Conditioned Variation in Heart Rate During Static Breath-Holds in the Bottlenose Dolphin (Tursiops truncatus)

Previous reports suggested the existence of direct somatic motor control over heart rate (f _H) responses during diving in some marine mammals, as the result of a cognitive and/or learning process rather than being a reflexive response. This would be beneficial for O₂ storage management, but would also allow ventilation-perfusion matching for selective gas exchange, where O₂ and CO₂ can be exchanged with minimal exchange of N₂. Such a mechanism explains how air breathing marine vertebrates avoid diving related gas bubble formation during repeated dives, and how stress could interrupt this mechanism and cause excessive N₂ exchange. To investigate the conditioned response, we measured the f _H-response before and during static breath-holds in three bottlenose dolphins (Tursiops truncatus) when shown a visual symbol to perform either a long (LONG) or short (SHORT) breath-hold, or during a spontaneous breath-hold without a symbol (NS). The average f _H (if _Hstart), and the rate of change in f _H (dif _H/dt) during the first 20 s of the breath-hold differed between breath-hold types. In addition, the minimum instantaneous f _H (if _Hmin), and the average instantaneous f _H during the last 10 s (if _Hend) also differed between breath-hold types. The dif _H/dt was greater, and the if _Hstart, if _Hmin, and if _Hend were lower during a LONG as compared with either a SHORT, or an NS breath-hold (P < 0.05). Even though the NS breath-hold dives were longer in duration as compared with SHORT breath-hold dives, the dif _H/dt was greater and the if _Hstart, if _Hmin, and if _Hend were lower during the latter (P < 0.05). In addition, when the dolphin determined the breath-hold duration (NS), the f _H was more variable within and between individuals and trials, suggesting a conditioned capacity to adjust the f _H-response. These results suggest that dolphins have the capacity to selectively alter the f _H-response during diving and provide evidence for significant cardiovascular plasticity in dolphins.