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 that is maintained until the following breath. RSA resulted in a maximum instantaneous f_H (if_H) of 87.4±13.6 beats min^-1 and a minimum if_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 if_H during 2 min submerged breath-holds where dolphins exhibited submersion bradycardia (36.4±9.0 beats min^-1) was lower than the minimum if_H observed during an average IBI; however, during IBIs longer than 30 s, the minimum if_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.