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

SUMMARY 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 (fH) to support increased metabolic demands. To assess the effect of the interaction between the diving response and underwater activity on fH, we integrated interbeat fH with behavioral observations of adult bottlenose dolphins diving and swimming along the coast of the Bahamas. As expected for the dive response, fH while resting during submergence (40±6beatsmin –1 ) was significantly lower than fH while resting at the water surface (105±8beatsmin –1 ). The maximum recorded fH (fH,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 fH 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 fH,max while active at depth and approached 57–79% of their fH,max during anticipatory tachycardia as the animals glided to the surface. fH 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