Other Environmental Themes

Ultimate Diving Machines

Until about 15 years ago, when researchers first started strapping electronic sensors to the backs of northern elephant seals, very little was known about the movements of these remarkable mammals. The animals were easily seen at their breeding and molting haunts on land, but were virtually invisible at sea. Data from implanted radio transmitters explains why elephant seals are so hard to catch offshore: They dive continuously, spending almost 90 percent of their time underwater. The seals also dive deeper than any other marine mammal, reaching depths of nearly a mile beneath the surface. To stay submerged for hours at a time, they slow their metabolism way down: The heart rate, which is normally 120 beats per minute, can dip as low as two beats per minute.

Elephant seals are champion long-distance migrators. Females log nearly 12,000 miles each year; males typically top 13,000 miles—the longest migration of any mammal on record. The nonstop diving adds another 5,000 vertical miles to their journeys. Elephants seals make two separate roundtrip excursions annually—the only vertebrate known to engage in a so-called "double migration." The first migration occurs after breeding: Males and females head off separately, In late February or early March, from California's Channel Islands to feeding grounds in the northern Pacific. Females embark on their second migration in late May, after spending a month on the beach fasting and molting, following a roughly similar route. Males begin their second jaunt in late August or early September, covering 6,000 to 7,000 miles in about four months. Marine biologists don't know why male and female seals swim separately, though human travelers—especially squabbling couples—may find the practice instructive.

Scientists are also mystified as to how elephant seals can maintain their amazing activity level for so long, apparently with little rest. Records collected to date contain no evidence that seals stop to sleep. Researchers speculate that the animals may sleep while swimming or get by on periodic catnaps—or sealnaps, as the case may be.