Stroke rates and diving air volumes of emperor penguins: implications for dive performance.

Emperor penguins (Aptenodytes forsteri), both at sea and at an experimental dive hole, often have minimal surface periods even after performance of dives far beyond their measured 5.6 min aerobic dive limit (ADL: dive duration associated with the onset of post-dive blood lactate accumulation). Accelerometer-based data loggers were attached to emperor penguins diving in these two different situations to further evaluate the capacity of these birds to perform such dives without any apparent prolonged recovery periods. Minimum surface intervals for dives as long as 10 min were less than 1 min at both sites. Stroke rates for dives at sea were significantly greater than those for dives at the isolated dive hole. Calculated diving air volumes at sea were variable, increased with maximum depth of dive to a depth of 250 m, and decreased for deeper dives. It is hypothesized that lower air volumes for the deepest dives are the result of exhalation of air underwater. Mean maximal air volumes for deep dives at sea were approximately 83% greater than those during shallow (<50 m) dives. We conclude that (a) dives beyond the 5.6 min ADL do not always require prolongation of surface intervals in emperor penguins, (b) stroke rate at sea is greater than at the isolated dive hole and, therefore, a reduction in muscle stroke rate does not extend the duration of aerobic metabolism during dives at sea, and (c) a larger diving air volume facilitates performance of deep dives by increasing the total body O(2) store to 68 ml O(2) kg(-1). Although increased O(2) storage and cardiovascular adjustments presumably optimize aerobic metabolism during dives, enhanced anaerobic capacity and hypoxemic tolerance are also essential for longer dives. This was exemplified by a 27.6 min dive, after which the bird required 6 min before it stood up from a prone position, another 20 min before it began to walk, and 8.4 h before it dived again.