What Are a Seal's Whiskers For?
― They Read the "Water Swirls" a Fish Left 30 Seconds Ago
A seal can follow the path a fish took, even with a blindfold on. Its clue is a line of tiny swirls that a swimming fish leaves in the water. The whiskers are the tool for reading these invisible "footprints".
Have you ever looked closely at a seal at an aquarium, face on? Dozens of thick, stiff whiskers grow forward from both sides of its nose.
The sea where seals live is not always clear. In the cloudy water of a river mouth, or in the dark of a night sea where no light reaches, they still catch fish and make a living.
How do they find prey in water where eyes are almost useless? The answer was in those whiskers.
Two reasons whiskers can follow a fish
When a fish swings its tail from side to side, small swirls form alternately in the water behind it. This line of swirls does not vanish at once. It stays in the water for a while.
A seal's whisker has a wavy surface. Thanks to this shape, the whisker itself shakes very little as the seal swims, so it can sense only the faint movements in the water.
In short, the fish leaves "footprints" in the water, and the seal reads them with a low-noise antenna. Let's take them one at a time.
A fish's path leaves "footprints" of swirls
A fish swings its tail from side to side, pushing water backward as it moves forward. When the tail swings right, a clockwise swirl forms. When it swings left, a counterclockwise one forms. So behind the fish, swirls that alternate in direction line up in a zigzag.
Look at Figure 1. A line of swirls stretches out behind a fish swimming to the right. The further left you go, the older the swirl. They weaken little by little, but they do not vanish right away.
Experiments have confirmed that seals really do read these "footprints". In 2001, a German research team put a blindfold and ear covers on a harbour seal. They then ran a small model submarine through the water and tested whether the seal could follow its path.
The seal tracked the submarine's route accurately. But when its whiskers were covered with a mesh mask, it could no longer follow it. The clue was not the eyes or the ears, but the whiskers. Later studies reported that seals could read which way a target had gone from a trail left more than 30 seconds earlier.
Whiskers must be "quiet" to read the footprints
Here is a problem. When you move a stick through water, swirls form behind it too. Each time a swirl peels off, first on one side and then on the other, the stick is pushed sideways and shudders. A power line "whistling" on a windy day works the same way.
A whisker is also a thin stick in the water. If the whisker itself shook every time the seal swam, the faint ripples a fish left behind would be drowned out by its own shaking. It would be like trying to hear a distant whisper while beating a drum next to your ear.
The harbour seal's whisker solves this problem with its shape. Its cross-section is a flattened ellipse, and from base to tip it swells and narrows again and again in a wavy form. With this shape, the swirls behind it do not line up, and the forces pushing on the whisker cancel out. Experiments report that the shaking is much smaller than with a round rod.
In Figure 2, try changing how fast the whisker itself shakes. A fish's footprint ripples slowly, a few times per second. A round whisker, by contrast, would shake hundreds of times per second.
The faster the seal swims, the harder a round whisker would shake. By keeping this shaking itself small, the wavy whisker is thought to let the seal pick out a fish's ripples even while swimming.
The base of a single seal whisker is said to connect to more than 1,000 nerves. That is an order of magnitude more than in the whiskers of rats or cats. It is even said that much of the nerve supply to the face is devoted to the whiskers.
Even among marine mammals living in the same sea, a sea lion's whiskers have a smooth surface and are not wavy. Sea lions also sense water movement with their whiskers, but harbour seals are reported to be better at following a long trail while swimming. The difference in shape may be linked to a difference in how they use them.
Summary
A fish leaves an invisible footprint behind it as it swims: a line of swirls. A seal reads that footprint with whiskers whose wavy shape keeps their own shaking down. This is why seals can chase prey even in cloudy water or at night.
A seal is not looking for fish with its eyes.
It reads the "footprints" left in the water with its whiskers.
How whiskers sense fine movements is covered in more detail in What Do a Cat's Whiskers Sense? For another story of an animal hunting with a sense that is neither sight nor smell, see How Can Sharks Find Prey Hidden in the Sand?
- Lower your hand, palm open, quietly into the bathtub. Keep it still and wait for the water to settle.
- With your other hand, sweep quickly across the water about 10 cm away. After a few seconds, notice the ripple reaching the hand you submerged.
- Next, hold a single chopstick and move it quickly sideways through the water. You should feel a fine shudder in the hand holding it.
The shudder in step 3 is the shaking of a stick being pushed each time a swirl peels off. A seal's whisker is shaped to keep this small.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from junior high science to university specialist courses
- Junior highCovered in junior high school science
- High schoolCovered in high school physics and biology
- High school+Advanced high school material, or textbook sidebars
- UniversityUniversity specialist courses (fluid dynamics, sensory physiology), not taught in high school
- ResearchNot yet taught as settled fact even at university; researchers are still investigating it
Junior highTerms: this phenomenon has names
- Vibrissa (whisker): a sensory hair with many nerves gathered at its base. It is a different thing from ordinary body hair.
- Kármán vortex street: a line of swirls, alternating in direction, that forms behind a rod placed in a flow.
- Vortex-induced vibration: the shaking of a rod as it is pushed sideways each time a swirl peels off.
Junior highHigh schoolChecking with a formula: how many times per second does a round whisker shake?
The number of swirls shed behind a rod depends on the speed of the flow and the thickness of the rod. Here we compare how often a round whisker would shake with the ripple of a fish's footprint.
| In symbols | f ＝ St × U ÷ d |
| In words | Swirls shed per second ＝ Strouhal number × flow speed ÷ rod thickness |
| Where it comes from | Experiments show that behind a round rod, the spacing between swirls stays almost constant at about 5 times the rod's thickness. If the spacing is fixed, the faster the flow, the more swirls are shed each second. The coefficient of this proportion is the Strouhal number, which is about 0.2 for a round rod. |
| f | Number of swirls shed per second (how often the rod shakes). Unit: times per second |
| St | Strouhal number. A number with no unit. About 0.2 for a round rod, and 0.2 to 0.4 for the tail of a swimming fish |
| U | Flow speed (swimming speed). Unit: metres per second |
| d | Thickness of the rod. For a fish, the width of the tail's side-to-side swing. Unit: metres |
| Speed of a seal swimming slowly (rough guide) | 1 m/s |
| Thickness of a whisker (rough guide) | about 1 mm ＝ 0.001 m |
| Strouhal number of a round rod | about 0.2 |
| Fish swimming speed and tail swing (small fish, rough guide) | 0.5 m/s and 0.05 m |
| Strouhal number of a fish's tail (rough guide) | about 0.3 |
| Whisker: Strouhal number × speed | 0.2 × 1 ＝ 0.2 |
| Whisker: divide by thickness (shakes per second) | 0.2 ÷ 0.001 ＝ 200 |
| Fish: Strouhal number × speed | 0.3 × 0.5 ＝ 0.15 |
| Fish: divide by swing width (ripples per second) | 0.15 ÷ 0.05 ＝ 3 |
| Difference in speed (how many times) | 200 ÷ 3 ≒ 67 |
| Length of footprint a fish leaves in 30 s (m) | 0.5 × 30 ＝ 15 |
A round whisker would shake about 200 times per second. A fish's footprint ripples about 3 times per second, so the whisker's own shaking would be nearly 70 times faster. And a fish's footprint, over 30 seconds, stretches about 15 metres. The wavy whisker is thought to cut this noise so that the seal can follow this long footprint.
High schoolHigh school+Why does a rod shake sideways when swirls form alternately?
High schoolWhen a swirl peels off one side of a rod, the flow on that side changes, and a sideways force acts on the rod. Next the swirl peels off the opposite side, so the force points the other way. Because this keeps alternating, the rod shakes from side to side. In terms of action and reaction, it can also be seen as the recoil of the rod pushing water sideways.
High school+When the shaking rate approaches the rod's own natural frequency, resonance occurs and the shaking grows large. This is an important effect in the design of bridges, chimneys and power lines. The harbour seal's whisker is thought to shift where swirls peel off along its length through its wavy shape, so that the forces of the shaking do not line up.
UniversityThe Kármán vortex street and the fluid dynamics of the wavy whisker
The alternating line of swirls behind a rod is called a "Kármán vortex street" in fluid dynamics. The dimensionless number that describes how often swirls are shed is the Strouhal number, which is roughly constant at about 0.2 for Reynolds numbers from about 1,000 to 100,000. For a rod 1 millimetre thick in water moving at 1 metre per second, the Reynolds number is about 1,000, right at the edge of this range. The harbour seal's whisker has an elliptical cross-section and an "undulating" shape whose thickness varies periodically along its length. Flow experiments and numerical simulations using whisker models have shown that this shape greatly weakens vortex-induced vibration. The swirl line made by a fish's tail runs in the opposite sense, a "reverse Kármán vortex street", and this provides the thrust that drives the fish forward.
📖 Derivations and further reading: Kármán vortex (Wikipedia, in Japanese) ／ Strouhal number (Wikipedia, in Japanese)
ResearchWhat is still unclear
- How far do wild seals rely on their whiskers? Most experiments were done in captivity, and how big a role whiskers play in real hunting in cloudy seas is still being studied.
- Can a footprint reveal a fish's size or species? There are reports that seals can tell differences in direction and shape, but how finely they can read it is not known.
- Can the wavy shape be put to use in engineering? Applications to sensors that measure underwater flow, and to supports that shake less, are being studied.
In other words, this article too is "an explanation of what is known so far". Values for whiskers and for how long footprints last vary with the study and the conditions.
Links to textbooks (by level)
| Level | Subject and unit | Where in this article |
|---|---|---|
| Junior high | Science: "Stimulus and response" | Nerves at the base of the whisker receive the water's movement as a stimulus |
| High school | Physics: "Waves and oscillations"; Biology: "Receptors" | Swirls peeling off alternately and shaking the rod; shakes per second |
| High school+ | Physics: "Resonance" | Shaking grows large as it nears the rod's natural frequency |
| University | Fluid dynamics, sensory physiology | Kármán vortex street, Strouhal number, suppressing vortex-induced vibration |
| Research | Biological fluid dynamics, bio-inspired sensors | Role in the wild, how fine the information in a footprint is |
| ― | Links to daily life | The sound of power lines on a windy day; the shudder when you move a stick through water |
- Dehnhardt, G., Mauck, B., Hanke, W., Bleckmann, H. (2001) Hydrodynamic trail-following in harbor seals (Phoca vitulina). Science 293, 102–104.
- Hanke, W., Witte, M., Miersch, L. et al. (2010) Harbor seal vibrissa morphology suppresses vortex-induced vibrations. Journal of Experimental Biology 213
- Wieskotten, S., Dehnhardt, G., Mauck, B. et al. (2010) Hydrodynamic determination of the moving direction of an artificial fish in harbour seals. Journal of Experimental Biology 213.
- Kármán vortex (Japanese Wikipedia)
※This article is a general-audience science explainer. The figures given are rough guides to help you understand the mechanisms. Figures on animal abilities come from reports based on captive experiments and vary by individual and conditions.