Comments on point: counterpoint: high altitude is/is not for the birds!

TO THE EDITOR: The debate between Scott et al. (6) and Llanos et al. (4) highlights the remarkable ability of birds and mammals to adapt and survive at high altitudes and cope with reduced oxygen (O2) availability. Scott and colleagues (6) make a compelling argument that birds are far superior to mammals at adapting and thriving in hypoxic environments, both at rest and during exercise. While their argument is supported by comparisons between birds and terrestrial mammals, it ignores marine mammals. Marine mammals such as seals are routinely exposed to and tolerate long bouts of hypoxia during breath-hold diving. For example, elephant seals may dive to depths of nearly 1,600 m and occasionally remain submerged for 2 h (3). Moreover, marine mammals can be exposed to partial pressures of arterial oxygen (PaO2) as low as 12 mmHg during free dives (5), which is less than the 20 mmHg observed in the bar-headed goose (6). Interestingly, a PaO2 of 12 mmHg observed in elephant seals corresponds to arterial saturations and O2 content of only 8% and 2.7 ml O2/dl (5). Thus marine mammals such as the elephant seal demonstrate a remarkable hypoxia tolerance. The integration of several key physiological adaptations including 1) substantially greater myoglobin concentrations compared to other mammals, 2) reductions in metabolism, 3) a greater reliance on aerobic metabolism (i.e., less lactate production), 4) a host of dramatic acute cardiovascular adjustments; and 5) an increased intrinsic cerebral hypoxia tolerance allow marine mammals to thrive in hypoxic environments and enable long deep dives (1–3).