ABSTRACT

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.