Dive response differs between shallow- and deep-diving Steller sea lions ( Eumetopias jubatus)

Abstract

Muscle exercise correlates with oxygen use, tissue perfusion and heart rate ( f H) in terrestrial animals, but the relationship between these physiological processes is less clear in diving animals. We found the mean heart rate of Steller sea lions trained to voluntarily dive to depths up to 40 m dropped by 40% while diving, and noted that mean bradycardia was 9% greater during shallow (10 m) compared to deep (40 m) dives. Longer dives resulted in lower heart rates, but only when they were shallow; on the other hand, minimum instantaneous f H decreased consistently with dive duration. In general, instantaneous f H did not reflect activity over short timescales. Our data suggest that our sea lions invoked a different dive response depending on whether they dove to shallow or deep depths. During shallow (10 m) dives only, the correlation between activity and f H was indicative of vascular compromise between diving and exercise. However, during deep dives (40 m), there was no such correlation, suggesting that locomotory activity was uncoupled from dive bradycardia, which was possibly mediated by an absence of blood flow to active muscle. For both diving scenarios, surface f H correlated with dive activity, suggesting that some underwater locomotory costs were deferred to the post-dive surface interval. Ultimately, our data support the speculation that Steller sea lion locomotory muscles become hypoxic during diving, regardless of dive depth.