Refugia at risk: microclimate degradation undermines food-hoarding adaptations in an endangered sky island endemic

Understanding responses of montane mammals to environmental change requires knowledge of individual species at multiple levels of biological organization. Red Squirrels (Tamiasciurus) serve as indicators of change due to their dependence on mature conifer seeds and a territorial tendency to larderhoard cones at middens to facilitate overwinter survival. Here, we integrate long-term population and behavioral data with mountain-wide ambient, surface, and internal midden temperature measurements to assess responses of Mt. Graham Red Squirrels (T. fremonti grahamensis) to changing conditions in the Pinaleño Mountains (Dził Nchaa Sí’an), Arizona, United States. Over the past quarter-century, we found a positive association between squirrel population size and food availability from the previous year. After accounting for annual variation in cone abundance, larderhoarding intensity was positively related to population density but not ambient temperature, implicating food competition or pilferage as important factors influencing caching behavior. Warmer internal midden temperatures linked to higher surface temperatures were associated with increased cone spoilage, thus reducing the efficacy of larderhoarding. In addition, population size decreased following warmer years, presumably related to reduced energy stores. In turn, middens with shorter squirrel occupancy histories were shallower and warmer compared to middens with longer occupancy histories. Thus, warming temperatures and lower population densities may act synergistically to reduce the efficacy of middens as energy stores. The persistence of Mt. Graham Red Squirrels will, in part, depend on their ability to successfully navigate alternative foraging or caching strategies, such as increased scatterhoarding. Such patterns highlight the vulnerability of species that cache perishable food, particularly at the southern edge of their geographic range, where temperatures are warmest. Similar long-term autecological studies of sky island mammals are needed to help elucidate species-specific mechanisms underlying response and resilience to environmental change.