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Portrait in Blubber

Fat, loud, and far from shy, elephant seals don't come across as mysterious creatures. But they hold many surprises.

By Carl Zimmer
Mar 1, 1992 6:00 AMNov 12, 2019 5:14 AM


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When Burney Le Boeuf started studying elephant seals in 1968, his operation was pretty low tech. Le Boeuf was surveying 4,000-pound bulls at a rookery on Año Nuevo Island off the coast of California, and he had to find a way to tag them so that he would know which bull was which. First he tried a dye-filled fire extinguisher, which he blasted at the sleeping bulls’ flanks. But that woke them up. Then he threw plastic sandwich bags filled with paint at them--too messy. Then he tried a paint roller on a pole, which afforded a bit more control over the shape and placement of the marks. But he ran out of enough colors to give each seal a distinctive mark. Finally he settled on squirt bottles of hydrogen peroxide mixed with Lady Clairol Blue, a hair bleach. It bleaches fast if it’s sunny out, says Le Boeuf. And you can form nice letters with it.

Today his grad students still use Lady Clairol Blue to mark the elephant seals, but Le Boeuf has moved on to more sophisticated equipment. And as his technology has improved, Le Boeuf has been able to get a better picture of what turns out to be a bizarre, often inexplicable way of life.

At first glance, elephant seals don’t seem like they have much to hide. Each year in late December hundreds of them come to Año Nuevo for their three-month mating season, and they sprawl on the beach in plain view of thousands of tourists. All Le Boeuf has to do to observe his subjects in action is get in his car on the campus of the University of California at Santa Cruz and drive 19 miles north on Route 1.

Yet when Le Boeuf began his studies, biologists knew almost nothing about the seals’ land-bound sex life, and even less about the animals’ time at sea. Le Boeuf changed all that; indeed, the facts he’s uncovered have led to a stream of research papers that seem as well suited to Ripley’s Believe It or Not as to Marine Mammal Science. As it turns out, the sex life of elephant seals is almost unmatched in its mixture of violence, frustration, and intensity. And their sea-faring existence boggles the mind, consisting of an endless series of dives that no animal should be able to make.

Le Boeuf’s first project on Año Nuevo revealed that for bulls, sex is an exhausting activity. A handful of dominant males, known as alphas, hold sway over harems of hundreds of females. Alphas win their status by battling lesser males in neck-slamming contests that spray the beach and ocean with blood. Under the alpha bull autocracy, it’s not unusual for 5 percent of the males at Año Nuevo to have 85 percent of the sex.

But abstinence can be exhausting, too. Love-starved males ring the harems in arduous vigils, waiting to ambush females that are trying to go to sea. For the fe-males, sex pre-sents a real danger. The libido of an unrequited bull can be so strong that he may inadvertent-ly kill a cow while mounting her. To avoid this threat, females try to sneak to sea at night; if they’re caught, they tend to cooperate.

We see the death rate at a minimum of one in a thousand females, says Le Boeuf. But it’s like rape--what you hear about is just the tip of the iceberg. When the females leave the rookery, frustrated males may even mount male and female pups left behind, crushing them in the process.

Le Boeuf spent many years documenting the perils of elephant seal sex. Then he decided to look at what they were doing when they weren’t mating. After all, the seals spend most of their time in the water.

In the early eighties Gerald Kooyman, a Weddell seal expert at the Scripps Institution of Oceanography, offered Le Boeuf a way to see what the seals were up to on their months-long journeys. Kooyman had developed a small, retrievable instrument that could measure depth by recording pressure. The instrument, a canister whose lid gets squeezed under the higher pressures of deeper waters, notes the compression. The two-pound device can be glued onto a seal’s back; then, when the animal comes back ashore to molt, the recorder is shed along with its fur and top layer of skin.

Relying on the depth recorders is still risky, says Le Boeuf, because the devices can fall prey to a seal’s own misfortunes. They go out there and get eaten by sharks and killer whales; they die of heart attacks; sometimes they just get lost and never come back. Males spend three to four months at a time at sea. But females leave the rookery for a predictable 70 days between breeding and molting. So Le Boeuf decided to better his odds by putting recorders on cows rather than bulls. He began with just two; both came back.

Even now, that first recording of their dives leaves Le Boeuf flabbergasted. The cows went down 2,650 feet, far deeper than any other seal and close to what was then the known mammalian record, set by sperm whales, of 3,700 feet. Even stranger, the seals dove nonstop. They made two or three descents an hour, surfacing for only two or three minutes at a time. These animals are diving continuously, which is unheard of, says Le Boeuf. They spend as much as ninety-four percent of their time underwater. No other animal dives that way.

Le Boeuf and his colleagues have since tagged more than 70 female seals with recorders and have shown that the pattern they initially found is normal for elephant seals. They broke records from the start, and they’re still breaking them, says Le Boeuf. One broke the sperm whale record by diving 4,150 feet.

Inspired by such tantalizing finds, Le Boeuf is intent on understanding every aspect of the seal’s diving ability by tracking its development from the animal’s birth to its death. This is the first time anything like this has been done for a seal, he says. Pups learn to swim in shallow water; by studying their blood, Le Boeuf and his colleagues have found that they go through dramatic internal changes as they paddle around. For example, they develop a high red-cell density, for binding a large amount of oxygen, and build a huge blood supply. After a couple of months of practice, pups abruptly leave their rookery. We know they’re diving continuously and diving a thousand feet down, which is rather amazing, says Le Boeuf. By the time a juvenile is two years old it’s diving as deep as any adult.

Le Boeuf also wanted to know where the dives were taking the seals. He put photocells on the depth recorders to register the time of dawn and dusk, which vary according to a seal’s longitude and latitude. To narrow down the location even more, he and his colleagues can compare the temperature recorded by thermometers on the device with published weather charts. We can get within sixty miles of the seal’s location, which is good enough for our work, says Le Boeuf.

Some bulls, it turns out, swim as far north as the Gulf of Alaska, taking advantage of their insulating bulk to feed in these cold, food-rich areas. They make shallow dives, probably, Le Boeuf thinks, to feed on the stingrays and other fish that populate the area’s underwater mesas, known as seamounts. Females don’t go so far north, but some are way out at sea, almost due north of Hawaii, he says. They tend to dive deeper and longer than the bulls, possibly because they have to reach food in the more temperate waters.

To get a better sense of what seals are actually doing on these journeys, Le Boeuf’s team built a propeller-equipped speedometer to measure swimming speed. We think it can tell us a lot, says Le Boeuf, since from the swim speeds you can calculate the angles at which the seals dive.

The device has revealed three distinct types of dives. In one, used for speedy migration, the seals make gradual, long-distance descents followed by sharp climbs. In another they appear to be catching food: when they bottom out, they either swim up and down in a sawtooth pattern, as if chasing a tasty fish, or in straight lines, as if skimming along the top of a seamount. There’s also an enigmatic dive in which the seals sink slowly, then drift back up to the surface. These dives, says Le Boeuf, may be a clue to how the seal’s physiology handles the extraordinary demands placed on it.

On the face of it, elephant seals shouldn’t be able to dive the way they do at all. Whales and other seals dive deep, but when they surface they have to relax for a while, to flush out the lactic acid built up in their muscles and to store more oxygen. In contrast, elephant seals spend almost no time recuperating on the surface. We can work out the amount of oxygen they can take down, says Le Boeuf, and it doesn’t work out very well unless you assume that the animal is reducing its me-tabolism perhaps as much as sixty percent, How can a seal in a zombie state chase after wide-awake prey? No one knows yet, he says.

Without enough oxygen to go around, elephant seals probably shut down the blood supply to most of their organs. Only the essential functions, such as those mediated by the heart and brain, continue. At some point, though, the seals have to switch their kidneys and livers back on to clean out the wastes that built up in the bloodstream. And sooner or later their stomachs and intes-tines have to go to work so the animals can digest their food. That kind of housekeeping may be what the seals do on the lazy dives.

The speedometer can’t explain another paradox, though. When most mammals experience the pressure of just 600 feet of water, they lose control of their nervous system. The mem-branes of the neurons get squeezed so hard that only a little voltage triggers a signal. Nerves misfire, causing seizures and death. Yet elephant seals have blithely dived seven times deeper than this danger zone.

Le Boeuf has an idea of how elephant seals stay in control. By squeezing the neuronal membranes, the great pressures the animals are subjected to essentially lower the voltage threshold at which nerves fire-- that is, they allow signals of even low strength to get through. Certain gases, however, Le Boeuf points out, can raise the voltage threshold. One of them is nitrogen, which is the basis of anesthesia. When a patient’s threshold is raised with nitrogen compounds, pain signals are blocked. The elephant seal may be storing nitrogen from the air in its blubber and sealing it off by shunting blood away from the fat. When the animal reaches dangerous pressures, it can draw nitrogen out to lower the effective pressure on the nerves and keep them functioning by allowing only the strongest signals to be transmitted.

The nitrogen hypothesis will be tough to test. Researchers have put animals such as rats in high-pressure chambers to see how their body chemistry changes. But a chamber that could handle a ten-foot long seal would be, as Le Boeuf puts it, a potential bomb.

For now, Le Boeuf is trying to figure out how to see what the seals are eating, and that involves inventing another instrument. You could do it a number of ways, he says. You could put a surface electrode over a chewing muscle that would register every time it was working. To see when it swallowed, you’d need to implant the electrode, which would be harder. Another way might be to have them swallow a temperature probe so that when they ate some cold food it could register the difference.

Once Le Boeuf and his colleagues know the longitude, latitude, and depth where the animals are catching food, he says, we could go out there and fish at the depth and see exactly what they’re eating. Then I’d think we’d really be getting somewhere.

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