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Mammals that cope with harsh environmental conditions by hibernating show seasonal variation in body mass, as fat reserves are accumulated during summer and depleted during winter. Hence, pre-hibernation mass is often considered a key phenotypic trait in hibernating species that can be an indicator of future survival and reproductive success. We used 30 years of demographic data from a high-elevation population of golden-mantled ground squirrels (Callospermophilus lateralis) to identify life-history and environmental variables that influence pre-hibernation mass in females, and to determine if pre-hibernation mass influences overwinter survival or subsequent reproduction. Growth curves revealed female-biased size dimorphism, with adult mass not attained until age 2 years, so we considered three age classes of females, juveniles, yearlings, and adults (≥2 years). We found that earlier snow melt during spring is associated with higher pre-hibernation masses of yearlings and adults, and that juveniles emerging earlier from their natal burrows had higher pre-hibernation masses than later-emerging juveniles. Measures of food quantity or quality did not affect pre-hibernation mass for any age class, nor was there an effect of reproductive status. Juvenile females with greater per- hibernation mass were more likely to survive overwinter and reproduce as yearlings, but we found no such effect for yearlings or adult females. We did not find an effect of snow depth or winter duration on either survival or reproduction. Our results indicate that time appears to be the key variable for golden-mantled ground squirrels to acquire sufficient body mass to survive and reproduce, but the effects vary by age class. ii
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