Female elephant seals spend most of their lives fishing in complete darkness. Now, by strapping infrared cameras to their heads, scientists have figured out how these sleek swimmers locate their prey: They move their whiskers like satellite antennas to sense water movement.
“This study is absolutely brilliant,” says Robyn Grant, a sensory biologist at Manchester Metropolitan University, whose research on sea lions in captivity has come to similar conclusions. But the new work is the first to show how seals use their whiskers in the wild while fishing, Grant says.
The new study included five northern elephant seals (Mirounga angustirostris) at Año Nuevo State Park, a coastal park south of San Francisco. Researchers outfitted them with headgear including cameras about the size of flash drives and other small sensors that detect light, depth, and the opening and closing of the mouth. The team recorded data in February 2015 and 2018, just after the breeding season, when the animals left the shore for a 2-month ocean migration in search of food. When the seals returned to shore, to shed fur and give birth, the scientists collected the devices and analyzed the footage.
At the start of each dive, still in shallow waters, the seals kept their whiskers retracted. But as they approached dark waters deeper than 200 meters, they extended their whiskers forward. At even greater depths, where their favorite meals of squids and lantern fish reside, the seals constantly moved their whiskers back and forth like a dish antenna scanning for a signal, the team reports today in the Proceedings of the National Academy of Sciences.
“Their whiskers are almost like hands reaching out in front of them,” says co-author Daniel Costa, a marine biologist at the University of California, Santa Cruz (UCSC), who has been studying elephant seals for more than 40 years.
“Cats and rats use whiskers to sense air flow, but seals do it underwater to sense water flow,” adds lead author Taiki Adachi, a marine biologist at UCSC.
The discovery didn’t come as a total surprise, as researchers in the field are used to seeing seals using their whiskers to investigate backpacks and other gear people leave on the ground while working, says co-author Rachel Holser, a marine biologist at UCSC. But she and her colleagues were not expecting to see such coordinated and rhythmic movement of the animals’ whiskers, which was never before observed in seals or any marine mammal in the wild.
The researchers believe the seal whiskers may detect not only the currents caused by the fish swimming, but also tiny movements in the water, such as the currents formed when fish open their gills to breathe. Previous studies with captive harbor seals estimated they could track a herring swimming as far as 180 meters away just by sensing the water movement. Seals’ whiskers have up to 1500 nerve fibers, seven times more than mice.
Marine biologist Guido Dehnhardt at the University of Rostock isn’t impressed by the results, however. His own work with blindfolded seals in tanks has found the animals use their whiskers to follow water tracks. But Dehnhardt argues the new study didn’t show this well, as the researchers only presented footage of the seals moving their whiskers close to fish; they did not link these movements to the water currents left by the prey (which could have been done if the seals also wore equipment to measure the water flow), he argues.
Grant remains bowled over, however. She says the study showcases the range of adaptations animals needed to evolve to survive in dark waters. Toothed whales, which also fish the deep waters, developed echolocation, using sounds to “see” in the dark, she notes, whereas seals developed an alternative strategy that’s common across land mammals. “Whisking probably evolved multiple times among mammals,” she says. “To see it in seals is really critical, as we are only starting to realize the importance of whisker movements in this group.”