Fat chance for a skinny squirrel: survival consequences of pre-hibernation fattening provide a model to understand foraging decisions and life-history strategies

Research Highlight: Allison, A., Conway, C., Goldberg, A., Morris, A., and Hakanson, E. (2025) Seasonal body mass dynamics mediate life-history trade-offs in a hibernating mammal. Journal of Animal Ecology. https://doi.org/10.1111/1365-2656.70160. Body mass is an important proxy of individual state, especially for animals in seasonal environments. Storage of energy, primarily as fat, decouples daily energy expenditure from current food availability, providing flexibility in how animals respond to temporal variability in conditions and resources. Hibernating mammals take this to the extreme among endotherms: By fattening extensively and entering prolonged bouts of energy-saving torpor, they can remain inactive for part of the year, which maximises their chances of survival until the productive season. Allison et al. (2025) used a large data set of annual body mass change to investigate among-individual variation in the extent and timing of pre-hibernation fattening, its ecological co-variates and survival consequences in the endangered northern Idaho ground squirrel (Urocitellus brunneus). Delayed fattening was a cost of reproduction for females. Relatively fat squirrels had increased survival over the hibernation season, which is likely explained by fatter individuals choosing to enter hibernation earlier to avoid predation risk, rather than an energetic constraint imposed by a short growing season. Allison et al. found support for hypotheses relating fattening dynamics to thermal conditions, conspecific density and possibly time limitation, but not to limited food availability or interspecific competition. These results provide insight into state-dependent behavioural decisions about activity in seasonal environments and their consequences for life-histories in an unusual model among endotherms. They are important also for informing management actions to help conserve this endangered species.