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. 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.

(Photo courtesy Ocean Oasis, San Diego Natural History Museum)

 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.

Male elephant seals venture into Alaska waters, while female prefer the vast open ocean during feeding migrations from breeding grounds in California. (Courtesy Russ Andrews, Alaska SeaLife Center; modified from Le Boeuf et al., Ecol. Monogr. 70:353-382, 2000, courtesy Drs. B.J. Le Boeuf and D.P. Costa.)

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.

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.

Elephant seals have a unique ability to store large amounts of oxygen in their blood. All that oxygen comes in handy during deep dives. (Courtesy Russ Andrews, Alaska SeaLife Center.)

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.

In this dive profile, an elephant seal's heart beat dramatically slows during descent. (Courtesy Russ Andrews, Alaska SeaLife Center.)

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.

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.

Elephant seals are diving machines. They spend very little time resting at the surface. (Courtesy Russ Andrews, Alaska SeaLife Center.)

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.

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.

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.