California sea lions are champion deep-divers, withstanding descents more than 300 meters deep. Now, a female California sea lion fitted with a data logger (pictured) has yielded the sea lions' secret, scientists report online today in Biology Letters. For decades, zoologists had suspected that marine mammals such as seals and sea lions collapse their lungs to withstand water pressures and endure deep dives. So scientists used the data logger to keep track of the partial pressure of oxygen in the sea lion's arterial bloodstream—a proxy to detect lung collapse—throughout her dives. During 48 dives in August 2011, each lasting an average of 6 minutes and reaching more than 300 meters deep, the sea lion's lungs collapsed at about 225 meters down—and then re-expanded at the same depth during the mammal's ascent. This technique not only staved off decompression sickness, by keeping nitrogen out of the bloodstream, but also reduced the amount of oxygen delivered from her lungs to her bloodstream—preserving the oxygen within the sea lion's upper airways. When she headed back to the surface, the preserved oxygen re-expanded into the lungs and prevented the sea lion from blacking out in the shallows. A trained, similarly outfitted California sea lion could help scientists gather even more data about this peculiar mechanism, the researchers suggest; we'll have to hold our breath for that.
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