Gaining weight to hibernate: Pre-hibernation body mass effects on survival in adult male northern Idaho ground squirrels

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RaeAnn E. Kirk, Austin Z.T. Allison, Kerri T. Vierling and Anne Yen, Department of Fish and Wildlife Sciences, University of Idaho, Moscow, ID, RaeAnn E. Kirk, DDCSP Collaborative, RaeAnn E. Kirk, ESA SEEDS Program, Courtney J. Conway, U.S. Geological Survey, Idaho Cooperative Fish and Wildlife Research Unit, Moscow, ID

Background/Question/Methods
The northern Idaho ground squirrel (Urocitellus brunneus) is a rare, federally threatened mammal endemic to Adams and Valley counties in Idaho. Potential causes of population decline are thought to be fire suppression, habitat loss and habitat fragmentation. The objective of this study was to understand the relationship between morphological traits on fitness parameters of adult male northern Idaho ground squirrels. We tested the hypothesis that pre-hibernation body mass would affect the chances of over-winter survival. Predictions included a positive relationship between pre-hibernation body mass and survival, a greater mean pre-hibernation mass of surviving squirrels compared to undetected squirrels, and variation in pre-hibernation body mass across study sites. Data was collected in 16 study sites located in Adams County. Squirrels were live-trapped and marked during the 2013-2019 field seasons. Body mass was considered the last weight recorded during the active summer (June-July) season. Recaptures in subsequent years allowed us to document survival. The true fate of squirrels not recaptured in subsequent years was not known with certainty; they either died, emigrated, or avoided capture. A summary statistics analysis of the data was conducted using RStudio.

Results/Conclusions
Squirrels that were caught the year after first capture weighed an average of 12 grams more than squirrels that were not recaptured in subsequent years. Pre-hibernation body mass of both groups (caught in subsequent years and not caught in subsequent years) varied among the study sites. The data supports the hypothesis that pre-hibernation body mass affects the survival of northern Idaho ground squirrels, as squirrels that survived the subsequent year after first capture had a higher mean body mass than squirrels that were not detected in subsequent years. These results may be used as indicators of survival at each site and help determine whether restoration treatments affect body mass and weight gain. Future considerations may include the analysis of other morphometric traits. Furthermore, other factors such as competition, predation, and nutrition should be considered beyond body mass when assessing the likelihood of overwinter survival.