There’s no 9-to-5 for female northern elephant seals. After the winter breeding season, the animals spend more than 19 hours—and up to 24 hours—per day hunting in the northern Pacific Ocean, killing up to 2000 small fish daily to survive, according to a new study of these elusive animals. The work, made possible by cameras and devices attached to the seals’ heads, could also help scientists monitor other deep-ocean life.
“This study is fascinating,” says Jeremy Goldbogen, a marine biologist at Stanford University who was not part of the research. “The advanced technology provides unprecedented levels of detail on where and when the elephant seals forage in a deep, dark ocean.”
Northern elephant seals (Mirounga angustirostris) are mysterious animals. They appear onshore, on some Pacific Coast beaches, only twice a year: in late December or early January to mate or give birth, and about 2 months later to shed their fur.
They spend the rest of their time, almost 10 months, fishing. Males, which can weigh up to 2 tons—about the weight of a small truck—hunt big fish close to the coast. Females, which are only about one-third of the size, hunt smaller fish in a deep-sea region known as the twilight zone. To get food from the zone, which reaches depths of 1500 meters, the females must hold their breath for up to 1.5 hours. “The physiological challenges that these animals face to meet their daily energetic demand is an extraordinary feat,” Goldbogen says.
To find out how the females survive on the small fish—some of which are just 2 centimeters long—Japanese and U.S. researchers attached infrared video cameras with depth sensors to the heads of 48 female elephant seals. They also attached GPS trackers and a special device that could count every time a seal opened its mouth. (The researchers called their device the Kami Kami Logger, after the Japanese sound for biting, similar to the English “chomp chomp.”)
After recording more than 200,000 dives during the 2.5 months between the breeding and molting seasons, the researchers found that the female seals spent day and night, from 80 to 100% of their time, continuously diving in search of fish, they report today in Science Advances. The females barely slept, and when they did, it wasn’t for more than 1.4 hours a day (see video, below).
The exhausting lifestyle—which scientists suspect lasts the entire time the females are at sea—eventually pays off. It helps them regain the fat they lose on land, when they forgo all food. During their first 2 months back at sea, the females gained about 100 kilograms on average.
Male elephant seals, meanwhile, have no chance surviving off the bounty of the twilight zone, the researchers conclude. Their huge size and lack of diving skills mean they would need to hunt almost four times as long as the females to get enough energy for 1 day. “They just could not make a living there,” says lead author Taiki Adachi, a marine biologist at the National Institute of Polar Research in Tokyo.
The work provides “stunning details” about the lifestyle of these creatures, says Barbara Block, a marine biologist at Stanford not involved with the research. She says this strategy of attaching instruments to the seals can tell scientists about more than just the seals themselves—tracking, for example, the abundance and type of fish in remote parts of the ocean. “Much of Earth’s biodiversity is located in the twilight zone,” she says. “It remains one of the unknown parts of our planet, and we can get a glimpse from the seals as our sentinels.”