In dark water, the difficult part of following a fish is not merely feeling movement. A swimming seal is already surrounded by movement of its own making. Water separates around its muzzle, vortices form behind projecting hairs, and every whisker risks becoming a tiny source of vibration.
The useful clue may be much fainter: the alternating swirls and pressure changes left in the water after prey has passed. To read that trail, a seal needs something like a microphone that does not roar whenever its owner moves.
A study published in npj Flexible Electronics on March 20, 2026, approached this biological puzzle with a striking hybrid. Researchers joined a real harbour-seal whisker to a soft electrohydraulic actuator that stood in for muscle, then watched how shape, angle and stiffness changed what the whisker could detect.
The results help connect two adaptations that are often discussed separately. A harbour seal’s wavy whisker suppresses much of the vibration generated by ordinary flow. Pushing that whisker forward, meanwhile, makes it more responsive to a wake. Geometry quiets the background; active movement raises the signal.
The whisker was real, but the seal was not
The experiment did not attach machinery to a living animal. The team used a 10.8-centimetre whisker from a deceased harbour seal and fitted its base into a three-dimensional printed, flexible imitation of the follicle-sinus complex that holds a whisker in a seal’s muzzle.
That assembly sat on a five-centimetre-long pouch made from thin plastic film. Printed silver electrodes covered part of the pouch, which contained silicone oil. Applying up to 8,000 volts caused the electrodes to pull together and move the oil, bending the pouch in a controlled way.
The actuator therefore behaved less like a hinge than a simplified muscle. It could move the whisker between a relaxed, retracted position and a stiffer, forward or protracted position. It responded in tens of milliseconds and retained about 90 percent of its bending amplitude at frequencies up to five hertz.
In water moving at 0.5 metres per second, it swept the whisker through 17.5 degrees. That closely matched the roughly 17-degree protraction measured in feeding tests with harbour seals. The engineering mattered because a rigid clamp cannot reproduce the changing stiffness at a living whisker’s base.
Why a straight whisker would hear itself
Put a smooth pole in moving water and alternating vortices tend to peel away from its sides. Those repeating forces can drive the pole sideways at a characteristic frequency. Engineers call the resulting motion vortex-induced vibration.
Harbour-seal whiskers complicate that process. They have an oval cross-section whose thickness and orientation vary down the shaft, giving the edge a wavy, bead-like appearance. Different parts of the whisker shed vortices out of step, disrupting the coherent pattern that would otherwise shake the whole structure.
“Cancels its own noise” is a useful shorthand, but it should not be read literally. The undulations suppress self-generated vibration; they do not eliminate every movement or every source of hydrodynamic noise.
To measure the effect, the researchers compared the harbour-seal whisker with one 11-centimetre whisker from a California sea lion. A laser Doppler vibrometer read movements of reflective tape on the whiskers with extremely high precision while water flowed past them at 0.4 to 0.7 metres per second.
The harbour-seal whisker’s vortex-induced vibration amplitude was more than three times lower under the tested conditions. That quieter baseline is only half the sensory trick, however. A useful whisker must still move strongly when the right disturbance arrives.
A cylinder stood in for fleeing prey
The team did not release a fish into the water tunnel. Instead, it placed a one-centimetre-wide cylinder ten centimetres upstream. At a flow speed of 0.53 metres per second, the cylinder produced a controlled Kármán vortex street: a repeating row of rotating patches of water.
A real swimming fish leaves a more complicated trail, often described as a reverse Kármán vortex street. The cylinder was not a realistic prey model in every detail. Its value was repeatability. The same predictable disturbance could be offered to whiskers in different mechanical states.
Without the cylinder, the harbour-seal whisker showed its own vibration peak around 170 hertz when rigidly held. With the cylinder present, another peak appeared at about eight hertz, close to the ten-hertz vortex-shedding frequency calculated for the cylinder.
That new low-frequency response was the wake signal. The researchers compared it with the whisker’s self-generated vibration to calculate a signal-to-noise ratio. Under this particular test, the harbour-seal whisker’s ratio was more than 50 times that of the smoother sea-lion whisker.
Earlier work has described a related response as “slaloming.” Rather than simply trembling in turbulence, a whisker can be pulled toward successive low-pressure vortices and lock onto the wake’s rhythm. Experiments with seal-whisker arrays have further shown that position and spacing across a muzzle shape the response.
Pushing forward made the signal clearer
The artificial muscle let the team ask what a fixed model could not: does protraction change sensitivity? They measured the same harbour-seal whisker while it was rigidly clamped, relaxed and retracted, and actively stiffened in the protracted position.
The protracted whisker produced the highest signal-to-noise ratio. Its value was more than three times the rigidly clamped result and roughly twice the retracted result. The ranking was protracted, retracted, then rigid.
Forward movement therefore does more than place the whisker nearer an object. Contraction changes the stiffness of the base and the orientation of the shaft, altering which frequencies become visible against its mechanical background.
There is likely an energetic trade-off. Maintaining protraction requires muscle contraction, whereas retraction is cheaper. A seal may gain sensitivity by sweeping its whiskers forward when prey cues are plausible, then allow them to fall back when prolonged high sensitivity is not worth the cost.
Wild observations fit that interpretation. In a camera study of northern elephant seals, animals repeatedly extended and rhythmically moved their whiskers during deep foraging. Protraction often began before prey appeared in the infrared video and lasted through pursuit and capture.
Where complete darkness enters the story
The new water-tunnel study did not test a seal hunting in darkness. That part of the headline rests on a larger behavioural record. Harbour seals have followed hydrodynamic trails while deprived of useful visual and acoustic cues, sometimes tracing the route of a moving object well after it had passed.
In a classic 2001 experiment, blindfolded harbour seals followed the trail of a small submarine. Deep-diving elephant seals offer evidence outside a pool: their cameras show prey capture far below the reach of sunlight, without the biosonar used by toothed whales.
The wavy whisker is thus not an eye, and it does not form a visual picture. Its heavily innervated base senses minute bending and vibration. Across an array, differences in timing, frequency and force could reveal where a trail lies and which direction it runs.
Rhythmic sweeping may help sample that changing field. It exposes the array to the wake in more than one position, while bouts of protraction temporarily tune the whiskers for a clearer response.
One whisker from each species is a real limit
The most important caution appears in the paper itself. The quantitative species comparison used only two samples: one harbour-seal whisker and one California sea-lion whisker.
The whiskers were similar in length but not diameter. The sea-lion whisker averaged about 1.3 millimetres across the flow-facing dimension, compared with 0.5 millimetres for the harbour-seal whisker. They also had different curvature and species-appropriate orientations in the tunnel.
Those are biologically meaningful differences, but they make it risky to assign every measured contrast to waviness alone. The team says complete sets containing whiskers of varying sizes are needed to establish whether the pattern holds across animals, species and whisker states.
California sea lions also catch prey successfully with relatively smooth whiskers. The study does not show that one shape is the only route to hydrodynamic sensing, or that the artificial actuator reproduces every action of living facial muscles.
It shows something narrower and still valuable: in this controlled comparison, undulation reduced the harbour-seal whisker’s background vibration, and active protraction markedly improved its ability to separate a repeatable wake from that background.
Sixty whiskers point toward a robotic future
After studying one moving whisker, the researchers built a full bionic muzzle. A printed seal face carried 60 real harbour-seal whiskers, 30 on each side, arranged with shorter whiskers toward the front and longer ones farther back.
The two sides could move independently. The system varied whisking angle and frequency, held whiskers forward for different periods, and demonstrated asymmetric movement that could eventually help determine which side of a robot encountered a wake first.
For now, it is a demonstrator, not a synthetic seal. It did not autonomously locate prey, and the paper’s strongest vibration measurements came from a single whisker. The next planned step is to place printed microelectromechanical sensors at the artificial follicles and record signals across the moving array.
If that works, an underwater vehicle might detect trails without emitting sonar and without depending entirely on cameras in turbid or lightless water. Such a sensor would not copy only the whisker’s outline. It would copy the decision to move and stiffen that outline at the right moment.
That is the experiment’s most revealing contribution. A seal’s whisker is not just a beautifully shaped passive antenna. It is part of an active system whose quietness and sensitivity change as the animal searches. The wavy shaft lowers its own hydrodynamic chatter; the sweep gives a fading trail room to be heard.