Respiratory sinus arrhythmia and submersion bradycardia in bottlenose dolphins (Tursiops truncatus)

Among the many factors that influence the cardiovascular adjustments of marine mammals is the act of respiration at the surface, which facilitates rapid gas exchange and tissue re-perfusion between dives. We measured heart rate ( f H ) in six adult male bottlenose dolphins ( Tursiops truncatus ) spontaneously breathing at the surface to quantify the relationship between respiration and f H , and compared this with f H during submerged breath-holds. We found that dolphins exhibit a pronounced respiratory sinus arrhythmia (RSA) during surface breathing, resulting in a rapid increase in f H after a breath followed by a gradual decrease over the following 15-20 s to a steady f H hat is maintained until the following breath. RSA resulted in a maximum instantaneous f H (i f H ) of 87.4±13.6 beats min -1 and a minimum i f H of 56.8±14.8 beats min -1 , and the degree of RSA was positively correlated with the inter-breath interval (IBI). The minimum i f H during 2 min submerged breath-holds where dolphins exhibited submersion bradycardia (36.4±9.0 beats min -1 ) was lower than the minimum i f H observed during an average IBI; however, during IBIs longer than 30 s, the minimum i f H (38.7±10.6 beats min -1 ) was not significantly different from that during 2 min breath-holds. These results demonstrate that the f H patterns observed during submerged breath-holds are similar to those resulting from RSA during an extended IBI. Here, we highlight the importance of RSA in influencing f H variability and emphasize the need to understand its relationship to submersion bradycardia.