Heart rate and oxygen consumption of northern elephant seals during diving in the laboratory

Many techniques have been employed to measure metabolic and cardiovascular changes in diving marine mammals. Each of these methods has its advantages, but the methods also have drawbacks when applied to phocid seals. The aim of this study was to investigate heart rate and metabolic responses to diving in juvenile northern elephant seals that are not associated with forced changes in exercise state, and, secondarily, to investigate whether heart rate could be used as an indicator of metabolic rate in this species. Six seals were allowed to dive freely in a metabolic chamber while simultaneous measurements of heart rate and oxygen consumption were made. Within each dive cycle (dive and surface interval), the seals spent an average of 74% of the time submerged. Mean dive duration was 6.43+/-0.6 (SD) min. Mean oxygen consumption during diving was 3.32+/-0.4 mL O2 min-1 kg-1, a decrease of approximately 26% from baseline values. An inverse relationship was observed between oxygen consumption and the percentage of time spent submerged in each dive cycle. The total amount of oxygen consumed during the surface interval increased with increasing dive duration, while the duration of the surface interval itself did not change, indicating that seals alter the rate of O2 uptake rather than the time spent at the surface. Mean heart rate during diving was 34.5+/-6.2 beats min-1, 36% lower than resting values. Mean diving heart rate was independent of dive duration, percent time submerged, and oxygen consumption. Mean surface interval heart rate was 66.6+/-11.1 beats min-1 and was not correlated with oxygen consumption. Average heart rate over the entire dive cycle increased with increasing oxygen consumption in all of the seals, but there was only a significant relationship in two seals, which casts some doubt on the usefulness of heart rate as an indicator of metabolic rate in this species. While providing important information on the changes in heart rate and oxygen consumption during diving in northern elephant seals, a complete understanding of the diving metabolic rate of these animals will require a combination of approaches that can be used in concert with data on freely living animals.