Diving behaviour of southern elephant seals: new models of behavioural and ecophysiological adjustments of oxygen store management.

Among pinnipeds, southern elephant seals (SESs, Mirounga leonina) are extreme divers that dive deeply and continuously along foraging trips to restore their body stores after fasting on land during breeding or moulting. Their replenishment of body stores influences their energy expenditure during dives and their oxygen (O2) reserves (via muscular mass), yet how they manage their O2 stores during their dives is not fully understood. In this study, 63 female SESs from Kerguelen Island were equipped with accelerometers and time-depth recorders to investigate changes in diving parameters through their foraging trips. Two categories of dive behaviour were identified and related to the body size of individuals, with smaller SESs performing shallower and shorter dives requiring greater mean stroke amplitude compared with larger individuals. In relation to body size, the larger seals had lower estimated oxygen consumption levels for a given buoyancy (i.e. body density) compared with smaller individuals. However, both groups were estimated to have the same oxygen consumption of 0.079±0.001 ml O2 stroke-1 kg-1 for a given dive duration and at neutral buoyancy when the cost of transport was minimal. Based on these relationships, we built two models that estimate changes in oxygen consumption according to dive duration and body density. The study highlights that replenishing body stores improves SES foraging efficiency, as indicated by increased time spent at the bottom of the ocean. Thus, prey-capture attempts increase as SES buoyancy approaches the neutral buoyancy point.