Re-evaluating the significance of the dive response during voluntary surface apneas in the bottlenose dolphin, Tursiops truncatus

Abstract

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.

Drivers of the dive response in trained harbour porpoises(Phocoena phocoena)

“…S1). Here, we investigated the drivers of these modes, which are ascribed to both the dive response and RSA (Fahlman et al, 2019).…”

Conditioned Variation in Heart Rate During Static Breath-Holds in the Bottlenose Dolphin (Tursiops truncatus)

“…Development of electronic devices that could record physiological changes continuously allowed the cardiovascular responses to be measured in freely diving birds and mammals. These studies confirmed that a diving bradycardia was observed during voluntary dives, but it was much more variable and not always as extreme as during forced dives (Elsner, 1965(Elsner, , 1966Elsner et al, 1966;Kooyman and Campbell, 1972;Jones et al, 1973;Butler and Woakes, 1979;Kanwisher et al, 1981;Blix and Kjekshus, 1983;Blix, 1987;Ponganis et al, 1991;Thompson and Fedak, 1993;Andrews et al, 1997;Ponganis et al, 1997;Houser et al, 2010;Fahlman et al, 2019b). Based on these data, the significance of the dive response has been argued.…”

“…The pre-dive f H was the average f H 60 s before the breath-hold (Pre). As past studies in the bottlenose dolphin have shown that f H , stroke volume, and cardiac output decreases with longer dive duration (Fahlman et al, 2019b), the minimum instantaneous f H (if Hmin ) during the breath-hold, and the average instantaneous f H (if Hend ) over the last 10 s of the breath-hold, were evaluated and expected to be lower for longer dives. To assess the anticipatory, or conditioned response, we compared the rate of change in instantaneous f H (dif H /dt), the average instantaneous f H (if Hstart ), and the minimum instantaneous f H (if Hstartmin ) during the first 20 s of the breath-hold.…”

“…Each trial consisted of an animal floating stationary in the water, dorsal side up and rostrum on deck with blow-hole out of the water. The trainer gave the dolphin a discriminative stimulus (SD) to turn its ventral side up to allow placement of 3 ECG electrodes on the sternum (Bickett et al, 2019;Cauture et al, 2019;Fahlman et al, 2019bFahlman et al, , 2020b. When the ECG signal was confirmed, the animal was given an SD to turn to floating dorsal side up, and the ECG signal was again verified.…”

“…To follow up on these previous studies with an experimental design that randomized the order of the dive duration, and to further evaluate the selective gas exchange hypothesis and the cardiorespiratory physiology in cetaceans, we wanted to determine the extent by which cetaceans are able to adjust f H , and thereby cardiac output (Bickett et al, 2019;Fahlman et al, 2019bFahlman et al, , 2020b, based on the expected dive duration. For this purpose, we conditioned 3 bottlenose dolphins (Tursiops truncatus) to voluntary participate in static breath-holds.…”