Elephant seals have a shockingly precise "internal GPS." We just discovered how it works.

How else would you navigate during a 6,000 mile journey?
Grant Currin
A map of the north Pacific Ocean showing dozens of journeys taken by female elephant seals.Beltran et al

An elephant seal dubbed Four Four Eight is probably — I hope — a few hundred miles off the coast of Northern California by now.

Marine biologist Roxanne Beltran tells IE that Four Four Eight was looking pretty rough when she saw the 15-year-old on Año Nuevo beach the week after Valentine’s Day.

Fortunately, that wasn't a surprise.

“It's the end of the breeding season, so the beach has really cleared out. She needs to leave soon,” says Beltran, a co-author of a study published this week that examines that journey. The research is published in the academic journal Current Biology. 

“It's time for her to get some food.”

In May, Four Four Eight will return to Año Nuevo beach, waddle up onto the sand, and slough off her skin and fur, like a huge scab.

Once that ordeal — a “catastrophic molt” in scientific terms — is out of the way, the elephant seal will set off on the year’s big adventure: a seven-month, 6,000-mile journey into the middle of the northern Pacific Ocean.

She’ll hardly rest.

Then, something remarkable will happen. At just the right moment, she’ll turn around. Timing is key because an error in either direction could send an entire year’s work up in smoke. If she turns around too early, her fat reserves won’t be enough for the calf she’s carrying to have a fair shot at survival. If she waits too late, she won’t make it in time for the annual mating event, missing out on her chance to reproduce next year.

Beltran’s study asks how seals like Four Four Eight know when, exactly, to turn back.

“We didn't know how they make decisions out at sea. We just knew that whatever decisions they made were always very good,” she says.

These seals are celebrities

Beltran, now a professor at the University of California Santa Cruz, first met Four Four Eight when she was an undergraduate research intern learning how to wrangle seals.

Four Four Eight, who once appeared on TV with Jeff Corwin, had been on researchers’ radar for eight years when she and Beltran met. The seal had been one of the 300 pups to get a tag the year she was born. She then returned to the beach every year.

“She was very good at returning to the beach, so we [were] OK putting expensive instruments on her,” Beltran explains.

Beltran was part of a crew tasked with isolating Four Four Eight, sedating her, taking measurements, and mounting tracking devices on her body.

“It's definitely a dance. Elephant seals are very protective of their space, and we have to get in their space” to get close enough to sedate them.

Most Popular

Those instruments are part of a long-term tracking project to collect data about elephant seals, whose lives are usually invisible to researchers.

In addition to the green tag on her flipper, Four Four Eight has a couple of more advanced instruments on her body. There’s a satellite tag on her head that reports her latitude and longitude a couple of times per day.

“We can see in real-time at our computers, sitting at our desks in Santa Cruz, where the seals are, even if they're 4,000 kilometers offshore,” Beltran says.

There’s also an instrument on the elephant seal's back that measures the depth and duration of her dives. That gives researchers insight into Four Four Eight’s diving patterns and her body condition.

“When they rest… they stop swimming. If they're fat, they float up through the water column. If they're skinny, they sink down in the water column,” Beltran says.

She used ten years of this data, collected by “hundreds of people,” to test two hypotheses explaining how elephant seals decide when to head back home.

Do they wait until they’ve eaten enough fish and squid to store away the energy needed to make the trip home and nurse their pup? Or do they calculate the time a return trip will take based on the distance they’ve already traveled?

The answer was clear.

Beltrans' research offers insight into elephant seals' extreme life history

A little more than a century ago, scientists thought elephant seals had gone extinct, hunted into oblivion by sealers in search of lamp oil.

Then, a group of about 20 individuals turned up on an island off the coast of Mexico. The Mexican government protected the seals — very successfully.

The population is now roughly 220,000.

How did the elephant seal manage such a dramatic turnaround? One reason, Beltran says, is that they produce pups “like machines.”

Most of a female elephant seal’s life is built around collecting food.

“The females take this kind of low-risk, low-reward strategy of going offshore,” Beltran says. Their aquatic journey consists of dive after dive to roughly 2,000 feet, where they eat primarily fish and squid.

“They dive for, on average, about 23 minutes followed by just two minutes of [rest at the] ocean surface,” Beltran says. “They do that over and over and over and over and over for seven months straight,” only breaking once per day, around sunrise, for a handful of rest dives.

There's a reason for the relentless strategy.

“Out in the open ocean, these fishes are like popcorn, they're just everywhere. And so I think it's a really reliable food source for the females,” Beltran says.

When a female returns to her home beach — a remarkable feat in its own right — she gives birth and spends three weeks pumping as much energy into her offspring as possible. At up to 55 percent fat, elephant seal milk is 45 percent richer than heavy whipping cream.

Females don’t eat or drink while on the beach, so after several weeks of nursing they grow “super skinny, [and] their pups are giant balloons,” Beltran says.

“They [also mate during] that nursing period” with aggressive males that can be up to five times bigger than the females, Beltran says.

Despite the relentless pace and physical demands of elephant seal reproduction, almost every seal that makes it to adulthood produces a calf every year of her adult life. About 97 percent of the females on Año Nuevo beach trudge back into the Pacific to repeat the cycle in any given year.

That’s why turning around in the right moment is so important. Arrive too early and you won’t have enough energy to give your current offspring a fighting chance. Arrive too late and miss the chance to breed. Other evidence has shown that elephant seals can’t make up time by picking up the pace, so there aren’t any second chances.

GPS guides the way

Once she’d crunched the numbers on a decade’s worth of journeys, Beltran found that the data strongly supported one of her hypotheses.

“It was surprisingly straightforward,” she says. We found that elephant seals have sort of internal GPS unit that tells them where they are and when it's time to turn around.”

The amount of food they’ve managed to eat doesn’t seem to factor into the decision. With a maximum speed of roughly 90 miles per day, they have hard constraints.

“If they need to get back to the beach by January 15 and they're still 3000 kilometers away from the breeding beach [on November 1], they're not going to be able to make it back on time,” Beltran said.

The data clearly show that this group of seals — Four Four Eight and her siblings, cousins, and friends — manage to “strategically schedule that turnaround point” to give them enough time to make it back and mate.

Because an elephant seal's job is never done.

“They get pregnant and do it all over again year after year after year," Beltran says.

Excerpt from the study

Many marine animals migrate between foraging areas and reproductive sites, often timing the return migration with extreme precision. In theory, the decision to return should reflect energy acquisition at foraging areas, energetic costs associated with transit, and timing arrival for successful reproduction. For long-distance migrations to be successful, animals must integrate ‘map’ information to assess where they are relative to their reproductive site as well as ‘calendar’ information to know when to initiate the return migration given their distance from home. Elephant seals, Mirounga angustirostris, migrate thousands of kilometers from reproductive sites to open ocean foraging areas, yet return within a narrow window of time to specific beaches. Each year, pregnant female elephant seals undertake a ~240-day, 10,000 km foraging migration across the Northeast Pacific Ocean before returning to their breeding beaches, where they give birth 5 days after arriving. We found that the seals’ abilities to adjust the timing of their return migration is based on the perception of space and time, which further elucidates the mechanisms behind their astonishing navigational feats3.

SHOW COMMENT (1)