Penguins maintain effort, despite reduced dive efficiency, with limited physiological response under increased swimming workload.

In highly variable marine habitat, it is key to understand how free-ranging animals cope with variation in the energy cost of locomotion. Here, we experimentally tested how an apex marine predator, the king penguins (Aptenodytes patagonicus) adjusted to an increase cost of locomotion when foraging at sea. We investigated the consequences of increased hydrodynamic drag on diving performance and physiology of a group of birds subject to externally-attached devices compared to control individuals. King penguins did not increase their locomotor effort to compensate for the additional drag but decreased swimming speed while maintaining dynamic body acceleration (VeDBA, a proxy of energy expenditure) constant. Treated birds showed no evidence of increased physiological stress in terms of body condition, oxidative stress, heterophil/lymphocyte ratio and glucocorticoid hormone levels, suggesting locomotory behavior was adjusted to minimize the costs of locomotion resulting from increased drag. Moderate consequences on mitochondrial aerobic metabolism suggest that treated birds may have had to adjust cellular metabolism. Our experimental study provides novel insight into the energetic and physiological constraints of locomotion in diving animals operating near their performance limits.