SUMMARYA 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 , 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.

Bradycardia is an important component of the dive response, yet little is known about this response in immature marine mammals. To determine if diving bradycardia improves with age, cardiac patterns from trained immature and mature bottlenose dolphins ( Tursiops truncatus) were recorded during three conditions (stationary respiration, voluntary breath-hold, and shallow diving). Maximum (mean: 117+/-1 beats.min(-1)) and resting (mean: 101+/-5 beats.min(-1)) heart rate (HR) at the water surface were similar regardless of age. All dolphins lowered HR in response to apnea; mean steady state breath-hold HR was not correlated with age. However, the ability to reduce HR while diving improved with age. Minimum and mean steady state HR during diving were highest for calves. For example, 1.5-3.5-year-old calves had significantly higher mean steady state diving HR (51+/-1 beats.min(-1)) than 3.5-5.5-year-old juveniles (44+/-1 beats.min(-1)). As a result, older dolphins demonstrated greater overall reductions in HR during diving. Longitudinal studies concur; the ability to reduce HR improved as individual calves matured. Thus, although newly weaned calves as young as 1.7 years exhibit elements of cardiac control, the capacity to reduce HR while diving improves with maturation up to 3.5 years postpartum. Limited ability for bradycardia may partially explain the short dive durations observed for immature marine mammals.