How seals use their whiskers to outsmart escaping fish

Fish have a getaway move that looks like misdirection: when they bolt, they shed a flurry of watery “smoke rings” that may throw a hunter off the trail.

New experiments show seals can see through the trick. Using only their whiskers, they can tell which of two vortex rings is bigger – even when the difference is nearly three-quarters of an inch – and use that clue to predict where the fish actually went.

In the new study, researchers in Germany trained a harbor seal to “feel” spinning rings of water without using sight or sound, then decide which ring was larger.

The work, led by the University of Rostock, suggests seals can read fine details in a wake that points them to a fleeing fish’s true direction, despite the confusing swirl a fish leaves behind.

Chasing fish underwater

Life below the surface is noisy with motion, but seals are built to sort it out. Their whiskers are exquisitely sensitive. As a fish swims, it carves a trail of turbulence that can be followed minutes later.

When a predator lunges, the fish counters with a rapid escape, firing three jets of water in quick succession and in different directions. At least two of those jets roll up into donut-shaped eddies – vortex rings – like smoke rings underwater.

One is larger than the other. If a seal can tell which is which, it can infer where the fish darted. That was the hunch. The question was whether a seal’s whiskers are precise enough to pull it off in real time.

“The seal will have a better chance of guessing a fish’s escape direction if it can tell the difference between the two vortex rings,” said co-author Wolf Hanke, an expert in sensory and cognitive ecology at Rostock.

“The challenge is that the difference a seal needs to read is small – on the order of a few centimeters across a fast, fragile swirl.”

Filou the seal takes the challenge

Enter Filou, a harbor seal (Phoca vitulina) living at the Marine Science Center in Rostock. Trainers taught Filou to submerge his head while blindfolded, eliminating visual cues, and to remain still as the team created a vortex ring beside him with a piston device.

At times, they dyed the water with uranine so the researchers could confirm the ring’s shape, but Filou never saw it. Moments later, they generated a second ring on the opposite side – slightly bigger or smaller than the first.

Filou’s job was simple in concept but tricky in practice: decide which side produced the larger ring by feel alone, then nudge one of two green balls mounted on either side of his head to report the answer. A correct choice earned a fish.

It wasn’t instant. “It took him quite a while to grasp the concept of different vortex ring sizes,” Krüger said. “But once the penny dropped, Filou became a remarkably consistent judge of watery geometry.”

Seals judge rings with whiskers

The team started with vortex diameters spanning about 3.5 inches (90 millimeters) down to about 1.8 inches (46 millimeters). Across hundreds of paired trials, Filou routinely chose the larger ring well over 80 percent of the time.

Crucially, the smallest gap he could resolve – nearly three-quarters of an inch (17.6 millimeters) – was still within his whiskers’ reach. That’s a tight threshold in a messy medium.

To make sure he wasn’t learning fixed pairings, the researchers reshuffled the matchups. A ring he had previously “ignored” at 2.7 inches (68 millimeters) became the larger ring in a new pairing. He still picked it out.

After months and thousands of switches, the pattern held. Harbor seals can discriminate surprisingly fine differences in the width of vortex rings using whiskers alone.

Why that matters for a fleeing fish

When a startled fish fires off those three jets, the second jet shoots opposite to its path of travel and tends to make the larger of the two rings the seal will feel. If a predator can tell those rings apart, the noisy, multi-ring wake becomes a readable signpost.

The largest ring says “the fish went that way – go the other way.” Seals, the study suggests, can do exactly that, turning a confusing flourish into a reliable hint.

The result also fits with what we know about pinniped whiskers. They’re tuned to subtle flow patterns, with surface structure that dampens self-induced noise and enhances sensitivity to external swirls. Discriminating ring width by less than 0.8 inches (two centimeters) pushes that ability into impressive territory.

Seal whiskers prove their power

The setup was elegantly spare. No sound cues. No vision. Just two rings, one decision, one tap. That parsimony strengthens the conclusion: Filou’s responses were based on hydrodynamic touch.

It also provides a clean behavioral benchmark for future work. Researchers can now ask how distance, speed, ring age, or background turbulence alter a seal’s read – and whether wild seals perform similarly amid waves, currents, and clutter.

There’s evolutionary logic here, too. For a predator, picking the right branch at the hydrodynamic fork saves energy and time. For prey, adding extra rings to the scene buys a split second to escape. The arms race proceeds at the scale of ripples and rings.

From the lab to the wild

This was one trained animal in controlled conditions. Wild hunts are messier. Rings can break down fast. Other cues – sound, sight, odor – may contribute. But the threshold the team measured sets a realistic lower bound on what seals can do with touch alone.

If a fleeing fish’s second-jet ring is larger than its partner by at least a couple of inches, a seal’s whiskers should, in principle, be able to tell.

That makes the fish’s “smoke rings” less a magic trick and more a riddle with an answer. The next steps will be to tie the lab results to free-swimming chases, and to watch how often seals actually bank on the big ring in the wild.

For now, the takeaway is clear. Seals don’t just feel wakes; they read them with surprising finesse. In the cold game of pursuit and escape, that tiny edge – nearly three-quarters of an inch (17.6 millimeters) – might be all the advantage a hunter needs.

The study is published in the Journal of Experimental Biology,

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–