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 ( ) 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 O storage management, but would also allow ventilation-perfusion matching for selective gas exchange, where O and CO can be exchanged with minimal exchange of N . 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 N exchange. To investigate the conditioned response, we measured the -response before and during static breath-holds in three bottlenose dolphins ( ) 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 (i ), and the rate of change in (di /dt) during the first 20 s of the breath-hold differed between breath-hold types. In addition, the minimum instantaneous (i ), and the average instantaneous during the last 10 s (i ) also differed between breath-hold types. The di /dt was greater, and the i , i , and i were lower during a LONG as compared with either a SHORT, or an NS breath-hold ( < 0.05). Even though the NS breath-hold dives were longer in duration as compared with SHORT breath-hold dives, the di /dt was greater and the i , i , and i were lower during the latter ( < 0.05). In addition, when the dolphin determined the breath-hold duration (NS), the was more variable within and between individuals and trials, suggesting a conditioned capacity to adjust the -response. These results suggest that dolphins have the capacity to selectively alter the -response during diving and provide evidence for significant cardiovascular plasticity in dolphins.