Seasonal climate effects on the survival of a hibernating mammal

Abstract

Global climate change and associated regional climate variability is impacting the phenology of many species, ultimately altering individual fitness and population dynamics. Yet, few studies have considered the effects of pertinent seasonal climate variability on phenology and fitness. Hibernators may be particularly susceptible to changes in seasonal climate since they have a relatively short active season in which to reproduce and gain enough mass to survive the following winter. To understand whether and how seasonal climate variability may be affecting hibernator fitness, we estimated survival from historical (1964–1968) and contemporary (2014–2017) mark–recapture data collected from the same population of Uinta ground squirrels (UGS, Urocitellus armatus ), a hibernator endemic to the western United States. Despite a locally warming climate, the phenology of UGS did not change over time, yet season‐specific climate variables were important in regulating survival rates. Specifically, older age classes experienced lower survival when winters or the following springs were warm, while juveniles benefited from warmer winter temperatures. Although metabolic costs decrease with decreasing temperature in the hibernacula, arousal costs increase with decreasing temperature. Our results suggest that this trade‐off is experienced differently by immature and mature individuals. We also observed an increase in population density during that time period, suggesting resources are less limited today than they used to be. Cheatgrass is now dominating the study site and may provide a better food source to UGS than native plants did historically.

Life history consequences of climate change in hibernating mammals: a review

“…Accordingly, results were often age‐ and season‐specific. For example, warmer winter temperatures decreased over‐winter survival for yearling and adult Uinta ground squirrels Urocitellus armatus (Falvo et al 2019). However, warmer winter temperatures conferred a survival benefit to juveniles (Falvo et al 2019).…”

“…For example, warmer winter temperatures decreased over‐winter survival for yearling and adult Uinta ground squirrels Urocitellus armatus (Falvo et al 2019). However, warmer winter temperatures conferred a survival benefit to juveniles (Falvo et al 2019). Juveniles have a larger surface‐to‐volume ratio than yearling and adults, likely forcing them to expend more energy while hibernating (Körtner and Geiser 2000, Sherman and Runge 2002); this could explain why they tend to survive over winter at a higher rate when ambient winter temperatures are above freezing.…”

“…An obvious reason for higher survival rates during hibernation than over the active season is that the hibernaculum keeps animals safe from predators; indeed, in warming climates overall survival may decline as the active season lengthens, accompanied by a higher predation rate (Turbill et al 2011, Turbill and Prior 2016). In the Uinta ground squirrel, however, pups experienced higher over‐winter survival when temperatures were warmer during hibernation, while older age classes suffered decreased over‐winter survival rates (Falvo et al 2019). Although metabolic costs decrease with decreasing temperature and increased snow insulation in the hibernacula, arousal costs increase with decreasing temperature.…”

“…Although metabolic costs decrease with decreasing temperature and increased snow insulation in the hibernacula, arousal costs increase with decreasing temperature. This result suggests this trade‐off is experienced differently by immature and mature individuals for some hibernating species (Falvo et al 2019), which may be tempered by climate change as winters and springs continue to warm at higher elevations.…”

Maternal survival costs in an asocial mammal

“…Even though hibernation increases annual survival overall, variation in resource availability during the active season plays an important role in determining adult mortality (Turbill et al, 2011 ). In early spring, above‐average precipitation can increase vegetative growth (Sharpe & Van Horne, 1998 ); however, rainy weather may preclude ground squirrels from seeking the resources needed for survival (Falvo et al, 2019 ; Kneip et al, 2011 ), perhaps because being wet challenges squirrel thermoregulation. High summer temperatures after the reproductive period also reduce foraging activity, resulting in diminished resource acquisition (Vispo & Bakken, 1993 ) which could reduce the probability of over‐winter survival.…”