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

Abstract

SUMMARYBlood gas analyses from emperor penguins (Aptenodytes forsteri) at rest, and intravascular PO2 profiles from free-diving birds were obtained in order to examine hypoxemic tolerance and utilization of the blood O2 store during dives. Analysis of blood samples from penguins at rest revealed arterial PO2s and O2 contents of 68±7 mmHg (1 mmHg= 133.3 Pa) and 22.5±1.3 ml O2dl–1 (N=3) and venous values of 41±10 mmHg and 17.4±2.9 ml O2 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 PO2 profiles obtained from birds equipped with intravascular PO2electrodes and backpack recorders during dives revealed that (1) the decline of the final blood PO2 of a dive in relation to dive duration was variable, (2) final venous PO2 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 PO2 values were indistinguishable in longer dives, and (4) final venous PO2 values of longer dives were as low as 1–6 mmHg during dives. Although blood O2 is not depleted at the ADL, nearly complete depletion of the blood O2 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 O2–Hb dissociation curve of the emperor penguin in comparison to those of most birds. A relatively higher-affinity Hb is consistent with blood PO2values and O2 contents of penguins at rest.