Abstract Balancing current reproductive investment with survival and future fecundity is a central challenge for long-lived animals, shaping life-history evolution. Although carry-over effects are well documented, the underlying energetic mechanisms remain unclear. Herein, we introduce the energetic flexibility concept, i.e. the ability of individuals to dynamically reallocate energy among competing life-history functions, which serves as a hidden axis influencing carry-over effects. We manipula