Effects of carbon dioxide accumulation on post-dive physiological recovery in odontocetes

Diving performance by marine mammals is associated with marked changes in tissue oxygen (O2) and carbon dioxide (CO2) levels. Yet, the primary metric for diving recovery in most studies has focused exclusively on restoring tissue O2, despite the importance of CO2 offloading as a major determinant for diving homeostasis. To assess the combined role of respiratory and blood gases, we compared post-exercise O2 and CO2 recovery rates in bottlenose dolphins (Tursiops truncatus, n=2) and beluga whales (Delphinapterus leucas, n=4). System-wide recovery mechanisms were also examined, including blood pH, breathing patterns and peripheral vasodilation. Following maximal swim repetitions, respiratory O2 and CO2 rates returned to resting levels within 8 min for belugas (V̇O2: 7.64±1.36 min; V̇CO2: 7.71±1.41 min; mean±s.d.) and 3.5 min for dolphins (V̇O2: 3.41±0.76 min; V̇CO2: 3.41±0.71 min). Blood O2 and CO2 recovery durations also varied by species. Belugas required 12-15 min to reach resting levels, whereas dolphins' blood O2 remained within resting levels and CO2 recovered in ∼4-7 min. Blood pH, driven by changes in PCO2, returned to resting levels between 12 and 15 min for belugas, but remained elevated throughout the recorded recovery period for dolphins. Blood lactate also remained near double the resting values for both species. Overall, we found that the compounding effects of CO2 with blood lactate appear to play a dominant role in odontocete dive recovery, which will dictate the duration of full physiological recovery by wild odontocetes following escape responses from anthropogenic disturbances.