Abstract
Hibernation is one of the most dramatic adaptations to seasonality, during which animals undergo drastic behavioral and physiological changes to survive periods of harsh environmental conditions and limited food availability. The physiological mechanisms underpinning hibernation have long intrigued ecologists, and recent advancements in wildlife tracking technology have facilitated myriad descriptive studies about where animals hibernate. However, the ecological processes governing hibernaculum site selection and the role of hibernacula in mediating survival of dormant animals has received considerably less attention. This gap in research is noteworthy given the vulnerability of hibernating animals to extrinsic sources of mortality and their inability to respond quickly to changes in temperature, extreme weather, or predation. Furthermore, many species undergo extended periods of torpor lasting many months and spanning a wide range of environmental conditions; the hibernaculum must remain a viable refuge as external conditions change. Greater focus on the mechanisms influencing hibernaculum site selection is essential to provide greater clarity about an essential but overlooked habitat requirement of hibernating animals, and thereby inform conservation strategies, particularly for vulnerable species.Given the ecological importance of hibernaculum site selection, we sought to better understand what factors influence hibernaculum site selection and the consequences of hibernaculum site selection in the northern Idaho ground squirrel (Urocitellus brunneus), a federally threatened species endemic to west-central Idaho, USA. The species experienced population declines in the late 20th century, likely due to habitat loss and fragmentation, and recent surveys estimate <1,400 adults remaining distributed among ~50 isolated populations. These squirrels inhabit open meadows, grasslands, and shrublands during the active season, and hibernate for 8-10 months annually. They are an ideal study species to study hibernaculum site selection because some squirrels move outside of their active-season range to hibernate in areas with higher canopy and shrub cover, while others remain within their active-season range to hibernate. We leveraged this behavioral variation among individuals to document the abiotic and biotic factors that explain intraspecific variation in hibernaculum site characteristics, and how site selection affects the likelihood of predation on hibernating squirrels.
In chapter 1, we test three mechanistic hypotheses that rely on abiotic processes to explain variation in hibernaculum site selection and migration to hibernacula: (1) thermal energetics, (2) post-emergence food limitation, and (3) spring flooding. We found strong support for the spring flooding hypothesis and limited support for the thermal energetics hypothesis. Squirrels with active-season home ranges located deeper within spring flood zones moved farther to hibernate compared to those living near or outside spring flood zones. Squirrels hibernating in areas with higher canopy closure experienced lower skin temperatures during torpor bouts; however, juveniles and squirrels in poorer body condition did not hibernate in areas with higher canopy closure (as would be predicted by the thermal energetics hypothesis) at rates significantly different from those of adults and squirrels in better body condition. Management actions for northern Idaho ground squirrels have primarily focused on thinning and burning forests to create more active-season habitat, but the impact of these actions on hibernation habitat is unclear. Our results suggest that preserving deep, well-drained soil is important for maintaining critical hibernation habitat for this range-limited species, while trees or forest per se may not be critical for hibernating squirrels.
In chapter 2, we examine how various hibernation habitat characteristics influence the likelihood of American badgers (Taxidea taxus) encountering and excavating hibernating northern Idaho ground squirrels, as well as how density dependence and interspecific competition with the congeneric Columbian ground squirrel (Urocitellus columbianus) influences hibernaculum site selection and predation risk. The probability of badgers encountering northern Idaho ground squirrel hibernacula increased with Columbian ground squirrel density and decreased with elevation. Additionally, badgers were more likely to encounter squirrels hibernating deeper within forests, but squirrels hibernating within 3 m of a tree were less likely to be excavated by badgers upon encounter. Northern Idaho ground squirrels living at sites with high densities of Columbian ground squirrels hibernated in more open habitats, moved shorter distances to hibernate, and were less likely to hibernate near a tree. Thus, aboveground habitat structure played a limited role in mediating predation risk, but both direct and apparent competition interacted to influence predation risk for hibernating northern Idaho ground squirrels. Our findings underscore the importance of preserving suitable hibernation habitat, which can inform targeted conservation strategies for this vulnerable species.