A fat chance of survival: Body condition provides life-history dependent buffering of environmental change in a wild mammal population

Environmental change often causes decreased food availability and/or increased foraging costs, putting wild animals at risk of starvation. Body-fat reserves can enable individuals to resist (buffer) periods of weather-driven food scarcity, improving their chances of survival and subsequent reproductive success. This capacity, however, is constrained by life-history factors and fixed long-term differences between individuals. Here, we use 29 years of data from a population of wild European badgers (Meles meles) to test how weather and population density affect individual body condition indices (BCIs), how BCI mediates survival rate and reproductive success, and whether long-term BCI phenotypes (fat vs. thin) provide life-history advantages. Maintaining body condition above a certain threshold was key to survival (reflecting a nonlinear relationship), especially when temperatures varied more between seasons (requiring greater tactical foraging and BCI adjustments) and following excessive rainfall (causing thermoregulative stress). BCI also affected survival more strongly in older individuals. Female reproductive success increased linearly with autumn BCI, and consistently fatter badgers (of both sexes) had higher lifetime reproductive success; however, substantial intra-individual body-condition variation remained after accounting for weather and individual factors, and 84% of individuals varied BCI substantially from year to year. Modelling BCI responses according to projected climate change through 2080 (Emissions Scenario RCP 8.5) revealed that even strong warming (as one-off events) would produce < 5% survival probability reductions, pushing few individuals below the BCI risk threshold. We thus demonstrate that life-history factors and individual body-condition tactics are fundamental to understanding population resilience under anthropogenic climate change.