| Summary: | The concept that most dives are aerobic and less than the aerobic dive limit (the onset of post‐dive blood lactate accumulation) is a fundamental concept in diving physiology and the interpretation of diving behavior and foraging ecology. The physiological basis of the aerobic dive limit (ADL) is hypothesized to be depletion of the muscle O 2 store. A dual‐wavelength near‐infrared spectrophotometer was developed and applied to the pectoral muscle to measure myoglobin O 2 saturation levels during dives of emperor penguins ( Aptenodytes forsteri ) at an isolated dive hole on the sea ice of McMurdo Sound, Antarctica. Data revealed several patterns of O 2 depletion. A monotonic decline to near complete O 2 depletion during dives at the ADL occurred in some birds. Based on myoglobin concentration in the pectoral muscle and an assumption of no muscle blood flow, this corresponds to an O 2 consumption rate of six to twelve ml O 2 kg muscle −1 min −1 . However, other birds demonstrated different O 2 depletion patterns suggesting maintenance of some muscle perfusion during dives. Results support the hypothesis that the onset of post‐dive blood lactate accumulation is secondary to muscle O 2 depletion during the dive but that the ADL may vary depending on muscle blood flow patterns. This research was supported by NSF grant OPP 0538594. CLW was supported by a NIH Marine Biotechnology Training Program Fellowship and a UC Regents Fellowship.
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