Some marine birds and mammals can perform dives of extraordinary duration and depth. Such dive performance is dependent on many factors, including total body oxygen (O2) stores. For diving penguins, the respiratory system (air sacs and lungs) constitutes 30-50% of the total body O2 store. To better understand the role and mechanism of parabronchial ventilation and O2 utilization in penguins both on the surface and during the dive, we examined air sac partial pressures of...