Each year the northern elephant seal migrates farther than any other mammal in the world, traveling as many as 13,000 miles (about 21,000 km). It is also the only mammal known to undertake two migratory journeys annually. All that traveling adds up to a lot of time spent underwater, which would seem to provide ample opportunity to lose track of where they're headed. Yet, like clockwork, northern elephant seals arrive at the same places, at the same times, year after year.
Perhaps because they do spend so much time underwater, those of us on land know very little about how the northern elephant seal (Mirounga angustirostris) manages to find the right migratory course in the first place, much less stick to that course and reach its final destination. Orientation cues would seem to play a role, and everything from visual to acoustic to magnetic cues has been suggested, but not proven.
Still, regardless of how it is done, it is done faithfully, and every February and March, when the seals leave their rookery beaches along the coasts of Mexico and southern California and disappear into the cool, gray Pacific, they have one thing on their mind—food. After nearly three months of food deprivation during the birthing and breeding season (during which they sometimes losing nearly half their fat mass) food is all they can think about, and to find it, they must swim north to their offshore foraging grounds in the North Pacific and the Gulf of Alaska.
The first stage of their migratory journey is made all the more remarkable by the fact that males and females actually follow different migratory courses, with each sex venturing to a different foraging destination. This is because male and female northern elephant seals feed on different kinds of prey. Males tend to stay tucked in along the continental shelf, tracking their favorite food—bottom-dwelling fish, small sharks, and rays. In contrast, females, which feed mainly on squid, swim further into the open ocean, in pursuit of their slippery prey.
The males’ route unfailingly delivers them into their central feeding grounds in the Gulf of Alaska near the eastern Aleutian Islands. The females, on the other hand, generally forage a little ways south of the gulf. And they, along with juveniles who have made the trip north, also are the first to return to the rookery beaches in late April or early May, which marks the onset of their annual molt, when they shed their outer layer of skin and hair. Young males are the next to arrive at the rookery, usually in early summer, followed by full-grown males in mid-summer.
After the molt, the seals begin the second leg of their journey, departing the sunny southern coast and heading once again for their northern feeding grounds. In December, following a few more months of feeding, they finally return to the rookery, ready to begin another season of birthing and breeding. It is a lot of travel, shrouded in still more scientific mystery.
Males are at sea for about 250 days each year and females about 300 days. Because the males’ route to their feeding grounds curves along the continental shelf, whereas the females’ route generally is more direct, males swim roughly 1,000 to 2,000 miles (about 1,600 to 3,200 km) further than females each year. The largest males, which may weigh as many as 5,000 pounds (about 2,270 kg)—several thousand pounds more than females—often swim the farthest.
But long-distance swimming is only part of what makes the physiology of northern elephant seals so extraordinary. They also are exceptional divers, plunging down to between 1,000 and 2,600 feet (roughly 300 and 800 meters) below the water surface and staying submerged for anywhere from several minutes to more than an hour. Dive-recorder devices have traced northern elephant seals diving to depths in excess of 4,000 feet (1,220 meters)!
It is difficult to imagine now, but in the late 19th century, the northern elephant seal was on the brink of extinction, having been hunted relentlessly by humans. The population was reduced to an estimated 100 to 1,000 animals. Thanks to protection laws implemented in both Mexico and the United States, however, the species rebounded, and today an estimated 125,000 individuals thrive in the North Pacific.
Kara Rogers is the senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is also a member of the National Association of Science Writers and a contributor to the Britannica Blog, where she runs a series called Science Up Front. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.
This post also appears on the Britannica Blog.