A perplexing thing about penguins is that they can stay underwater longer than we can, even though their blood is no more rich in oxygen than ours. Now scientists may have figured out how penguins do it: The birds cut blood flow to inactive organs and go into hypothermia.
By comparing the rate of oxygen consumption in body tissues to blood oxygen levels, scientists can calculate the maximum amount of time that an aerobic animal can stay submerged. In general, the larger the animal, the longer the dive. Trained people can stay down for about 4 minutes, reaching a depth of about 100 meters. According to their body size, penguins should have to surface in half that time. A king penguin, however, can stay down for more than 7 minutes, diving up to 300 meters.
This discrepancy was a mystery until a team of researchers from France, England, and Germany implanted temperature sensors in 12 penguins and monitored their feeding habits. They found that during deep-sea dives, the penguins' stomach temperatures drop from a toasty 38 degrees Celsius to as low as 19oC--not surprising, as the temperature of the fish the birds consume is only about 4oC. However, they also found that the temperature of the rest of the abdomen drops to a mere 11oC, much cooler than could be explained by the penguin's frigid diet.
The team concludes in tomorrow's issue of Nature that the penguin body alters its metabolism by restricting blood flow to nonessential tissues during a deep dive. This would account for the penguin's ability to stay underwater longer. "If you decrease perfusion of blood into inactive tissue, you decrease oxygen consumption, heat production, and temperature," says team member Yves Handrich, a physiologist at the Centre National de la Recherche Scientifique in Strasbourg, France.
"It's a perplexing observation, a very exciting observation," says Gerald Kooyman, an animal physiologist at the Scripps Institution of Oceanography in La Jolla, California. "The idea that it's not food [causing the cooling] is a little bit of a surprise to me." The penguins seem to undergo a more drastic metabolic shift than deep-diving mammals such as whales and seals. "As whales dive, they restrict blood flow, and there's a metabolic decline in the periphery," says Zoe Eppley, an evolutionary physiologist at the University of Maine. Penguins, however, "may very well cut blood flow down to the core," she says.