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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 (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.

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