Returning on empty: extreme blood O2 depletion underlies dive capacity of emperor penguins.

Blood gas analyses from emperor penguins (Aptenodytes forsteri) at rest, and intravascular P(O(2)) profiles from free-diving birds were obtained in order to examine hypoxemic tolerance and utilization of the blood O(2) store during dives. Analysis of blood samples from penguins at rest revealed arterial P(O(2))s and O(2) contents of 68+/-7 mmHg (1 mmHg= 133.3 Pa) and 22.5+/-1.3 ml O(2) dl(-1) (N=3) and venous values of 41+/-10 mmHg and 17.4+/-2.9 ml O(2) dl(-1) (N=9). Corresponding arterial and venous Hb saturations for a hemoglobin (Hb) concentration of 18 g dl(-1) were >91% and 70%, respectively. Analysis of P(O(2)) profiles obtained from birds equipped with intravascular P(O(2)) electrodes and backpack recorders during dives revealed that (1) the decline of the final blood P(O(2)) of a dive in relation to dive duration was variable, (2) final venous P(O(2)) values spanned a 40-mmHg range at the previously measured aerobic dive limit (ADL; dive duration associated with onset of post-dive blood lactate accumulation), (3) final arterial, venous and previously measured air sac P(O(2)) values were indistinguishable in longer dives, and (4) final venous P(O(2)) values of longer dives were as low as 1-6 mmHg during dives. Although blood O(2) is not depleted at the ADL, nearly complete depletion of the blood O(2) store occurs in longer dives. This extreme hypoxemic tolerance, which would be catastrophic in many birds and mammals, necessitates biochemical and molecular adaptations, including a shift in the O(2)-Hb dissociation curve of the emperor penguin in comparison to those of most birds. A relatively higher-affinity Hb is consistent with blood P(O(2)) values and O(2) contents of penguins at rest.