Science / Medicine : Mysteries of the Very, Very Deep : Experts Try to Fathom the Depths of Elephant Seals’ Astounding Dives
Prowling the Pacific for exotic species in 1892, the Smithsonian Institution’s famous wildlife collector, C.H. Townsend, startled the scientific world when he discovered eight northern elephant seals on Mexico’s remote Guadalupe Island, 150 miles west of Baja California.
Rapaciously hunted for their mountains of oil-rich blubber, these heavyweight champions of the seal world--adult males tip the scales at 3 tons and are identifiable by their comical elephantine noses--were widely thought to be extinct by the end of the 19th Century. So what did Townsend do? Good museum man that he was, he killed seven of the giant seals so they could be skinned and displayed, even while acknowledging that they were probably among “the last of an exceedingly rare species.”
In these more ecologically enlightened times when they are legally protected by both the United States and Mexico, elephant seals get much better treatment from scientists and the public. And they have responded with what UC Santa Cruz biologist Burney J. Le Boeuf calls “one of the most remarkable recoveries of any marine or terrestrial mammal in history.” Their numbers doubling every four or five years, there are now as many as 140,000 northern elephant seals, all descendants of the tiny colony on Guadalupe, which totaled only a few dozen seals in Townsend’s day. (Most, happily, were probably at sea when he visited the island.)
As old breeding grounds become increasingly jammed, the seals have spread rapidly northward, establishing new rookeries on offshore islands and beaches from Baja to Point Reyes National Seashore near San Francisco. About 27,000 pups have been born this year alone, estimates marine biologist Brent S. Stewart of the Hubbs-Sea World Research Institute in San Diego, who works on San Miguel Island, off Santa Barbara, site of the largest rookery and birthplace of 14,000 seals annually. What’s more, said Stewart, “we can expect continued substantial growth in the years ahead.”
No longer preoccupied with the seal’s possible extinction, scientists have turned their attention to the colorful habits of these giant pinnipeds. Most recently they have found the seals to be prolific divers that plummet to astonishing depths for long periods of time.
The northern elephant seals--three-quarters of which call California home--are a welcome addition to the state’s wildlife population and a popular tourist attraction. Ever since the seals settled on its beaches in 1961, visitors have been flocking to Ano Nuevo State Reserve, 18 miles north of Santa Cruz, to witness the bloody mating battles between mature bulls for control of harems of a hundred or more cows. Public interest is so high in the dramatic spectacle that state park authorities must raffle off tickets during the winter breeding season.
Mirounga angustirostris, as the elephant seal is known technically, is already the focus of keen scientific awareness. Not only because of its fecundity--female elephant seals, starting around age 3, produce pups at the rate of one a year over lifetimes of up to 19 years--but also because of its extraordinary physiological abilities. Unlike its more familiar pinniped cousins, such as the California sea lion and the harbor seal, the elephant seal does not spend much time in coastal waters or on shore. Except for two annual haul-outs--one to breed or give birth (between December and March), another to molt (spring for females and juveniles, summer for males)--it lives continuously at sea up to 10 months a year.
Until recently, scientists knew almost nothing about the elephant seal’s life in the water. The animals were rarely spotted in the ocean and even their diet was a mystery, since by the time they come ashore, when they cease eating altogether, few clues remain of their well-digested meals.
But that began changing a few years ago when scientists such as Le Boeuf began attaching a new generation of electronic instruments to the animals that could record their activities at sea, including the duration and depth of their dives. The phlegmatic elephant seals were exceptionally cooperative. They didn’t flee into the water on the approach of humans, a fearlessness that made them easy prey for sealers in the 19th Century. So the scientists could mark the animals for future identification--Lady Clairol Ultra-Blue hair rinse seemed to work best--and glue the recorders to their fur.
Also, since the seals usually return to the same beaches where they were born at known times of year, the scientists could be reasonably sure of getting back their expensive instruments (cost, about $2,000 apiece). From the scientists’ point of view, said Le Boeuf, who has been studying the growing colony at Ano Neuvo since the late 1960s, “they are very obliging subjects.”
To the astonishment of the scientists, who gathered here last month for the world’s first international conference on elephant seals, the animals turned out to be extraordinarily gifted divers. Recovering depth gauges from 49 different animals, Le Boeuf and his colleagues reported that the seals were submerged up to 92% of the time while at sea, significantly more than other well-known diving mammals such as the Atlantic right whale, which spends 73% of its day under the waves.
Even more surprising, they achieved spectacular depths. Le Boeuf’s recorders showed that his animals had gone down to an average of 1,650 feet and a maximum of 4,125 feet. Stewart and his colleague, Robert DeLong of the National Marine Fisheries Service’s Seattle laboratory, recorded two seals--one male, another female--that went even deeper to about 5,120 feet--almost a mile. This exceeded the record for all air-breathing vertebrates, including the sperm whale (3,740 feet), which until now had been considered the top diver among marine mammals.
Typically, Le Boeuf’s animals stayed down about 20 minutes, though a few recorded by other investigators stayed down more than an hour. Between these plunges, the seals spent only a minute or two on the surface, barely enough time to get a few gulps of air. By contrast, other species of seals and even such adept swimmers as dolphins take long rest periods on the surface between bouts of diving.
How elephant seals manage to hold their breath for so long remains a puzzle to physiologists. By collapsing their lungs, the animals don’t have to worry about the release of nitrogen bubbles into their bloodstreams when they surface--which causes the bends in humans, a crippling ailment accompanied by sometimes fatal convulsions. It is also known that their heartbeats slow to less than half their surface rate of 90 beats per minute.
“The big hole in our understanding,” said Stewart, “is what is going on metabolically during deep dives. If you go through the dive records, you find durations far greater than should be theoretically possible without the seals exceeding their aerobic limits.” This is the point beyond which an air-breathing animal cannot metabolize its foodstuffs. Yet, Stewart noted, the animals clearly are not going beyond this barrier, because they are not staying on the surface long enough between dives to get rid of the lactic acids that are normally produced when aerobic limits are exceeded.
“The challenge is to find out what’s going on in their bodies when we can’t see them,” Stewart said.
Whatever its secrets, deep-diving gives elephant seals a distinct advantage. It lets them feed in a portion of the sea where they have few competitors--mostly, it appears, on squid and a few deep-dwelling fish, such as Pacific whiting. Both are bioluminescent, and the elephant seals apparently home in on their prey’s dim glow in the darkness of the deep. “That’s probably the function of these beautiful big eyes,” said DeLong.
By remaining far below the surface, the seals also reduce the risk of encounters with their major enemies, the great white shark and the killer whale. Studying a relatively new colony of elephant seals on Southeast Farallon Island, just off San Francisco, William Sydeman of the Point Reyes Bird Observatory spotted 60 shark attacks on seals in the surrounding waters during one breeding season. Only half of the pups may survive their first year.
David Slip, an Australian investigator studying diving among the equally talented southern elephant seals (close kin of the northern seals), found that one animal stayed deeply submerged for two hours--possibly because killer whales were on the surface.
Though the scientists are just starting to collect data on heartbeats and metabolism during dives, the seals seem marvelously adapted for life in the deep, expending a minimum of energy. Tomohiro Asaga of the Tokyo School of Fisheries, who studied 5,600 separate dives, reported that at times the seals just drift slowly downward, without any purposeful motion. He concludes that the seals may well be taking snoozes of five or 10 minutes.
Though the animals are packed together like sardines while on shore, they appear to travel by themselves as they prowl the deep for food. The scientists found that the animals traveled as far as Hawaii and Alaska, yet returned unerringly to their home beaches at the end of foraging expeditions of several thousand miles.
“Do they use the stars, or the sun, or some other means of navigation?” asked Gerald Kooyman of the Scripps Institution of Oceanography in La Jolla. “We really have no idea.” Like so many other things about these marvelous animals, this remains one of the many mysteries about them.
