Acoustic measurements of post-dive cardiac responses in southern elephant seals ( Mirounga leonina ) during surfacing at sea.

Measuring physiological data in free-ranging marine mammals remains challenging, owing to their far-ranging foraging habitat. Yet, it is important to understand how these divers recover from effort expended underwater, as marine mammals can perform deep and recurrent dives. Among them, southern elephant seals ( Mirounga leonina ) are one of the most extreme divers, diving continuously at great depth and for long duration while travelling over large distances within the Southern Ocean. To determine how they manage post-dive recovery, we deployed hydrophones on four post-breeding female southern elephant seals. Cardiac data were extracted from sound recordings when the animal was at the surface, breathing. Mean heart rate at the surface was 102.4±4.9 beats min-1 and seals spent on average 121±20 s breathing. During these surface intervals, the instantaneous heart rate increased with time. Elephant seals are assumed to drastically slow their heart rate (bradycardia) while they are deep underwater, and increase it (tachycardia) during the ascent towards the surface. Our finding suggests that tachycardia continues while the animal stays breathing at the surface. Also, the measured mean heart rate at the surface was unrelated to the duration and swimming effort of the dive prior to the surface interval. Recovery (at the surface) after physical effort (underwater) appears to be related to the overall number of heart beats performed at the surface, and therefore total surface duration. Southern elephant seals recover from dives by adjusting the time spent at the surface rather than their heart rate.