Time Domains of Hypoxia Adaptation-Elephant Seals Stand Out Among Divers.

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In this Opinion, adaptations to hypoxia are examined during the short time domains of breath holds from three accomplished diving animals: northern elephant seals (Mirounga angustirostris), California sea lions (Zalophus californianus), and emperor penguins (Aptenodytes forsteri). Review of dive behavior, oxygen (O2) storage, and arterial blood O2 profiles during dives reveals that the elephant seal undergoes the most frequent and extreme hypoxemia. Exceptional breath hold durations, routine hypoxemia, established research protocols, and accessibility to the animals make the elephant seal stand out for physiological investigation and evaluation of biochemical/molecular adaptations in hypoxemic tolerance, protection against re-perfusion injury, and O2 transport during dives. The northern elephant seal and southern elephant seal (M. leonina) are the premier pinniped divers (Le Boeuf et al., 1988; Hindell et al., 1991, 1992; Hassrick et al., 2010; Robinson et al., 2012). During several month-long trips to sea, these animals spend 80–90% of their time underwater, perform routine dives of 20–30 min duration to average depths >400 m, have short inter-dive surface intervals that average two min, and typically gain about one kg d−1 in body mass. In contrast to elephant seals, California sea lions only spend about 30% of their time at sea diving (Feldkamp et al., 1989). Most dives are < 100 m in depth and between 1 and 4 min in duration (McDonald and Ponganis, 2013; Tift et al., 2017). However, dependent on geographic location, climate variability, and prey distribution, these sea lions can regularly perform 10-min dives to 400–500 m, with the current longest reported dive of 16 min (Melin et al., 2008; McHuron et al., 2016, 2018). Emperor penguins are the premier avian divers; they exploit the entire water column to depths >500 m. Shallower dives are up to 5–6 min in duration while deep dives are about 10 min (Kooyman and Kooyman, 1995; Kirkwood and Robertson, 1997; Sato et al., 2011). The longest dive documented by a continuous dive profile in an emperor penguin is 27.6 min. During foraging trips to sea, emperor penguins spent about 31% of their time resting on the sea ice (Watanabe et al., 2012).