A harbor seal sports about a hundred whiskers — which are more than just for show. They provide the animals with key intel on their surroundings.
"These are sensory hairs in the facial region," says Yvonne Krüger, a biologist at the University of Rostock in Germany. "And with these whiskers, they can sense the water movements that are generated by fish."
That's because those movements leave behind underwater trails that a seal can follow to find the fish and gobble them up. "If you look at an airplane, you can see a trail left behind," says Krüger by way of analogy. "And this is similar to what you have if a fish is swimming through the water column. You can't see it — but you can sense it, with the whiskers."
Seals can use their whiskers to pull all sorts of information out of disturbances in the water. They can determine the direction in which something has traveled. Lab studies suggest seals might even distinguish between the movements of different types of fish.
Krüger has done research that suggests that harbor seal whiskers may even help the animals outmaneuver the evasive behavior of the fish they're hunting.
A fishy escape
In one experiment, published in the Journal of Experimental Biology, Krüger focused on the feats of an adult male harbor seal named Filou. He's a dark gray and lives at a marine science center in Germany. "He looks very beautiful," says Krüger.
"We have a very strong bond," she adds. "Filou likes to do everything correct. He likes learning new things. If he has one mistake, he gets frustrated."
In a word, "he's a nerd," she says with a smile.
Krüger trained Filou to test out a working theory that seals use their whiskers to outmaneuver fish trying to escape them.
Harbor seals are talented predators — but it can be hard work. After all, fish don't want to be eaten. Just take the rainbow trout, a favorite food of the harbor seal.
"Rainbow trout are able to camouflage their swimming direction by bending into a C-shape," says Krüger, "and then they swim away in a different direction [than] they have been swimming before."
This new posture and evasive swimming behavior cause the trout to spin off two underwater vortex rings in opposite directions. Think of them like smoke rings made out of water. And only one of those rings — the slightly smaller one — moves in the direction in which the fish is swimming, potentially confusing a harbor seal in pursuit.
Krüger wanted to know if a harbor seal can read through that camouflage and still follow the fish since both vortex rings would point the seal in different directions: one roughly parallel to the swimming fish and one roughly opposite.
She thought that if a seal like Filou could differentiate between the sizes of the vortex rings, it might be able to learn the correct one to chase. Filou did not disappoint.
Training pays off
Krüger spent almost two years training Filou to select the bigger of two vortex rings generated artificially underwater. "You have to be patient with animals," she says. "You have to give them time to learn."
Ultimately, Filou got it and was able to distinguish between the rings — even when the size difference was less than the width of a human thumb and far less than what they'd need to discern in the wild.
"Which tells me that Filou would still be able to follow the correct direction to successfully hunt and prey on the rainbow trout," says Krüger.
And when she covered up Filou's whiskers with a nylon stocking, he could no longer complete the task, suggesting it was his whiskers that were giving him the information he needed to differentiate between the two vortex rings.
It's just one animal, but Krüger and her colleagues say it's an ability she suspects is likely shared by harbor seals more generally — and one they might well deploy in the wild.
"If we think about a harbor seal swimming in the ocean, trying to find its food, they are actually able to read where a fish had been and where it's going to," she says. "So if they hunt in murky waters or if they hunt at night, they don't have to see the fish."
Robyn Grant, a sensory biologist at Manchester Metropolitan University, says this work "is a really important step in working out how the seals [use] their whiskers to extract tiny bits of information from these hydrodynamic trails."
"I would have loved to have some information about the precise movements of the whiskers," says Grant, who wasn't involved in the research. "But this is a great start to tell us what they're capable of doing."
To Grant, it's worth understanding how an animal like a harbor seal senses its surroundings because it may reveal how they might be affected by changes to their environment, including extreme weather events.
"You can imagine that that could mask some of these critical stimuli that the seals would like to pick up," she says.
Plus, Grant observes that these findings may inspire sensors that could help aquatic robots navigate their surroundings to do work related to underwater archaeology, mining or biological surveys.
In other words, Filou appears to be making a trail of his own that others may be eager to follow.
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