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Diving seems effortless for penguins. Plunging beneath the surface, emperor penguins regularly remain submerged for up to 12 min by carefully managing their oxygen reserves. Paul Ponganis from the Scripps Institution of Oceanography explains that emperors diving from isolated ice holes fuel the dive aerobically for the first 5.6 min and supplement the remainder of the dive with anaerobic metabolism. However, when Ponganis compared the aerobic dive limit for ice hole diving penguins with estimates of the aerobic dive limit for freely foraging animals, it appeared that the free ranging birds were able to sustain the aerobic portion of a dive for up to 8 min. What were the free ranging birds doing to eke out their oxygen supply for an additional 2.4 min? Ponganis and an international team of collaborators travelled to the Antarctic to find out how the birds extend their aerobic dive limit (p. 2854).

Attaching swim speed/acceleration data loggers to penguins diving in open water and through an isolated ice hole, Katsufumi Sato, Greg Marshall, Gerald Kooyman and Ponganis allowed the free ranging birds to venture off foraging for a couple of weeks while the ice hole divers dipped in and out of the water. `From the acceleration data you can see a surge every time the animal strokes with its wings, so you can count the number of peaks per dive to get the stroke rate pattern,' explains Ponganis. He adds, `We expected that stroke rate would be lower in dives at sea and because of that there would be less muscle work and less oxygen consumption and that would explain how these birds dive as long and as frequently as they do.'

However, the freely diving birds were stroking faster. The birds were not extending their aerobic dive limit by beating their wings more slowly to conserve oxygen. And when the team compared the length of time spent by birds at the surface recovering from dives, the free divers spent no more time at the surface than the ice-hole divers. `Then we became interested in looking at the diving air volume, how much air they take down with them, because it is a significant proportion of the oxygen store,' explains Ponganis.

`Knowing the swim speed, depth and body angle during a penguin's passive glide to the surface at the end of a dive, we can make calculations – based on a buoyancy model developed by Katsu Sato – as to how much air is in the respiratory system,' says Ponganis. Assuming that the penguins did not exhale while submerged, the team found that the penguins carried more air as they extended their dives down to 300 m. The penguins seemed to anticipate how deep they would dive and adjusted the amount of air they carried down accordingly.

However, penguins that dived between 400 and 500 m appeared to be carrying less air than the birds that only dived to 300 m. `They probably exhaled prior to the final segment of the dive and that is why we were getting the low volumes, and we are trying to pursue that,' Pongains says.

Most amazingly, the team recorded one dive where an emperor penguin remained submerged for a record breaking 27.6 m. According to Ponganis, the accelerometery data show that after it emerged from the water the penguin just lay on the ice for 6 min before it stood, took another 20 min before it started walking and then waited a further 8.4 h before it ventured back into the water. `This animal was exhausted,' says Ponganis, who suspects that the dive was extended when the pack ice shifted above the penguin's head, blocking its escape route.

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