Sleeping Giants : A Santa Cruz Researcher Asks: Can Elephant Seals Nap Under Water?

A male elephant seal, named Plover by research students, is lying on the beach at Ano Nuevo State Park, 20 miles north of Santa Cruz. Lazing in the sun, he keeps a sharp watch over his harem of half a dozen females and their offspring napping close by. It is not an easy job.

Other males continually threaten to invade his small enclave, approaching from the water or from the dunes, provoking the two-ton seal to rise and sound a harsh warning call and, if necessary, to scoot across the sand with surprising speed to chase them away. When one challenger refuses to be intimidated, Plover charges into battle. The two huge sea mammals face each other, chest against chest, with their heads held high and their pendulous snouts raised in the air. Plover delivers several quick blows with his head to the neck of his rival and bites at his neck with his sharp canines, driving the hapless suitor off to try his luck elsewhere.

That scene is one that is repeated often this time of year at Ano Nuevo State Park, as hundreds of elephant seals gather on the beach for the breeding season--from December to April. The species’ rigid social structure permits only a few males to dominate the breeding each year, with many males never getting the chance to mate at all. Competition for females is so fierce at times that fighting between bulls can last for 20 minutes or more, turning the sand red with blood. “The best fighters are the best lovers,” says Burney J. LeBoeuf, a professor at UC Santa Cruz. A leading expert on elephant seals, LeBoeuf has studied the species since he joined the university two decades ago.

A century ago, the elephant seal was on the verge of extinction. During the 1800s, sealers killed hundreds of thousands of them in the north Pacific for their oil-rich blubber. When the slaughter ended, all that remained were 50 to 100 animals on the island of Guadeloupe, off Baja California.

Since then, the species has made a strong comeback. Scientists estimate that there are now about 100,000 elephant seals in the north Pacific. The animals have reclaimed most of their original range from Baja California to Point Reyes and breed in a dozen locations, including the islands of Ano Nuevo, San Miguel, Santa Barbara, San Nicolas and San Clemente. Because of overcrowding, they have recently started to establish their rookeries on the mainland--in places such as Ano Nuevo State Park--providing a rare opportunity for study at close range.

Despite the elephant seal’s increasing numbers, researchers, including LeBoeuf, are worried about the animal’s long-term survival. The near extermination of the species, combined with the system in which only selected males mate, apparently has caused serious in-breeding among the descendants of the original survivors. LeBoeuf, a specialist in reproductive physiology, says that tests have shown that there is no variation in the genetic makeup of the seals, which could limit the species’ ability to adapt to changes in its environment. “When a species recovers from near extinction,” he says, “does it really ever recover? Elephant seals don’t have the genetic variability they had before the crash. They have come back in numbers and range, but they may not be the same kind of animal.”

The males have several well-documented habits that don’t do the species any favors. Bull seals, so aggressive in their efforts to breed with the females and father the next generation, occasionally trample and kill newborn pups--sometimes even their own offspring. That, according to LeBoeuf, is the leading cause of death among young elephant seals. When mating, the males have even been observed to unintentionally kill the females by biting them. And males who are left out of the mating--as all of them are some of the time--try to compensate by attempting to mate with the much smaller yearlings. Not only do they terrify the youngsters but they sometimes crush them to death as well.

Elephant seals are ideally suited for studying social behavior because they are generally tolerant of humans and can be tagged fairly easily, so individuals can be tracked throughout the mating season. These seals also provide a dramatic illustration of sexual dimorphism: The male can weigh as much as 5,400 pounds--as much as seven times the size of the female. The trunk-like snout, which can grow as long as 2 1/2 feet, occurs only in the males, and is the feature for which elephant seals are named.

One of the frustrating aspects of studying elephant seals has been the lack of knowledge about how they live at sea, where they spend seven months of the year. “It’s like they go into a black box,” LeBeouf says. During the last two mating seasons, however, he has attached depth recorders to 12 female elephant seals in the hopes of learning more about their lives under water.

LeBoeuf found that elephant seals are incredibly deep divers. One dive he measured was 894 meters deep--more than half a mile under water. On the average, the seals dive between 400 and 650 meters and stay under water as long as 45 minutes at a time. Given their weight gain during the period of study, it is clear that they are feeding. Much of the time, however, they are diving hundreds of meters below the normal range of the creatures they eat (squid, fish, small sharks and stingrays) and the animal that usually eats them (the great white shark).

The recorders also showed that the seals dive continually, raising the question of when they sleep. “They’re like a machine--just nonstop,” LeBoeuf says. He theorizes that diving elephant seals may enter a sleep-like state that resembles sleep periods observed when the animals are on land. When they sleep on the sand, they stop breathing for 20 minutes at a time, their bodies relying on the oxygen stored in their numerous red blood cells. Similarly, when they dive, their metabolisms slow down, they stop breathing and they use less oxygen.

LeBoeuf speculates that during dives the seals may be not sleeping at all, sleeping briefly while diving or napping at the bottom of deep dives. “It might not be sleep as we know it,” he says. This spring, LeBeouf plans to conduct more experiments using the depth recorders, which have become a valuable tool in his research. “They are going to revolutionize our understanding of the underwater behavior of seals and whales,” he says.