Testing the Depths of Life

Northern elephant seals migrate farther than any other mammal, spending much of their time at bone-crushing depths

  • Michael Tennesen
  • Feb 01, 1999
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 Ôguess 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.

One of their secrets is an ability to conserve energy underwater. Their metabolism slows by as much as 40 to 50 percent during dives. "It´s sort of like holding your breath while doing an aerobics class," says Michael Castellini, a marine scientist at the University of Alaska at Fairbanks.

Typically an elephant seal dives two to three times per hour. Each dive lasts about 20 minutes, followed by 2 to 3 minutes at the surface, followed by another dive. Dives can go longer: Researchers recorded one female elephant seal diving for two hours. At the root of this amazing ability are some unique physiological adaptations. To decrease oxygen consumption, scientists believe that diving elephant seals gradually close off the flow of blood to muscle, tissue and internal organs, restricting it to the brain and central nervous system. They even decrease their heart rates, gradually dropping from 100 to 120 beats per minute at the surface to 30 to 35 at depth. The seal´s heart rate can slow to 2 to 3 beats per minute for short periods--close to cardiac arrest for a human.

The animals are able to decrease oxygen consumption in part because swimming requires little effort. Elephant seals are shaped very much like submarines, and have a fine coat of hair that may help decrease their drag in the water. Castellini believes that diving underwater and chasing fish may cost elephant seals only 1.5 times as much energy as sleeping.

Sleep is another of the great mysteries of these animals. If they dive all the time, when do they sleep? Castellini has evidence, from studies of seal pups, that elephant seals may be able to sleep while swimming. Sleeping and diving may pose no danger to elephant seals because their main predators, great white sharks and killer whales, are rarely found below 130 feet.

Elephant seals often stop breathing during sleep on land, a phenomenon called sleep apnea. These periods last 10 to 20 minutes, interspersed with 2 to 3 minutes of regular breathing. Castellini is looking at sleep apnea in elephant seal pups as a way of understanding sleep apnea in human infants. Though suspended breathing while sleeping may be natural to elephant seals, it can be lethal to humans. The work has broadened to look at sleep apnea in other pinnipeds in the hopes of understanding and controlling this human infant killer.

Elephant seals´ lungs collapse and become airless as they dive, although scientists do not know at what depth this occurs. With no air, there can be no exchange of gas (especially nitrogen) between lungs and blood, and thus elephant seals avoid the blood-chemistry imbalances such as the bends (nitrogen bubbles) or rapture of the deep (nitrogen poisoning) that plague human divers.

How the animals avoid high-pressure nervous syndrome is a mystery. That malady affects most animals between 350 and 700 feet, where increasing pressure squeezes nerve cells. "It leads to involuntary tremors, seizures and death--you literally get racked apart, because your neurons are all firing at once," says Burney Le Boeuf, a biologist at the University of California at Santa Cruz. Elephant seals, however, glide through this zone with no effect. The creatures may either have a different nervous system or may use inert gases to neutralize the effect, Le Boeuf says.

Most elephant seal dives are between 1,650 and 1,815 feet, a zone called the "deep scattering layer" for the effect it has on sound waves. This zone is rich in marine life, containing hundreds of species of fish and squid. "It´s where most of the biomass in the ocean is concentrated," says Le Boeuf. "The elephant seals are diving to the center of the richest part."

At this depth, elephant seals consume an estimated 100 to 200 pounds of food a day to regain fat stores vital for their breeding and molting fasts. One advantage of deep-sea feeding is minimal competition with other marine mammals. Only sperm whales dive this deep. Even humans don´t drop their nets or lines this low. But seeing can be a problem. Le Boeuf put video cameras on diving elephant seals and viewed the results. "You are looking at a black screen 90 percent of the time," he says.

In this dark environment, some creatures are bioluminescent and others have light-emitting organs they use to signal one another. The seals may use the lights to locate their prey. "I really think that´s what these great huge elephant seal eyes are all about," says DeLong. "They are great light-gathering instruments."

Though the majority of dives are around 1,700 feet, the animals regularly go much deeper. Number 666´s record of 5,150 feet is equivalent to four Empire State Buildings stacked one on top of the other. The pressure at this depth is a crushing 160 atmospheres. The first recording devices placed on elephant seals were metal boxes that imploded from those pressures.

To gauge the effects of such pressures, Le Boeuf´s students paint Styrofoam mannequins and lower them to about 600 feet. "They come back looking like shrunken heads," Le Boeuf says.

The skull of an elephant seal is fused solid around the brain, but facial bones and rib cages are built to collapse. Despite these adaptations, the pressure may take a toll on the animal´s body. Le Boeuf says this may be why elephant seals live only 13 to 20 years while other seals, which don´t dive as deeply, may live more than 40 years.

"If you dive deeply for a living you may pay for it with a shorter life," says Le Boeuf.

But deep dives may also be the reason for the species´ survival. The animal taps a rich deep-water resource that neither people nor most of the great whales can exploit. As Le Boeuf suggests, they may have survived simply because no other animal was able to steal their dinner.

Much of the deep dark world of the elephant seal remains to be discovered. And the more researchers learn about this world, the more impressed they are.

"When I first saw them in the late 1960s and ´70s, I looked at them as big slugs on the beach," says DeLong. "But then, when we started to see their diving behaviors and these tremendous long migrations, I began to realize how truly phenomenal these animals are."

California writer Michael Tennesen visited several elephant seal rookeries for this story, but his habituation to air and an allergy to crushing pressures kept him from accompanying seals on their dives.

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