Physiological limits in the world's extreme divers: how marine mammals manage CO2 accumulation and its effects on dive recovery
Authors: Nazario Emily , Kendall Traci , Williams Terrie 
Published: May 2023

To maintain a daily cycle of extended apnea interspersed with brief recovery breathing periods at the surface, diving mammals utilize a suite of adaptations based on 50 million years of evolution that allow them to withstand rapid, marked fluctuations in metabolic gases (O2, CO2). Although considerable research has been conducted on the use of O2 stores by marine mammals, far less is known about the effects of CO2 accumulation during a dive or its depletion during the post-dive recovery period. Here, we investigated the interrelationships between metabolic CO2 production, blood chemistry (e.g., PCO2, PO2, pH), and the rate of recovery in diving bottlenose dolphins ( Tursiops truncatus). We hypothesized that 1) higher post-dive CO2 accumulation in the blood leads to increased respiratory rates, shorter inter-breath intervals, and longer breaths to promote rapid recovery to resting gas levels, and 2) elevated CO2 accumulation in the blood correlates with extended recovery periods. Using trained dolphins, we simultaneously assessed CO2 production via open-flow respirometry and blood PCO2 to evaluate the influence of CO2 accumulation on post-dive respiration rates and overall dive recovery. Recovery patterns for metabolic CO2 production, ventilation, and total blood CO2 were monitored after sequential dives simulating dive intervals observed in wild populations. We found that immediately following the highest level of exercise, CO2 accumulation in the blood (max = 66.4 mmHg) coincided with some of the lowest breath frequencies (mean = 2.42 breaths/min), longest inter-breath intervals (mean = 13.54 sec), and shortest breath durations (mean = 0.67 sec). Additionally, CO2 accumulation in the blood increased with dive activity and correlated with extended recovery times for inter-breath intervals and breath durations, and acid-base balance metrics such as pH and HCO3-. Interestingly, following all dive counts, both metabolic CO2 and O2 showed similar recovery durations before the return to resting values (at maximum exercise mean CO2 = 204.62 sec, mean O2 = 204.52 sec). In comparison, the recovery of blood gases to baseline lagged behind metabolic and ventilation equilibration. As for other mammals, these marine-living mammals are sensitive to deviations in CO2 that are tightly linked to changes in post-dive ventilation to maintain acid-base balance. Overall, this study demonstrates that perturbations in blood gases require the longest durations to reach pre-dive values compared to changes in rates of oxygen consumption and carbon dioxide production. This study also provides new insights regarding the ability of bottlenose dolphins to manage more considerable deviations in their blood gases despite slower readjustment rates relative to their mammalian counterparts, such as human free-divers. Office of Naval Research, Office of Naval Research Defense University Research Instrumentation Program, National Science Foundation Graduate Research Fellowship Program This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.