Seasonal Fattening Among Bat Populations Globally: Storing Energy for Survival in a Changing World

Seasonality is a fundamental challenge for life on Earth and energy storage prior to colder and drier periods by fattening is a common strategy for survival. Fattening should reflect a trade-off between an expected seasonal energy deficit and the costs of increased body mass, which are particularly important to flying endotherms. We examined body mass change (ΔM_b), a proxy of fat storage, among bat populations over low productivity periods with global variation in yearly average and seasonality of local climates. We found that ΔM_b increased with decreasing mean annual surface temperature (MAST) but ΔM_b also increased at higher MAST with higher seasonality of rainfall. Seasonal use of body energy reserves by bats is predicted to be widespread in warm, seasonal climates at low latitudes but is poorly studied compared to cold temperate regions. In colder climates only, females lost less mass than males over winter, supporting the 'thrifty females' hypothesis, and ΔM_b has increased with year of study in warm climates, possibly linked to effects of global climate change on their energetics. Our quantitative synthesis highlights how intrinsic and environmental factors shape seasonal fattening in bats, and its global importance for survival in this diverse and widespread mammal group.