Energetics link long-term environmental variations to breeding success in a wild penguin population

Environmental variability shapes species' population dynamics. Yet, the mechanisms linking environmental changes to individual-level metrics (e.g. foraging behaviour, body condition) and reproductive outcomes in the wild remain poorly understood. Energetics play a central role in mediating trade-offs between self-maintenance and reproduction under fluctuating environmental conditions. As such, it provides a powerful framework for identifying how individual responses to environmental variation scale up to influence population dynamics. Using a unique long-term monitoring and bio-logging dataset spanning over 25 years providing continuous measures of diving behaviour, feeding activity and daily energy expenditure, this study investigates how individual responses to environmental variation affect population dynamics. Focusing on Adélie penguins (Pygoscelis adeliae) during the energetically demanding chick-rearing phase, we integrated individual-level foraging and energetics data with colony-wide reproductive metrics to elucidate how environmental cues lead to life-history trade-offs. Winter sea-ice conditions exhibited a quadratic relationship with key individual behavioural and energetic parameters. Specifically, increased sea-ice concentration and delayed ice retreat led to longer foraging trips, reduced time spent diving and poorer body condition. At the population level, while energy expenditure was not associated with changes in reproductive outcome, increased foraging effort (time spent feeding per day) led to enhanced fledging success. Adverse on-land conditions, such as higher snowfall, had negative impacts on reproductive outcomes. These findings support the central role of energy as a common currency of maintenance and reproduction. By linking individual energetics to demographic performance, our work advances our understanding of how energy allocation strategies in response to environmental stressors shape population dynamics. These insights are crucial for improving predictive models of population trajectories and offer valuable guidance for conservation strategies aimed at mitigating the impacts of global change on ecosystems.