Going to Extremes

Somehow diving animals routinely plunge to astonishing depths, seemingly without enough oxygen. How do they do it?

  • Caroline Harding
  • Aug 01, 1993
The 650-pound leatherback sea turtle must have sensed danger. Perhaps it saw the sleek, menacing form of an approaching shark. For the turtle took a dive. Straight down. The streamlined body plunged into the dark depths until it had left the rays of light near the surface far behind-along with any foraging sharks.

As an evasive maneuver by a teardrop-shaped creature endowed with powerful front flippers, such a move may be standard behavior. But to marine biologist Scott Eckert, who had attached a time-depth recorder (TDR) to the turtle's carapace, this particular dive was anything but routine. The year was 1985. The place was the warm waters of the U.S. Virgin Islands. And Eckert was in for a shock. The turtle descended far past the instrument's ability to measure depth, to an estimated 4,265 feet.

"We thought the TDR was broken," Eckert recalls. "So we retested it and now we're convinced. The leatherback went straight down and straight back up-and it was fast." Eckert's calculations used the dive's recorded portion and the amount of time the turtle stayed down. At the time, no one else had come close to measuring a dive that deep.

For the past 30 years, scientists studying diving have often turned up the unexpected. In the early 1960s, marine biologist Gerald Kooyman of Scripps Institution of Oceanography found that Weddell seals routinely dive to depths at which water exerts more pressure than they should be able to withstand-at least given biologists' understanding of seal physiology at the time. Ever since, researchers have been exploring the underwater capabilities of marine vertebrates from elephant seals to penguins. The findings have intrigued not only biologists and marine ecologists, but also medical researchers studying heart attacks and strokes.

Physiologists have searched for more than a century for the answer to how air-breathing divers function on limited oxygen. In 1870, French physiologist Paul Bert found that heart rates of diving ducks slowed dramatically. Seventy years later, in the 1940s, American scientist Laurence Irving and medical doctor and physiologist Per Scholander, originally from Sweden, collaborated to develop the classic "dive response" theory. When a seal dives, they reasoned, its heart rate slows, and blood flows only to the organs that need the oxygen most: the brain and the heart.

Kooyman had that theory in mind in 1960 when he developed the first time-depth recorder for use on marine vertebrates. He started with an ordinary kitchen timer. "I worked out the basic design and then had a watchmaker-repairman build it," Kooyman says. The crude instruments "certainly weren't streamlined," he adds, "but they worked extremely well." He measured several hundred Weddell seal dives-and found that the seals were going far deeper than he expected. "There was nothing known about the capabilities of these animals," he says. "It was thought that they didn't dive very deep or very long. I was recording dives over 1,000 feet."

Weddell seals inhabit Antarctic coastal waters and spend about half of their lives diving, entering the water through narrow breathing holes in the ice. They can venture more than 3 miles away from a hole and return to it before running out of oxygen. Kooyman found that although most of the dives lasted five minutes or less, when the seals were searching for new breathing holes, they often stayed under for 20 minutes or more. The longest dive lasted more than 48 minutes. The biggest surprise came in 1964, 'when one seal went to 1,969 feet. "That," Kooyman says, "was unheard of."

At 1,969 feet, pressure exerts 882 pounds of force over every inch of the seal's body, 14.7 pounds per square inch for every 32.8 feet of descent. How could a seal withstand such pressure and remain submerged for so long?

Kooyman and other scientists were surprised to find that the lungs of the deepest divers have relatively small capacities. Weddell seals, for example, do not use their lungs as the major oxygen storehouse. Endowed with higher concentrations of red blood cells and hemoglobin than other animals, such divers store large amounts of oxygen in their blood and muscles-as much as 13 times the amount held in the lungs. In contrast, short-term divers like sea lions have about the same blood-oxygen storage capacity as do terrestrial mammals, and they rely on their lungs.

To cope with extreme pressure, most deep divers have cartilage-reinforced air passageways that allow the lungs to collapse and then reinflate. The lungs contract slowly as pressure increases during descent, and they gradually open back up during ascent. Because air doesn't reach the gas-exchange surfaces of the lungs when they are collapsed, the blood no longer absorbs oxygen. Nor can the blood take up the nitrogen that can cause decompression sickness (commonly called "the bends").

Scott Eckert's interest in diving began after he and his wife Karen, also a marine biologist, moved to the Virgin Islands in 1982 to head a leatherback conservation project. The turtles spend most of their lives in the oceans of the cold-temperate latitudes. Females typically migrate thousands of miles to nest on tropical shores. For four months, they visit the beach every ten days to lay eggs.

Interested in studying the turtles' dives, Eckert contacted diving authority Kooyman, who thought leatherback behavior during nesting season-including the days at sea-seemed similar to the routine land visits of fur seals he had studied. The turtles might be doing some deep diving, and they might even be formidable divers: As cold-blooded animals, they do not heat their own bodies. Unlike seals, they do not use part of their precious stored oxygen for heating energy during dives. "I thought the dives could be very long and extremely deep," Kooyman recalls. "They could break the recorders." In 1984, he packed up his TDRs and flew to St. Croix.

The scientists established that leatherbacks spend much of their time between 500 and 1,500 feet deep in the ocean, likely following their food source of luminescent jellyfish. The jellyfish are part of a migrating zone of zooplankton called the deep scattering layer. During the day, the layer stays below 1,800 feet, where less than 1 percent of the sun's rays penetrate. At night, the jellyfish move toward the surface to feed on phytoplankton. When the turtles dive to extreme depths-like the 4,000-plus-foot dive Eckert estimated-they are probably evading predators.

To Kooyman, the recorders' tracings looked familiar. "The patterns looked exactly like seal dives," he says. "Routine dives to foraging depths, back up to the surface for a short period, then another dive. Had someone handed me the records and asked what animal made them, I would have said seal."

Soon after investigating leatherback dives with the Eckerts, Kooyman returned to seals as subjects, this time elephant seals. Marine biologist Burney Le Boeuf, of the University of California, Santa Cruz, had studied the on-shore behavior of northern elephant seals, but in the early 1980s, the seals' aquatic life remained a mystery. They spend as much as ten months a year at sea and sightings are rare.

The largest breeding colonies of northern elephant seals occupy islands off southern California. After periods at sea, a seal usually returns to the same island, a habit that allows scientists to retrieve the TDRs. The females, which Le Boeuf studied, spend five weeks on shore after the breeding season for the birth of pups and mating, go to sea for about 70 days and then return to molt.

The first study, in which Le Boeuf and Kooyman collaborated in 1983, had a rough start. The elephant seals were shedding their TDRs within a day. The scientists finally rigged an attachment above the hind flippers of two females. But when they didn't return from sea within the expected two months or so, the project seemed over. Then, after 126 days, the seals showed up, recorders still in place. One of the elephant seals had dived to 2,067 feet.

"It was bizarre behavior," Le Boeuf says. "The animals were continuous divers, they were deep divers and they had long duration dives with short surface intervals." The seals had averaged 18 to 20 minutes per dive, the longest measured at 35 minutes. But it had been an El Nino year, in which food chains of the North Pacific were disturbed by aberrant wind patterns and warm water currents that suppressed nutrient-rich waters from surfacing as usual. "We didn't know whether the dive pattern was due to the elephant seal," says Le Boeuf, "or due to the weather."

In the next two years, Le Boeuf attached recorders to nine elephant seals using an alternative technology: marine epoxy. Glued to the seals' coats, the recorders would now be shed on shore during molting. The new data matched the old. The seals had spent 90 percent of their time underwater at depths between 1,640 and 2,300 feet. The deepest dive went to 4,100 feet and the longest lasted 62 minutes. How much time were the seals spending at the surface recuperating? Usually less than three minutes.

Seal lungs collapse on descent at about 130 feet and reinflate on ascent at about the same depth. "That means they're spending most of the time at sea, indeed most of their lives, with their lungs collapsed!" Le Boeuf says in disbelief. Other deep divers, such as Weddell seals, spend as much as an hour at the surface resting after long dives. On short dives, their metabolisms remain aerobic (enough oxygen is stored in blood and tissues), so surface time need only he brief. But on long dives, their metabolisms become anaerobic, producing an oxygen debt, which divers must repay by breathing.

The brief period elephant seals spend at the surface does not seem long enough to repay the debt. "How can the GI tract shut down for 90 percent of the time, 22 and a half hours a day?" asks Le Boca."How can the animal dive for over an hour and spend just a few minutes at the surface? It continues to be a puzzle."

Short-term divers have also puzzled researchers. In the early 1960s, veterinarian and neurobiologist Sam Ridgway, head of the Biomedical Division of the Naval Undersea Center in San Diego, discovered that bottlenose dolphins can also function anaerobically.

With the cooperation of a dolphin named Tuffy, Ridgway found that the animals were capable of dives lasting six minutes--with what should have been only enough oxygen for three minutes. Not only that, Tuffy swam down a 1,000 foot cable and rang a dive buzzer at the end. Along the way, a camera recorded the fact that Tuffy's chest had completely collapsed at 230 feet.

The dive response, which slows the heart rate for maximum conservation of oxygen, plays a major role in prolonging the amount of time a diver can remain submerged. In the early 1970s, Ridgway devised a way of determining whether the diving response is a mechanical response at the brain-stem level, or whether it is controlled by higher centers of the brain. By actually teaching California sea lions to simulate dives on command while sitting high and dry on foam rubber pads in a laboratory, he found that his subjects could consciously control their own heart rates.

"The trainer gave the animal the signal to 'lower your heart rate,' " Ridgway says. "And we watched the monitor and when the heart rate was to our criteria, say 20 beats per minute, the trainer gave the sea lion a reward." Ridgway's conclusion: "The animals have the capability to willfully control the degree of response they make to different diving situations."

To explore the question of how divers survive with depleted oxygen stores, in the late 1970s, physiologist Peter Lutz turned to the loggerhead turtle. Lutz, based at Florida Atlantic University, found the loggerheads' dive physiology confounding. "They were diving too long," he says, "staying under when no oxygen was left in the blood. To our surprise, we found their brains can function in the absence of oxygen."

Lutz's discoveries may have applications in human medicine. When the brain runs out of oxygen (the major cause of death from heart attack and stroke), it releases excitatory neurotransmitters that in turn excite the brain, causing death. But in the turtle, inhibiting neurotransmitters actually slow down the brain, allowing the turtle to survive. Says Lutz, "If you had told one of the classical physiologists that a brain could survive for 48 hours without oxygen he would have said it's not possible. But it does work."

You might also encounter such skepticism if you told a classical physiologist that one of the key stages of a heart attack can aid divers. Yet that's just what physiologist Robert Elsner, now at the University of Alaska, Fairbanks, has found in seals. Among his discoveries is that humans and many other species have diving responses like that of a seal's, only less developed. "You have heard of the cases where small children have been immersed in cold water for a long time and have been revived without brain damage," he says. "The physiological events in such children may be similar to those which take place in diving seals."

Recently, Elsner has established that during a dive, the blood supply to a seal's heart becomes intermittent, sometimes stopping for as long as one minute. "One could say that the seal has a heart attack every time it dives," Elsner says. The halted blood flow may bring about a conversion to anaerobic metabolism, which allows the seal to make more complete use of available energy sources. "In that regard, says Elsner, "the condition which is our main killer can be considered a beneficial adaptation in the seal."

As for studies of the dives themselves, recent technology has yielded increasingly fantastic data. Some highlights:

Using microprocessor-controlled dive recorders, in 1987 Robert DeLong of the National Marine Mammal Laboratory in Seattle and Brent Stewart of Hubbs-Sea World Research Institute in San Diego studied male elephant seals. The results: One seal descended a record-breaking 5,016 feet, and all of the seals descended further than 3,280 feet. The longest dive lasted 77 minutes; most were about 23 minutes. The seals are likely following prey.

To find out where the seals were going, DeLong and Stewart, working with electronics designer Roger Hill, designed and built a "geolocation" time-depth recorder. Its main feature: recording light levels at the surface to calculate location. At the end of the breeding season in March 1989, DeLong and Stewart glued the new instruments to the coats of eight bulls before they went to sea.

When the seals returned in July, the researchers found that one had traveled nearly 3,000 miles in 46 days to the Aleutian Islands. There it foraged for more than a month before heading back in almost a straight line, this time traveling 2,300 miles in 37 days-diving continually to an average depth of 1,500 feet. Another seal also traveled to the Aleutians, and two swam to the Gulf of Alaska. "We don't know how they do it," DeLong says. "They obviously have exquisite navigational skills, the complexity of which we don't yet comprehend."

Satellite technology has recently aided ecologist John Bengtson in studies of the crabeater seal. Leader of the Antarctic Ecosystem Research Program at the National Marine Mammal Laboratory, NMFS, in Seattle, Bengtson is looking for clues about how commercial krill harvesting could affect the ecosystem.

The most abundant seal in the world, the crabeater dives at night to eat krill and sleeps during the day on floating pack ice. Bengtson has found that at dusk and dawn, crabeaters descend as far as 1,400 feet-probably to Catch krill in the dark. At night, the dives are as shallow as 80 feet. The new instruments have eliminated the need to follow the seals on moving ice with ships and helicopters.

Microprocessor time-depth recorders are also helping Gerald Kooyman observe emperor and king penguins, which are too small to carry his older instruments. Penguins have solid bones that reduce buoyancy; tightly packed, scale-like feathers; and narrow, flattened wings for underwater "flight." At first, Kooyman thought their diving behavior would resemble that of the Galapagos fur seal, which is about the same size and dives for only a few minutes. But emperors, he discovered, dive for as long as 20 minutes, and they descend as deep as 1,640 feet. Their dives are longer than their oxygen stores should allow, and the amount of nitrogen in their blood would ordinarily cause the bends in mammals. He is now studying how the birds accomplish such dives.

As advanced as the new recorders are, this veteran of diving research apparently dreams of technology beyond most researchers' wildest dreams. Could we one day actually witness what we must now trace with instruments? "The imagery of a squadron of emperor penguins going by at a depth of 1,640 feet," says Kooyman. "Can you imagine what that must be like?"

Los Angeles writer Caroline Harding's interest in diving started with her own unusual ability to descend as deep as 60 feet with only a mask and fins.

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