Emperors of Diving

Watching emperor penguins plod along the frozen landscape of Antarctica, descriptors such as “agile” or “nimble” would never be used to characterize their wobbly movements.

That changes the moment the giant birds dive into the frigid waters of Antarctica. Under the ice-covered surface, emperor penguins swim after prey with an ultra-efficiency that has mesmerized biologists. Most intriguing is the emperor’s remarkable ability to dive deeply, beyond 500 meters (1,640 feet), and for extraordinarily long durations (more than 23 minutes in a single dive!).

Scientists at Scripps Institution of Oceanography at UC San Diego have analyzed the emperor penguin’s diving prowess and recently gained new knowledge about the physiological adaptations that contribute to its impressive diving ability.

Scripps graduate student Cassondra Williams, who recently defended her Ph.D. dissertation on penguin physiology, has spent the past six years probing oxygen regulation during emperor penguin dives. She spent several years developing a specialized instrument from scratch for measuring muscle oxygen and tested the device during field expeditions to Antarctica in 2007 and 2008.

“The best part of working in Antarctica is that it’s the purest way to look at science,” said Williams. “You are away from the everyday things so you can really focus on your science…. It’s so beautiful to walk out in the middle of the night, look around, and it’s perfectly quiet. You can’t see anything that’s man-made, just white snow and ice.”

Diving emperor penguins initially carry oxygen in three stores—the blood, lungs, and myoglobin in muscle—to sustain aerobic metabolism. But more than five minutes after a penguin leaves the surface, lactate, which follows bursts of energy, begins appearing in its blood and the bird crosses its aerobic dive limit, switching to anaerobic metabolism in some tissues.

What triggers this transition? The animals were thought to cross the aerobic dive limit when one of the three oxygen stores became exhausted. However, when Scripps research physiologist (and Williams’ advisor) Paul Ponganis measured oxygen levels in the blood and air sacs of penguins after long dives, the animals had oxygen to spare. That only left the muscle as the potential trigger. Williams says that diving animals were thought to isolate their muscle from the circulatory system, leaving oxygen stored in the tissue as its only source of aerobic metabolism while submerged and forcing it to switch to anaerobic respiration once the supply was exhausted. The accumulation of lactate during anaerobic respiration may require the birds to spend more time at the surface and less time foraging in order to recycle the lactate.

Williams and Ponganis teamed with Scripps alumna Jessica Meir, currently at the University of British Columbia, to measure muscle oxygen levels directly inside diving emperor penguin muscles and discovered that depleted muscle oxygen supplies trigger the aerobic dive limit. Their findings, supported by the National Science Foundation, a National Institutes of Health Marine Biotechnology Program Fellowship, and a UC Regents Fellowship, were published in the Journal of Experimental Biology.

From the moment she arrived at Scripps in 2005 Williams began developing a new type of “spectrophotometer,” an instrument that uses light intensity to measure the penguins’ muscle oxygen stores as they dive in the wild. After two trying years of technical development and testing, Williams and her colleagues traveled south with her colleagues to surgically implant the spectrophotometers in the pectoralis muscles of the penguins. They also attached time-depth recorders to the animals’ backs to track their dive profiles. The team ensured that the animals would return with their precious equipment by drilling an isolated hole in the sea ice—to which the penguins were guaranteed to return—at the team’s “penguin ranch” site, before releasing the implanted animals to go foraging for a day or two.

After successfully retrieving all of the spectrophotometers and dive recorders and returning the penguins to their colony, Williams began analyzing the data and found that the penguins had been actively foraging beneath the ice. Of the 50 dives that Williams successfully recorded, 31 exceeded the emperor penguin’s calculated aerobic dive limit.

Next, Williams graphed the muscle oxygen profiles over the course of each dive and identified two distinct patterns. In the first pattern, the oxygen levels fell continually, approaching zero around the point when the birds crossed the aerobic dive limit.

“This profile certainly supports the hypothesis that muscle oxygen depletion is the trigger of the aerobic dive limit,” said Williams.

However, the team was surprised when they saw the second pattern, which showed that after initially falling, the oxygen levels plateaued for several minutes before falling again to almost zero. They concluded that blood must be flowing into the muscle to replenish the oxygen supply during the middle phase of the dive, delaying the onset of lactate accumulation.

Ponganis, a practicing physician who specializes in anesthesiology, believes the penguin’s remarkable diving adaptations hold promise not only for understanding the biology of the emperor penguin, but for human health and medicine in cases of low oxygen such as stroke or heart attack.

“I think their metabolic rate is impressive,” said Williams. “You can see how hard they are working underwater but they are efficient swimmers and very hydrodynamic.”

—Mario C. Aguilera and Journal of Experimental Biology