The dive response is well documented for marine mammals, and includes a significant reduction in heart rate ( f H ) during submersion as compared while breathing at the surface. In the current study we assessed the influence of the Respiratory Sinus Arrhythmia (RSA) while estimating the resting f H while breathing. Using transthoracic echocardiography we measured f H , and stroke volume (SV) during voluntary surface apneas at rest up to 255 s, and during recovery from apnea in 11 adult bottlenose dolphins ( Tursiops truncatus , 9 males and 2 females, body mass range: 140–235 kg). The dolphins exhibited a significant post-respiratory tachycardia and increased SV. Therefore, only data after this RSA had stabilized were used for analysis and comparison. The average (±s.d.) f H , SV, and cardiac output (CO) after spontaneous breaths while resting at the surface were 44 ± 6 beats min −1 , 179 ± 31 ml, and 7909 ± 1814 l min −1 , respectively. During the apnea the f H , SV, and CO decreased proportionally with the breath-hold duration, and after 255 s they, respectively, had decreased by an average of 18%, 1–21%, and 12–37%. During recovery, the f H , SV, and CO rapidly increased by as much as 117%, 34%, and 190%, respectively. Next, f H , SV and CO rapidly decreased to resting values between 90–110 s following the surface apnea. These data highlight the necessity to define how the resting f H is estimated at the surface, and separating it from the RSA associated with each breath to evaluate the significance of cardiorespiratory matching during diving.

Man-made environmental change may have significant impact on apex predators, like marine mammals. Thus, it is important to assess the physiological boundaries for survival in these species, and assess how climate change may affect foraging efficiency and the limits for survival. In the current study, we investigated whether the respiratory sinus arrhythmia (RSA) could estimate tidal volume ( V T ) in resting bottlenose dolphins ( Tursiops truncatus ). For this purpose, we measured respiratory flow and electrocardiogram (ECG) in five adult bottlenose dolphins at rest while breathing voluntarily. Initially, an exponential decay function, using three parameters (baseline heart rate, the change in heart rate following a breath, and an exponential decay constant) was used to describe the temporal change in instantaneous heart rate following a breath. The three descriptors, in addition to body mass, were used to develop a Generalized Additive Model (GAM) to predict the inspired tidal volume ( V Tinsp ). The GAM allowed us to predict V Tinsp with an average ( ± SD) overestimate of 3 ± 2%. A jackknife sensitivity analysis, where 4 of the five dolphins were used to fit the GAM and the 5th dolphin used to make predictions resulted in an average overestimate of 2 ± 10%. Future studies should be used to assess whether similar relationships exist in active animals, allowing V T to be studied in free-ranging animals provided that heart rate can be measured.