Overwintering Strategies Drive Climate Change Impacts on Body Mass in Northern Mammals.

Climate change drives variable shifts in mammalian body sizes, yet the mechanisms underlying this variation remain unclear. Two explanations prevail: (1) higher temperatures reduce thermal constraints, favouring smaller bodies; or (2) warming enhances primary productivity, supporting larger bodies. We propose that species' overwintering strategies, which mitigate exposure to winter temperatures, determine which mechanism predominates. Using ~50 years of body mass data, we tested these hypotheses in three small mammals with contrasting natural histories in the rapidly warming Yukon, Canada. Snowshoe hares (Lepus americanus), exposed to ambient winter conditions, declined in mass over time and responded negatively to temperature and snow. In subnivean species, deer mice (Peromyscus maniculatus) showed no long-term trend but increased body mass with growing degree days, whereas red-backed voles (Clethrionomys rutilus) increased with temperature and snow. The results demonstrate that overwintering ecology mediates climate-driven body mass change, emphasizing the role of natural history in shaping species' responses to warming.