Testing The Depths Of Life - research on elephant seals

National Wildlife, Feb-March, 1999

Northern elephant seals migrate farther than any other mammal, spending much of their time at bone-crushing depths. How do they do it?

It's late february, early morning. Strong northerly winds blow across San Miguel Island, one of the Channel Islands off Southern California, an important breeding ground for northern elephant seals. Bull elephant seal number 666 lumbers across the beach like an overstuffed caterpillar. When he enters the water, however, he transforms from an awkward animal to a graceful creature--one of the most powerful swimmers of all marine mammals.

The bull is emaciated. For the two- to three-month breeding season, he has fasted and fought continual bloody battles on land with other males for the right to breed. As a result, he has lost half of the two tons he weighed when he appeared on the beach in December. Now he must start putting that weight back on, to compete in the next breeding season nine months hence.

Number 666 is part of a ground-breaking study of elephant seal behavior. For the past seven years, Brent Stewart, a biologist at Hubbs-SeaWorld Research Institute in San Diego, and Robert DeLong, a biologist for the National Marine Fisheries Service in Seattle, have attached tracking devices to number 666 and his cohorts to find out what elephant seals do during the eight to ten months they spend at sea. Those instruments have revealed that elephant seal females travel nearly 12,000 miles annually, and males more than 13,000 miles each year--the longest migration of any mammal on Earth.

But this lengthy migration is not the only surprise the devices reveal. Seal number 666 will, during his journey, dive to 5,150 feet, a record for his species. "We've been educated to believe there are set limits to the mammalian system," says Stewart, "but then we get back these recorders from the elephant seals and the recorders say Oguess again.'"

By studying elephant seal diving behavior, scientists are learning more about a rich zone of marine life so deep that elephant seals and sperm whales are the only mammals known to venture there. And they are learning about some unique adaptations elephant seals have to survive at these depths, adaptations that may one day help prevent sudden infant death syndrome (SIDS) in humans.

Elephant seals breed on islands off California and Baja California, Mexico. They get their name from the long trunklike proboscis that hangs down from the bull's face. Elephant seals are the largest and most powerful of all the four-flippered marine mammals known as pinnipeds. Males stretch 16 feet long and weigh as much as two tons, while females average 10 feet long and weigh only half as much as males.

In the late 1800s, elephant seals were almost hunted to extinction by sealers who sought them for their fat, which the sealers converted into oil for machinery lubricants. Now these protected animals--which range from Baja California to the Aleutian Islands--have made a remarkable comeback; current numbers are estimated at 150,000.

Part of what makes elephant seals unique is that they migrate twice a year between southern beaches and northern feeding grounds--once after mating and again after molting (shedding their fur and skin). And these animals do not just swim straight to feeding grounds; they dive continuously throughout their journeys, going to extreme depths to feed on deep-sea fish and squid. These dives add an average of 5,000 vertical miles to their lengthy horizontal journeys. "They are basically on the move the whole time," says DeLong.

Elephant seal males and females travel to different locations, though scientists do not understand why. Female elephant seals tracked by Stewart and DeLong leave the Channel Islands and head for spots about 2,000 miles off the Washington coast. Males go farther, to the Gulf of Alaska and the eastern Aleutian Islands.

Elephant seals travel alone, covering about 60 nautical miles a day. It takes males 45 days to travel to Alaskan waters, where they spend about 35 days before heading back south. Back in California, the seals shed their hair and upper layer of skin during another fast, lasting three to four weeks, before returning to Alaska. DeLong speculates that all this molting and fine-tuning of the fur may be necessary to protect their skin on land or to decrease drag in the water.

Scientists are not sure why the seals make this dual round-trip, when there are many beaches in Alaska, British Columbia and the Pacific Northwest that appear to be suitable molting sites. DeLong says it may be because most of these northern beaches have had bears and wolves on them, and, more recently, people.

At sea, elephant seals spend about 90 percent of their time underwater, diving continually. Antarctica's Weddell seals, which are about the size of female elephant seals, dive regularly for about 11 hours, but they then must spend the next 11 to 13 hours resting on the ice. Even humpback and sperm whales alternate bouts of deep diving with periods of slow surface travel or rest, lasting from 30 minutes to several hours. Elephant seals, however, never stop.