| Summary: | To investigate the physiological enigma of the impressive dives of emperor penguins and elephant seals, blood oxygen (O₂) transport and depletion in these species were addressed with a three tiered approach. First, the transport of O₂ was examined by assessing heart rate (the principal determinant of blood O₂ depletion) during dives of emperor penguins. Secondly, O₂ transport was investigated at the biochemical level by characterizing the O₂-hemoglobin (Hb) dissociation curves for these species. Finally, the actual depletion of O₂ in the blood was documented by measuring blood O₂ partial pressure (Po₂) and temperature continuously during dives with a backpack recorder on translocated, juvenile elephant seals. Application of the O₂-Hb dissociation curve to Po₂ dive data also allowed for calculation of % Hb-saturation during dives. These studies revealed physiological responses and biochemical adaptations that contribute to the remarkable dive capacity of these species, including: 1) In contrast to any other diving bird, but similar to that of seals, emperor penguin heart rate while diving is significantly lower than resting rate, with values as low as 6 beats per minute in longer dives. This suggests parsimonious O₂ utilization and allows extended dive time. 2) The hemoglobin of the emperor penguin has a significantly higher affinity for O₂ as compared to other birds. It is in the range of seals and other marine mammals, allowing for more complete utilization of the respiratory O₂ store and increased blood O₂ content when Po₂ is low. 3) The elephant seal possesses exceptional tolerance to low Po₂ in the blood, with arterial Po₂ of 12 -23 mmHg and venous Po₂ of 2-10 mmHg at the end of routine dives. These Po₂ values correspond to hemoglobin saturations as low as 1-26% and O₂ contents of 0.3 (venous) and 2.7 ml O₂ dl⁻¹ blood (arterial). Temperature data collected during elephant seal studies revealed that core body temperature is preserved during diving, inconsistent with previous assertions of hypometabolism and a cold induced Q₁₀ effect during diving. Such results support the hypothesis that these species routinely "push the envelope" of the usual physiological limits of homeotherms to achieve such extraordinary dives
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