Inside Elephant Seal Pups’ Race to the Depths
While northern pups dive right in, their southern cousins take their time
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Elephant seals can tip the scales at more than 8,000 pounds and don’t look particularly nimble on the beach. But in the ocean these incredible athletes boast world-class swimming and diving abilities. The air-breathing mammals can dive to depths of over a mile, despite darkness and crushing water pressure, and to do so they can spend nearly two hours holding their breath.
These amazing abilities help the seals find fish and other food—and avoid becoming a meal for predators like orcas. But how do juveniles develop their diving skills when they first head to sea on their own? Scientists have learned that it depends on which end of the planet a seal lives. “We discovered that northern elephant seals appear to develop their diving capabilities more quickly than southern elephant seals, which allows them to reach deeper depths during their first oceanic migration,” says Roxanne Beltran, a physiological ecologist at the University of California, Santa Cruz.
On the first day they head out to sea, juvenile northern elephant seals can dive for up to 11 minutes, a duration that their southern relatives don’t achieve for 125 days, according to a study published recently in Royal Society Open Science. They also dive deeper sooner. After just 30 days at sea, the northern elephant seals reached depths of over 850 feet, while it took southern elephant seals more than five times as long, 160 days, to start reaching the same depths.
“This is a fascinating study, and it is the opposite of what I would have expected, given that southern elephant seals are known to reach greater depths as adults compared to northern elephant seals,” says Heather Liwanag, a biologist at California Polytechnic State University not involved in the research.
The adults of the two closely related species are relatively similar in their reproductive systems and behaviors. But they inhabit different environments: the northeast Pacific Ocean and the Southern Ocean. Differences in those habitats, from ocean currents to prey availability, may influence the different ways elephant seals develop the diving skills so important for their survival.
For the study, researchers used tracking technology to observe how young elephant seals in each hemisphere developed their diving abilities while out at sea. Beginning in 2018, scientists sedated two dozen juvenile northern elephant seals at California’s Año Nuevo Natural Reserve and epoxied small recording tags to their backs that measured time, depth and light during the animals’ first trip to sea. When the animals returned to land an average of 230 days later, four of the devices were recovered with data intact. Researchers similarly tagged 20 juvenile southern elephant seals on the Kerguelen Islands, in the sub-Antarctic French territories. Nine devices were recovered when those animals returned to land from their first ocean migrations, which averaged about 175 days at sea.
Some of the elephant seals returned without tags, while others never returned at all. At sea, the seals covered a lot of territory. They swam around 5,000 miles, and sometimes more, during their first foraging adventures. Some elephant seals have been known to cover more than 13,000 miles during a migration season. “Without any prior experience or learning from adults, surviving such feats by getting enough food, avoiding predators and navigating back to their natal sites is quite remarkable,” says zoologist Trevor McIntyre of the University of South Africa, who wasn’t involved with the study. “And, ultimately, we don’t know how they do it.”
Many of them don’t succeed.
“Juveniles survive much less often than adults, probably because of their inexperience, small body sizes and limited breath-holding capabilities,” says Beltran, the senior author of the study.
University of California, Santa Cruz, postdoctoral researcher Joffrey Jouma’a, working with a French team, led the effort to uncover the differences in diving behavior during the seals’ first migrations to sea. The trip, which can last from 6 to 12 months, is a crucial one that often doesn’t end well; around 30 to 40 percent of juveniles don’t survive their first year. Beltran and the research team don’t know why northern elephant seals began diving deeper, and longer, so much sooner at this time. However, they do have some intriguing ideas.
One key difference between the northern and southern species is that the northern elephant seals spend a few more days fattening up on mother’s milk. Northern elephant seals have a lactation period of 24 to 28 days, versus 22 to 23 days in the south. This means mother seals provide more fat-rich milk to their pups, and so the youngsters build up bigger reserves that will provide them with both energy and warmth. This feeding frenzy is followed by an important period of fasting after mothers stop feeding their pups. The fast is longer among the northern elephant seals, at about 56 days, compared to 37 days for their southern counterparts.
The larger fat reserves may allow northern elephant seals to enjoy the longer fasting period. During that time they build social skills with other pups and, crucially, practice breath-holding cycles that they will need during diving. As the fasting period goes on, the seals practice holding their breath longer and longer by lowering their metabolic rate, Beltran explains. “This is similar to humans training for a marathon—after more training runs, our bodies fine-tune their ability to use less oxygen for muscle movement and to store more oxygen,” she says. “We think the longer fasting period in northern elephant seals allows them to develop their oxygen stores and diving metabolic rates more quickly than southern elephant seals, and get a head start on diving abilities during the first trip to sea.”
Scientists may conduct a future study measuring oxygen stores and diving metabolism to test this idea, says Beltran.
Why southern elephant seals head to sea more quickly than their cousins to the north is an open question. The environments at each end of the Earth may play a role. On land, weather may influence behavior. And the number and density of females and the proximity of brawling males could be a factor—crowded, less desirable sites could cause seals to spend less time feeding. At sea, differences in currents and eddies of the northeast Pacific Ocean and Southern Ocean could impact the seals’ movements and the locations and availabilities of different prey.
However, McIntyre points out the study tracked only a handful of seals, and other factors may play a role in their dive behavior. “For example, the southern elephant seal juveniles at Kerguelen often spend a lot of time foraging on the Kerguelen plateau,” he says. “This shallow area would potentially also result in generally shorter and shallower dives being performed irrespective of physiological capabilities.”
Liwanag also wonders whether the differences in diving behavior could somehow be linked to the incredible genetic bottleneck that occurred when northern elephant seals were hunted to the brink of extinction by the 1880s. The species rebounded in a stunning conservation success story thanks to protections in the United States and Mexico.
Northern elephant seals don’t always dive deeper than their southern relatives, the study shows, or stay down longer. In fact, it appears that the northern animals often feed at similar depths as southern elephant seals. It was the dives seals used to rest or to transit from one area to another that were longer and deeper among the northern elephant seals.
McIntyre says deep diving likely allows elephant seals to avoid predators like orcas and great white sharks. “If you imagine an animal that spends greater than 90 percent of its time at sea diving, with the majority of this time at depths below what orcas are known to dive to, that would seem to be a good strategy to avoid them.”
Each species seems to develop diving skills in a way that works where it lives—even if we humans don’t understand exactly why. “Mortality rates are similar across species,” Beltran says, “so it doesn’t appear that one species is more successful.”
Elephant seals have quite a few incredible adaptations that enable their deep-diving lifestyle. Their blood is rich in carbon monoxide—at ten times higher than the levels of an average human. The gas is dangerous to us because it slows oxygen delivery throughout the body. But for elephant seals, that same property allows them to remain underwater longer with the same amount of oxygen and surface less frequently to breathe. Researchers also recently reported an incredible discovery that northern elephant seals sleep for just two hours a night, and do so in ten-minute underwater power naps, during the months they spend at sea.
As they put technology to work, researchers continue to learn more about the amazing abilities of elephant seals and how they thrive in the open sea. And by comparing the behaviors and adaptations of similar seals living on either end of the globe, scientists can discover much about the animals and the different marine environments they call home.