What triggers the aerobic dive limit? Patterns of muscle oxygen depletion during dives of emperor penguins

The physiological basis of the aerobic dive limit (ADL), the dive duration associated with the onset of post-dive blood lactate elevation, is hypothesized to be depletion of the muscle oxygen (O-2) store. A dual wavelength near-infrared spectrophotometer was developed and used to measure myoglobin (Mb) O-2 saturation levels in the locomotory muscle during dives of emperor penguins (Aptenodytes forsteri). Two distinct patterns of muscle O-2 depletion were observed. Type A dives had a monotonic decline, and, in dives near the ADL, the muscle O-2 store was almost completely depleted. This pattern of Mb desaturation was consistent with lack of muscle blood flow and supports the hypothesis that the onset of post-dive blood lactate accumulation is secondary to muscle O-2 depletion during dives. The mean type A Mb desaturation rate allowed for calculation of a mean muscle O-2 consumption of 12.4. ml O-2. kg(-1) muscle. min(-1), based on a Mb concentration of 6.4. g 100. g(-1) muscle. Type B desaturation patterns demonstrated a more gradual decline, often reaching a mid-dive plateau in Mb desaturation. This mid-dive plateau suggests maintenance of some muscle perfusion during these dives. At the end of type B dives, Mb desaturation rate increased and, in dives beyond the ADL, Mb saturation often reached near 0%. Thus, although different physiological strategies may be used during emperor penguin diving, both Mb desaturation patterns support the hypothesis that the onset of post-dive lactate accumulation is secondary to muscle O-2 store depletion.