For example, higher concentrations of hemoglobin and the blood protein myoglobin allow emperor penguins to store about 2.5 times more oxygen per unit of body mass than humans, Ponganis noted.
The birds also conserve oxygen by slowing their heart rate to as low as five beats a minute on long dives, even while chasing after fish.
"If we can understand why a diving animal does not incur tissue damage when exposed to low oxygen levels and low blood flow, [we may gain] a better understanding of how such damage occurs in humans and how it might be prevented," Ponganis said.
For the new work, Ponganis and colleagues monitored oxygen levels inside the blood vessels of wild birds living temporarily at the "Penguin Ranch" research facility on McMurdo Sound in southern Antarctica.
(Read "'Penguin Ranch' Reveals Hunting, Swimming Secrets" [January 30, 2004].)
The thick layer of sea ice on the ranch is broken only by diving holes drilled by scientists. Since the birds are too far away from other openings, they must return to the same location after fishing trips.
The study penguins were equipped with tiny sensors inside their air sacs. These sensors transmitted data to special recorders strapped to the birds' backs.
Most of the recorded dives were under six minutes long, but some were substantially longer—including a 23-minute descent that stands as the longest known emperor penguin dive.
David R. Jones has studied the physiology of diving ducks at the University of British Columbia in Vancouver. He was not involved in the penguin research.
"I think it's just amazing that these animals are able to push themselves to such a degree that they use up virtually all of their oxygen stores in their respiratory system and blood," Jones said.
And the ability of Ponganis and colleagues to collect blood-oxygen data in the bitter cold of Antarctica is "amazing," Jones added.
"This is a really important contribution to the field of diving physiology."
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