and recorded the temperaturesexperienced by tortoises during winter hibernation.Tortoises avoid cold temperatures in winter by using underground cover sites or hibernacula, burrows excavated in soil or natural rocky caves. Hibernacula generally have higher temperatures than the open environment during winter, providing buffering from daily temperature fluctuations and protection from potentially lethal cold temperatures.

The timing of hibernation bydesert tortoises differed among sites and years. Environmental cues acting over theshort-term did not appear to influence the timing of the hibernation period. Such potential cues for hibernation onset include reduced day length and photoperiod, cooler temperatures, reduced forage availability, and timing of precipitation events. Differentindividual tortoises entered hibernation over as many as 44 days in the fall andemerged from hibernation over as many as 49 days in the spring.

This range ofvariation in the timing of hibernation indicates a weak influence at best of externalcues hypothesized to trigger and terminate hibernation. However, there do appear to be regionaltrends in hibernation behavior as hibernation tended to begin earlier and continuelonger at sites that were higher in elevation and generally cooler. The emergence datewas generally more similar among study sites than the date of onset.

While the climateand the subsequent timing of hibernation differed among sites, the averagetemperatures experienced by tortoises while hibernating differed by only about fivedegrees from the coldest site to the warmest site.

Management Implications

Nussear, K.E., T.C. Esque, D.F. Haines, and C.R. Tracy. 2007. Desert tortoise hibernation: temperatures, timing, and environment. Copeia:378386.

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URL: http://www.werc.usgs.gov/pubbriefs/nussearpbjun2007.html
Last update: 18 June 2007