When they are in the deep, dark ocean, seals use their whiskers to track down their prey, a study has confirmed after observing the sea mammals in their natural habitat.
It’s hard for light to penetrate the gloom of the ocean’s depths, and animals have come up with a variety of adaptations in order to live and hunt there. Whales and dolphins, for example, use echolocation – the art of sending out clicky noises into the water and listening to their echo as they bounce off possible prey, to locate them. But deep-diving seals who don’t have those same acoustic projectors must have evolutionarily learned to deploy another sensory technique.
Scientists have long hypothesised that the secret weapons are their long, cat-like whiskers, conducting over 20 years of experiments with artificial whiskers or captive seals blindfolded in a pool, given the difficulties of directly observing the hunters in the tenebrous depths of the ocean.
Now a study may have confirmed the hypothesis, according to Taiki Adachi, assistant project scientist of University of California, Santa Cruz, and one of the lead authors of the study published in Proceedings of the National Academy of Science.
Adachi and his team positioned small video cameras with infrared night-vision on the left cheek, lower jaw, back and head of five free-ranging northern elephant seals, the Mirounga angustirostris, in Año Nuevo state park in California. They recorded a total of approximately nine and a half hours of deep sea footage during their seasonal migration.
By analysing the videos the scientists noted that diving seals held back their whiskers for the initial part of their dives and, and once they reached a depth suitable for foraging, they rhythmically whisked their whiskers back and forth, hoping to sense any vibration caused by the slightest water movements of swimming prey. (Elephant seals like to snack on squid and fishes, and spend a long time out at sea.) Then, on their swim back to the surface, the whiskers were curled back towards the face again.
For less than a quarter of the time the seals were hunting, they could also see some bioluminescence – the light that some creatures deep underwater can emit thanks to chemicals in their bodies – to track down their meals using sight. But for the remaining 80% of their hunting spree, they were presumably just using their whiskers, according to Adachi. This technique isn’t dissimilar to rodents, Adachi noted. It’s just that, since water is much denser than air, the whisking speed is much slower in elephant seals.
“This makes sense,” said Sascha Kate Hooker, a pinniped researcher from the Sea Mammal Research Unit at University of St Andrews, who was not involved in the study. “Among the deep-diving marine mammals, the elephant seal reaches the same depths as sperm and beaked whales, often well over a kilometre below the surface.”
Guido Dehnhardt, the director of the Marina Science Center at the University of Rostock, and a pioneer in whisker-research who was not involved in the research, welcomed the findings but was cautious about how much new information they represented. “It was my group who had shown more than 20 years ago that the seal’s whiskers represent a hydrodynamic receptor system, and that the seals can use it, for example, to detect and follow the hydrodynamic trails of fish,” Dehnhardt said.
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The study is particularly interesting from a technical point of view, especially with regard to the cameras used being so small, said Dehnhardt, but there’s still too much speculation. “It would be a great story if the seals in addition to a head-mounted camera wore a hydrodynamic measurement system [a machine that can measure the movement of fluids] so that whisker movements and hydrodynamic events could be correlated.”
In future Adachi would like to start comparing how other mammals use their whiskers, in order to better understand how some animals’ whisker superpower has shaped the foraging habits of the animal kingdom.