Survival of the fattest: How body fat and migration influence survival in highly seasonal environments

Abstract

Abstract Energy stores and migration are important adaptations for animals in seasonal environments, but their roles may vary relative to an animal's endogenous and exogenous environment. In partially migratory populations, migrants and residents experience different seasonal environments; thus, the influence of energy stores on survival may differ relative to migratory tactic, with potential consequences to survival and fitness. Using data from Sierra Nevada bighorn sheep (Ovis canadensis sierrae; hereafter, Sierra bighorn), we tested the hypothesis that body fat (energy stores) buffers animals against their environment, but that buffering capacity differs across environments experienced by high‐elevation residents (using a single range year‐round), traditional migrants (making 1 round‐trip movement between high‐ and low‐elevation ranges during winter) and vacillating migrants (making ≥2 round trips between high‐ and low‐elevation ranges during winter). We predicted that: for animals with high levels of body fat, survival would be high regardless of migratory tactic; residents would require larger stores of body fat to survive than migrants; energy stores would be least influential to survival for vacillating migrants. High levels of body fat in autumn (≥14% for females and ≥19% for males) largely buffered animals against harsh environments (survival >0.90) regardless of migratory tactic. At lower levels of body fat, traditional migrants had higher survival than residents. Vacillating migrants exhibited nearly 100% survival with no detectable effect of body fat on survival. Collectively, these results support the hypothesis that body fat buffers animals against harsh environments but that the buffering capacity differed relative to the environment and highly flexible behaviours (i.e. vacillating migration) can allow animals to decouple survival from body fat. Our work reveals that synergies between physiological and behavioural adaptations of animals in highly seasonal environments carry potential fitness consequences for individuals and demographic consequences for populations. Interactions among these adaptations are key to understanding nutritional prerequisites for persistence in variable environments. Read the free Plain Language Summary for this article on the Journal blog.