Elephant Seal Slumber
INTRO: All marine mammals have the remarkable ability to dive and forage for extended periods below the ocean surface. But few marine mammals dive deeper or stay longer below the surface than do elephant seals. As Doug Schneider reports in this week's Arctic Science Journeys Radio, one Alaska scientist is unraveling the mysteries of the elephant seal's amazing diving ability.
STORY: Alaska is famous for its walrus, its seals and its Steller sea lions. But few people know that Alaska plays host to thousands of northern elephant seals, too. That's probably because elephant seals don't usually come ashore in Alaska. And they don't spend much time lounging about on the surface, either, so few people ever catch a glimpse. Rather, they spend the summer foraging the deep ocean seafloor—usually from 1,000 to 2,500 feet below the surface, but sometimes as deep as 5,000 feet. Exactly how these two-tonned hulks are able to dive to depths that would crush other animals is a mystery that scientist Russ Andrews is trying to unravel.
ANDREWS: "You can imagine that it's really difficult to go to those really great depths. So we know their bodies are put together to allow them to be squeezed. I'm sure that even if you just swim to the bottom of the pool, you sense your ears starting to squeeze. And if you're a scuba diver, you know that your sinuses really start to hurt. The same thing happens in your lungs because your lungs are not made to be squished by that high pressure. And if they can't squish because of the physical structure, like the way we're put together, blood has to fill that space. And that means blood is coming out of blood vessels—that shouldn't happen. This doesn't happen in marine mammals, and especially elephant seals because they are put together differently to be able to handle going down really deep."
Russ Andrews is a marine biologist at the Alaska SeaLife Center in Seward, Alaska, and a research professor at the University of Alaska Fairbanks School of Fisheries and Ocean Sciences. He's spent years studying elephant seals, and he says this unusual marine mammal has physiologically evolved into an amazing diving machine.
ANDREWS: "They are sort of prepared to leave the surface because they have built up a lot of oxygen storage. They breathe really fast so they store up a lot of oxygen. Surprisingly they don't do what we do—take a really big deep breath as our last breath and go down. Well they don't do that. They have so much ability to store oxygen in their blood that they actually exhale on their very last breath. By exhaling, that means there's less air in their lungs, and it's easier for their lungs to collapse. So as they start to swim down, the first thing they do is slow their heart rate. So it can cut off blood flow to parts of its body to slow down how fast its overall body is using that oxygen that's now only stored up inside them. Then they start to dive down. As they're diving down their lungs are getting squeezed more and more and more. That means they're getting more and more negatively buoyant. So they sink. And after a while they don't even have to use their flippers anymore to push them down. They can just start gliding down. It'd be like you and I having a whole bunch of lead weights. We'd just fall to the bottom."
Throughout the descent, the heart rate continues to decline. In some seals the heart rate slows to just three beats per minute. Once on the bottom, however, their heart rate increases as they swim about to find food. Elephant seals typically stay underwater for about 30 minutes, but scientists have monitored individuals that stayed below the surface for two hours.
Russ Andrews says elephant seals dive almost constantly, even when they're just traveling from place to place. Going to the surface just long enough to get air is probably an adaptation aimed at avoiding surface predators like sharks and killer whales.
ANDREWS: "There are some elephant seals that never spent more than 10 minutes at the surface at any one time. A few elephant seals had extended surface intervals of maybe one or two hours at the surface at a time. But this is one or two hours out of five or six or seven months at sea."
And since elephant seals seem always to be diving, Andrews wondered when they would have time to sleep. And while he hasn't yet proved it, he thinks elephant seals have evolved a unique solution.
ANDREWS: "If they are indeed gliding on the way down, that means they can probably go to sleep, too. And so that's what we want to look at next."
Whether elephant seals actually sleep during their dives is a question he hopes to solve by monitoring the brain waves of several seals during their descents. If it's true that seals actually sleep while sinking to the seafloor, Andrews will have even more questions.
ANDREWS: "That raises the question of what keeps them from gliding into the abyss of the ocean? Even though they're built for being squeezed by pressure, they aren't perfect. They need something to wake them up and keep them from going too deep. There's some tricks there we don't understand."
OUTRO: This is Arctic Science Journeys Radio, a production of the Alaska Sea Grant Program and the University of Alaska Fairbanks. I'm Doug Schneider.
Audio version and related Web sites (sidebar at top right)
Thanks to the following individual for help preparing this script:
Russ Andrews, Research Assistant Professor
Arctic Science Journeys is a radio service highlighting science, culture, and the environment of the circumpolar north. Produced by the Alaska Sea Grant College Program and the University of Alaska Fairbanks. The shortcut to our ASJ news home page is www.asjnews.org.
URL for this page is http://seagrant.uaf.edu/news/
Northern Elephant Seal Facts
While a rare event, northern elephant seals sometimes haul out onto Alaska shores. In March 1999, a juvenile elephant seal hauled out just down the street from the Alaska SeaLife Center in Seward, Alaska. It was brought to the center for rehabilitation because it was suffering from a disease called "scabby molt." This elephant seal was released back into the wild a few months later.
Some 200,000 northern elephant seals roam the waters of the Gulf of Alaska, during months-long feeding migrations from breeding grounds off California.
Only sperm whales dive deeper and stay longer below the surface than elephant seals.
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