Top Story: April 19, 2010

Resting Dives

Northern Elephant Seal, Piedras Blancas, San Simeon, CA 02feb200

Most of us encounter northern elephant seals only when they come ashore. If you’ve driven the coast of California at the right time of year, you’ve probably seen them in Año Nuevo or San Simeon where they’re noisy and plentiful.

But it’s what they do in the water might surprise you. They dive to extraordinary depths for feeding - down to between 1000 and 2600 feet! During their migrations in the Pacific, they will travel the open ocean for several months at a time. According to Dan Costa, Professor of Ecology and Evolutionary Biology at UC Santa Cruz and supervisor of elephant seal research at Año Nuevo, one migration (after they wean their pups) lasts two to three months and the other (after they molt) lasts seven months.

This fact has long puzzled researchers.  How do elephant seals rest while migrating at sea for such long periods of time?

Dr. Costa and other researchers believe they have found the answer to this question, and they published their theory in the April 23rd issue of Biology Letters.

According to the paper, northern elephant seals rest as they perform drift dives, which resemble a type of scuba diving and allow the seal to drift with the currents.  “We found that seals rolled over and sank on their backs during the drift phase, wobbling periodically so that they resembled a falling leaf… this allows them time to rest, process food or possibly sleep during the descent phase of these dives where they are probably less susceptible to predation.”

These drift dives are slower than their other dives. On average, they may last 25 minutes and reach about 1400 feet below sea level. How often are they making these dives? “Mostly they do it more after a series of feeding dives. A few times a day if things are going well,” according to Dr. Costa via email.

Sounds a little like siestas or catnaps, doesn’t it? Making me sleepy… Yawn.

Creative Commons image by Mike Baird

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