Heart rates of Steller sea lions drop slowly and oscillate while diving.

Lowering heart rate while diving helps marine mammals regulate blood pressure while redistributing blood flow and conserving oxygen during extended dives. However, the classic characterization of this dive response-as a pronounced and abrupt reduction in heart rate observed during forced dives in laboratory settings-contrasts with the higher minimum heart rates and oscillatory patterns observed in freely diving marine mammals in the wild. To assess this apparent discrepancy in cardiovascular control, we measured the heart rates of three Steller sea lions (Eumetopias jubatus) using subcutaneous cardiac monitors during trained stationary dives (3-4 m) in an aquarium pool. During 12 of the longest dives (76-161 s), heart rates decreased from an average of 95 to 34 bpm within the first 26 s of submergence. However, while mean HR at depth eventually averaged around 31 bpm, it also oscillated between 27 and 39 bpm every ~ 5-6 s (0.2 Hz) for much of the dive, before rising prior to surfacing. The observed relatively slow drops in heart rates were similar to those seen in other marine mammals, suggesting an optimal rate of decline that reflects the anticipated conditions of voluntary dives. We further hypothesize that the oscillating minimum heart rates of freely diving marine mammals reflect time delays in the baroreceptor reflex due to the prolongation of circulation time. Our findings suggest these delays shape the rate and pattern of heart rate decline, ultimately influencing cardiovascular control, gas management, and breath-hold duration in diving mammals.