The dive response redefined: underwater behavior influences cardiac variability in freely diving dolphins

A hallmark of the dive response, bradycardia, promotes the conservation of onboard oxygen stores and enables marine mammals to submerge for prolonged periods. A paradox exists when marine mammals are foraging underwater because activity should promote an elevation in heart rate ( f H) to support increased metabolic demands. To assess the effect of the interaction between the diving response and underwater activity on f H, we integrated interbeat f H with behavioral observations of adult bottlenose dolphins diving and swimming along the coast of the Bahamas. As expected for the dive response, f H while resting during submergence (40±6 beats min−1) was significantly lower than f H while resting at the water surface (105±8 beats min−1). The maximum recorded f H ( f H,max) was 128±7 beats min−1, and occurred during post-dive surface intervals. During submergence, the level of bradycardia was modified by activity. Behaviors such as simple head bobbing at depth increased f H by 40% from submerged resting levels. Higher heart rates were observed for horizontal swimming at depth. Indeed, the dolphins operated at 37–58% of their f H,max while active at depth and approached 57–79% of their f H,max during anticipatory tachycardia as the animals glided to the surface. f H was significantly correlated with stroke frequency (range=0–2.5 strokes s−1, r =0.88, N =25 dives) and calculated swim speed (range=0–5.4 m s−1, r =0.88, N =25 dives). We find that rather than a static reflex, the dive response is modulated by behavior and exercise in a predictable manner.