|Emperor penguins in Antarctica. |
Photo by StormPetrel1.
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But first of all, how do you measure oxygen levels in a penguin's muscles? It was no easy task - one of the study's authors, Cassondra Williams from the Scripps Institution of Oceanography, had to design a special probe which could be surgically implanted in a penguin's pectoralis muscle. After two years of technical testing and development, the probes were ready for use on wild birds, so Williams and fellow researchers Paul Ponganis and Jessica Meir headed to McMurdo Sound in Antarctica.
In Antarctica, the researchers implanted probes in selected emperor penguins and attached time-depth recorders to their backs to track their dive profiles. The penguins were then released to forage for a day or two through an isolated hole that the researchers had drilled in the sea ice, which ensured the penguins (and the expensive equipment) would return.
Once the probes and time-depths recorders were retrieved, and the penguins had been returned to their colony, the scientists began to analyse the data from the 50 successfully recorded dives from three penguins. Looking at the patterns of oxygen use over the course of each dive, two distinct patterns emerged.
In the first type of dive, the muscle's oxygen levels fell continually until they approached zero - the penguin appeared to cut off blood flow to the muscle, so it had to rely on its own oxygen supplies, leaving the blood oxygen for the rest of the body, such as the brain and heart.
Williams calculated that the mean muscle oxygen consumption rate for these types of dives was only 12.4ml of oxygen per kilogram of muscle per minute - 1/10th the value calculated for penguins swimming in an artificial flume.
"I think this metabolic rate is impressive. You can see how hard they are working underwater but they are efficient swimmers and very hydrodynamic," she said.
In the second type of dive, the muscle's oxygen levels initially fell and then plateaued for several minutes before falling again to almost zero. The researchers realised that the blood must flow back into the muscles to replenish the oxygen supply during the middle phase of the dive. While this stops the muscles from getting tired, the penguins can only do this until the blood oxygen levels become too low for the rest of the body.
In both types of dive, when the oxygen levels in the muscles approach zero, the muscles start to make energy using anaerobic respiration - that is, without oxygen. Unfortunately, a byproduct of this type of respiration is the production of lactic acid. Researchers believe that if the penguins let the lactic acid accumulate in their muscles, it takes them longer to recover after a long dive. This might be why on some dives, the penguins send extra oxygen to their muscles.
Williams told LiveScience, "They don't want to hit their aerobic limit and accumulate lactic acid, but it's not clear how or why they do that."
Penguins continue diving long after muscles run out of oxygen, 12 May 2011, Journal of Experimental Biology
Penguins' oxygen trick: How they survive deep dives by Jennifer Welsh, 12 May 2011, LiveScience
Journal of Experimental Biology citation
Williams, C. L., Meir, J. U. and Ponganis, P. J. (2011). What triggers the aerobic dive limit? Patterns of muscle oxygen depletion during dives of emperor penguins. J. Exp. Biol. 214, 1802-1812.