At the impressionable age of seven, Birgitte McDonald packed up and headed out on a vacation from her Bakersfield home to San Diego with her family. The family's collective passion for critters of all types led to a jam-packed whirlwind tour of the San Diego Zoo, Wild Animal Park, and Sea World.
That trip sparked a lifelong interest in biology that continues today at Scripps Institution of Oceanography at UC San Diego, where, as a postdoctoral researcher, McDonald recently made a significant contribution to science's understanding of marine mammals and their astonishing diving capabilities.
For a recently published study, McDonald and her colleagues, including her advisor, Scripps research physiologist Paul Ponganis, obtained key physiological measurements from a single sea lion, which was tagged with a data-logging backpack that measures oxygen and records time and depth during dives.
Sea lions are capable of holding their breath for up to 10 minutes and reaching 500 meters (1,640 feet) or more. Scientists have pondered how these animals accomplish such diving feats and rapidly rise to the surface without experiencing a dangerous rise of nitrogen in their tissues, which can lead to deadly decompression sickness, or "the bends."
Such scientific speculation has endured for decades. Per Scholander, the Scripps marine physiologist for whom the building housing Ponganis' laboratory is named, was a forerunner in documenting the unusual anatomy of sea lion and seal lungs. Scripps research physiologist Jerry Kooyman has documented the stunning diving ability of Emperor penguins, which employ their own method of deep diving.
McDonald and Ponganis recently described for the first time how sea lion lungs collapse during deep dives at sea. Their data painted a scenario in which sea lion lungs collapse at about 220 meters (730 feet) and then re-expand at a similar depth on ascent, thus avoiding dangerous nitrogen levels and allowing them to tap into an oxygen reservoir at the conclusion of the dive. While at the deepest portions of the dive, the sea lions switch to alternate oxygen stores in their blood and muscles.
"Although they are not known to be the best divers, like the elephant seal, California sea lions are a very good representative of sea lion species in general and we wanted to know how these animals are able to do these long duration dives to pretty great depth," McDonald said. "By collapsing their lungs and saving oxygen for the end of the dive, sea lions decrease the risk of blacking out in shallow water due to low oxygen levels during the final segment of the dive. Also, lung collapse at depth reduced nitrogen absorption, decreasing the risk of the bends."
McDonald's data revealed that once sea lions reach the surface, they take a quick breather and then head down again as part of a series of dives that regularly reach 300 meters (984 feet).
"They reload and do it all again for another dive," said McDonald.
Her field research, centered at a deserted beach on San Nicolas Island off the Southern California coast, demands McDonald and her fellow researchers to be physically fit as well as stealthy in order to scrutinize the highly skittish California sea lions. Even a wayward seagull can alert the jumpy sea lions of the visitors' presence, so the researchers are required to covertly navigate down a seaside bluff, then crawl along the sandy beach to approach the sea lion colony.
On Monday, Nov. 5, McDonald will continue her study by leading a group of researchers back into the field to study sea lions in the wild.
"The first study looked at what's going on in the blood and oxygen during their dives," said McDonald. "And now we're trying to get more of the big picture."
- Mario C. Aguilera