When aquatic mammals and birds plumb the depths, they have only the oxygen reserves that they carry on board to sustain them; yet, some animals can remain immersed for tens of minutes on a single lungful of air. All divers have evolved a suite of physiological responses to eke out their oxygen reserves, including increasing oxygen stores in venous blood and lowering the heart rate (bradycardia) to conserve oxygen stored in the blood. Birgitte McDonald and Paul Ponganis from the Scripps Institution of Oceanography, USA, explain that although the cardiac response of diving phocid seals (such as grey seals and Weddell seals) have been studied extensively, less was known about the cardiac responses of the otariids, such as fur seals and sea lions. As California sea lions are good divers, McDonald and Ponganis headed to San Nicolas Island, off the coast of Southern California, to measure the heart rates of the diving animals to find out how they cope with submersion (p. ).
The duo captured five females and attached ECG electrodes, a pressure sensor, accelerometers and a radio transmitter to each animal; they then released the sea lions to venture off foraging. After retrieving the data loggers several days later, the team could see that the animals had two distinct heart activity patterns, depending on the dive depth. In short shallow dives lasting less than 2 min, the heart rated dropped rapidly during the descent, remained constant while the animal was at depth and increased rapidly as the seal ascended. However, when the seals undertook lengthier dives that exceeded 3 min and went as deep as 300 m, they raised their heart rate while at the surface prior to the dive before rapidly reducing their heart rate during the initial descent and slowing the heart rate decrease as they neared the bottom. The seals' heart rates then increased slightly while they foraged at depth, before gently increasing during the initial ascent and increasingly rapidly just before the seals returned to the surface. And when the sea lions remained submerged for more than 6 min, their heart rate fell as low as 6 beats min−1.
The team suspects that the deep-diving sea lions use the pre-dive period when their heart rates are high to maximise their venous oxygen reserves in preparation for the long submersion, and they suggest that the very low heart rates experienced at the bottom of the dive might force the animals to consume oxygen stored in the muscle and protect them from absorbing too much nitrogen. The duo also points out that deep-diving emperor penguins use the same strategy, suggesting that this might be a common tactic used by endotherms that dive carrying a lungful of air.
