Breathing between the waves: how elephant seals (<i>Mirounga angustirostris</i>) manage oxygen while sleeping and diving

Elephant seals spend much time underwater during extended pelagic trips, with only brief surface intervals, making efficient oxygen management critical for their survival. Renowned for their breath-holding abilities and deep dives, they also exhibit apneic periods while asleep. The role of sleep-associated apneas in breath-hold divers is not clear, with several hypotheses for their occurrence. Nonetheless, terrestrial sleep apneas provide a logistically tractable experimental system in which to investigate oxygen management and hypoxia tolerance in diving seals. Our aim was to examine differences in blood oxygen during sleep and diving apneas, along with consecutive eupneic periods to determine their cardiovascular similarities and distinctions. We tested the null hypothesis that rates of oxygen depletion (during apnea) and subsequent reoxygenation (during eupnea) would be similar in dive and sleep-associated apneas. We analyzed venous PO 2 and ECG in juvenile elephant seals (mean 170.6kg, n=5 individuals, n=16 apneas), comparing rates of deoxygenation and reoxygenation and their durations. We selected dive and sleep apneas of similar lengths (7.07±1.13min, T=7.94, df=14, p=0.55). Apneic durations were longer than subsequent reoxygenation periods during both sleep and diving (F 1,14 =111.0, p&lt;0.0001), consistent with use of a “dive response” to minimize the drawdown of body oxygen reserves. Deoxygenation rates (mmHg O 2 depletion/min) did not differ between sleep and diving (posthoc p=0.08). However, in the eupneic phase, surface reoxygenation rates were 2-fold higher than reoxygenation during sleep (posthoc p&lt;0.0001). This indicates that while deoxygenation is similar during apneas on land and underwater, reoxygenation is expedited between dives. This observation was further supported by time required for complete reoxygenation following apnea, with surface reoxygenation occurring twice as fast as during sleep (posthoc p&lt;0.0001). These data highlight cardiovascular strategies needed to minimize surface time and maximize underwater duration. Preliminary ECG analysis during sleep-associated apnea shows a 15% higher heart rate during the reoxygenation period (T=7.94, df=9, p&lt;0.0001). This study provides new insights into the distinctions between sleep and dive apneas and the utility of sleep-associated apneas in studying hypoxia tolerance of diving mammals. Understanding these mechanisms is essential for comprehending how these animals adapt to the demands of their pelagic lifestyle. Our data indicate that deoxygenation rates are consistent during matched-duration breath-holds on land and at sea, but that these types of apneas differ substantially in their reoxygenation. Further analysis of heart rate will help evaluate the role of the heart in contributing to the observed differences in reoxygenation. Funding by NSF # 2020706, 1921742 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.