In the darkness of the deep ocean, vision is limited for predators seeking out their next meal. Some animals, like toothed whales, use echolocation to find their prey. Others have glowing lures that attract their food to them.
But seals, which can dive down to depths of more than 5,000 feet in search of food, don’t seem to use either. Instead, researchers have suspected that they use their whiskers to detect subtle movements in the water, similar to how rats use whiskers to detect movements in the air.
To test this hypothesis, scientists attached small infrared cameras to five elephant seals in the wild and recorded their whisker movements while they dove for prey. The team captured over nine hours of video footage. The researchers saw that seals moved their whiskers back and forth when swimming at hunting depths, but in shallower waters, when they weren’t hunting, they kept their whiskers retracted, writes Sasha Warren for Scientific American.
“[In] natural conditions, animals will use all of the information from many sensory systems and integrate it to shape behavior in the wild....they may use eyes; they may use whiskers; they may use hearing,” said Taiki Adachi, lead author and marine ecologist at UC Santa Cruz to Scientific American.
The only source of light in deep, dark waters is often organisms themselves. However, in the seals’ hunting trips, bioluminescence—a chemical reaction that produces light within organisms—was present only in about one in five successful catches, per the publication. It’s unlikely, then, that the seals were using bioluminescence to catch prey. The researchers conclude, in a new study published in Proceedings of the National Academy of Sciences, that rhythmic whisker movements play the primary role in sensing prey.
“This makes sense,” Sascha Kate Hooker, a pinniped (seals and walruses) researcher from the Sea Mammal Research Unit at the University of St Andrews, who was not involved in the study, tells the Guardian’s Sofia Quaglia. “Among the deep-diving marine mammals, the elephant seal reaches the same depths as sperm and beaked whales, often well over a kilometer below the surface.”
Previous research has shown that seals in captivity, when blindfolded, can even use their whiskers alone to hunt, without the aid of sight. Seals could use their whiskers to detect movement as far as 130 feet away, tens of seconds after the object creating motion in the water had already passed by. But the rhythmic whisker movement has never been observed before in any marine mammal in the wild, reports Sofia Moutinho for Science.
“These field studies are really urgently needed,” Guido Dehnhardt, a marine scientist at the University of Rostock in Germany, who was not involved in the study but coauthored the blindfolded captive seal whisker research over 20 years ago, tells Scientific American. “To do this in the wild with free-ranging animals is really a great challenge—and the results are very impressive.”
The authors argue that the study has resolved a “decades-long mystery about how deep-diving seals locate prey in the darkness.”
The scientists believe that whiskers can detect movements in the water from prey nearby, but Dehnhardt tells Science that the new research doesn’t directly link the whisker use to movement in the water; the video footage simply shows whisker movements close to fish. Further research is needed to measure both simultaneously.