What Do a Cat's Whiskers Feel?
― The Hair Has No Nerves; the Base Reads the Movement of a Lever
The hair of a cat's whisker has not a single nerve running through it. The sensing happens at the base. The whisker is a long lever, and it carries even the faintest touch at its tip to the nerve-packed base as a big twist.
At night, in a room with the lights off, a cat slips through the gaps between furniture. There is no sign of a bump.
Look closely at a playing cat's face. The moment it pounces on a toy, its whiskers sweep forward, like a hand reaching out.
Touch the tip of a whisker gently with a finger, and the cat blinks at once or turns its face away. It is only a hair. Why is it so sensitive?
Just two reasons whiskers are so sensitive
A whisker is buried much deeper in the skin than an ordinary hair. Many nerves gather in the pocket around its root. The hair serves as a rod that passes movement down to the base.
A thin rod bends under a small force. The longer the rod, the bigger the "twist" a force at its tip makes at the base. A whisker is shaped to move its base even under the force of a speck of dust.
So a whisker is not a "hair that feels". It is a "lever that carries force to the base". Let's go through it step by step.
The hair itself has no nerves
Cutting your hair doesn't hurt, and the same goes for a cat's whisker: the hair part has no nerves. It has no living cells either. The hair is a hard rod of protein made by the skin.
The secret is at the base. A whisker is buried far deeper than an ordinary hair. A small pocket filled with blood surrounds the root, and nerve endings line it thickly. More than a hundred nerve fibres are said to connect to the base of a single whisker.
When something touches the tip, the hair bends around its base as a pivot. The base then tilts slightly inside the pocket and presses on the nerves around it. Look at Figure 1. The slider changes how far the tip is pushed down.
In Figure 1's starting state, the tip is pushed down 1 millimetre. The force on the tip is then estimated at about 1/3000 of the weight of a 1-yen coin, which is roughly 0.3 milligrams. Even so, the base is properly twisted, and the nerves pick it up.
The longer and thinner, the more sensitive
This is what it means to call a whisker a lever. The twist at the base equals the force on the tip multiplied by the whisker's length. Touch with the same force, and a whisker twice as long twists the base twice as much.
How easily it bends depends even more strongly on thickness. Double the thickness of a rod and it becomes 16 times harder to bend. A cat's whisker is thick at the base and tapers to a fine point. The thinnest tip therefore bends under the very slightest force.
Meanwhile, because the base is thick, the whole hair doesn't just flop or snap. The tip is sensitive, and the base passes force on firmly. The thin, tapered shape manages both.
What do cats use their whiskers for?
Cats are good in dim places, but in total darkness even their eyes don't work. Then the whiskers tell them where things are. The whiskers spread out beside the face are thought to help check how wide a gap is.
The surprise is what happens right in front of the face. Cats are said to have trouble focusing their eyes on things that are too close. Sweeping the whiskers forward as they pounce is thought to make up for that hard-to-see distance by feeling it.
Whiskers are also said to sense faint flows of air. Airflow changes near objects, so a cat may notice something is there before touching it.
Cats also have hairs of the same kind above the eyes, behind the cheeks and under the chin. A few more sit on the back of each front leg near the wrist, and are said to sense the movement of prey being held. On each side of the mouth there are usually 12 whiskers, about 24 in all.
Cutting a whisker doesn't hurt, because the hair has no nerves. But the length that works as a lever is lost, so the cat has a harder time sensing its surroundings. Whiskers fall out naturally and grow back, so there is no need to worry if you find one on the floor.
Summary
The hair of a cat's whisker has no nerves. The whisker is a long, thin lever that carries the faint force of a touch at its tip to the nerve-rich base as a twist. In the dark, and at close range where their eyes can't focus, cats check the world by touching it with their whiskers.
A whisker is not a hair that feels.
It is a long lever that carries faint forces to the base.
The mechanics of a cat's body also appear in how a cat twists in mid-air to land on its feet and why cats survive falls from high places. For another way of sensing in the dark, try how snakes find prey by heat. Underwater, seals use their whiskers to read the swirling footprints a fish leaves behind.
- Gently move just the tip of a fine hair on your arm with a fingertip, without touching the skin. The hair has no nerves, yet you can tell "something touched it" near the root.
- Let a 30 cm ruler stick out about 20 cm from the edge of a desk, and hold it down on the desk with your hand. Press the tip lightly with a finger and see how much it bends.
- Now shorten the part sticking out to 10 cm and push with about the same force. You will see that the longer one bends a lot under a much smaller force.
Please don't try this on a cat's whiskers. Cats often dislike having their whiskers touched.
For those who want more ― terms, formulas and links to textbooksEach part is labelled with its level, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics or biology
- High school+Advanced high-school material, or textbook sidebars
- UniversityUniversity-level courses not taught in high school (mechanics of materials, neuroscience)
- ResearchNot yet settled textbook knowledge; researchers are still studying it
Middle schoolTerms: this phenomenon has names
- Sinus hair (vibrissa) / tactile hair: a sensory hair whose base is wrapped in a blood-filled pocket and packed with nerves. A cat's whiskers are of this type.
- Lever / moment of force: how strongly a force tries to turn a rod. It is set by force × distance from the pivot.
- Cantilever beam: a rod fixed at one end only, with the other end free. A whisker, a diving board and a ruler sticking out from a desk all have this shape.
Middle schoolHigh schoolChecking with a formula: how much force bends the tip by 1 mm?
Here we make a rough estimate by treating the whisker as a uniform round rod with the thickness of its base. A real whisker thins toward the tip, so it should in fact bend under an even smaller force than this. Lengths are in millimetres and forces in newtons (N).
| In symbols | F = 3 × E × I × δ ÷ L³ (where I = π × d⁴ ÷ 64) |
| In words | Force on the tip = 3 × stiffness of the material × bending resistance of the cross-section × drop of the tip ÷ (length cubed) |
| Where it comes from | The bending formula for a cantilever beam. It follows from the condition that, at each point of the rod, the bending moment balances the force with which the material tries to spring back |
| Symbol | Meaning and unit |
| F | Force pushing the tip (N) |
| E | Young's modulus = stiffness of the material (N/mm²) |
| I | Second moment of area = bending resistance set by the cross-section shape (mm⁴) |
| δ | How far the tip drops (mm) |
| L, d | Length of the whisker and thickness at its base (mm) |
| Whisker length L | 60 mm (a typical long whisker beside the mouth) |
| Base thickness d | 0.2 mm (said to be about 0.2–0.3 mm) |
| Hair stiffness E | 3000 N/mm² (keratin, the main component of hair, is said to be a few thousand N/mm²) |
| Tip drop δ | 1 mm |
| Gravity on a 1-yen coin | 0.0098 N |
| Thickness squared | 0.2 × 0.2 = 0.04 |
| Thickness to the 4th power | 0.04 × 0.04 = 0.0016 |
| Multiply by π | 3.1416 × 0.0016 ≒ 0.005027 |
| Divide by 64 to get the cross-section's bending resistance I (mm⁴) | 0.005027 ÷ 64 ≒ 0.0000785 |
| Multiply by the material stiffness (E × I) | 3000 × 0.0000785 ≒ 0.2355 |
| Multiply by 3 (δ is 1 mm, so it stays as is) | 3 × 0.2355 ≒ 0.7065 |
| Length squared | 60 × 60 = 3600 |
| Length cubed | 3600 × 60 = 216000 |
| Force on the tip F (N) | 0.7065 ÷ 216000 ≒ 0.0000033 |
| What fraction of a 1-yen coin's weight | 0.0098 ÷ 0.0000033 ≒ 2970 |
| Moment twisting the base (N·mm) | 0.0000033 × 60 ≒ 0.000198 |
The force to push the tip of a whisker down 1 mm is about 0.0000033 newtons (3.3 micronewtons). That is about 1/3000 of the weight of a 1-yen coin, the weight of something around 0.3 milligrams sitting on it. Because the formula contains d⁴, doubling the thickness needs 16 times the force, and halving the length needs 8 times the force. A thin, long whisker is sensitive thanks to this shape.
High schoolHigh school+The base reads a "twist", not a "force"
High schoolIn high-school physics, the effect that tries to turn a rod is called the "moment of force", given by force × arm length. When a force F acts on the whisker tip, a moment of F × L acts on the base. This is why a longer whisker twists its base harder under the same force.
High school+Studies using rat whiskers suggest that the direction and size of the moment at the base may reveal the distance and direction of the object touched. The moment differs for the same force depending on whether the touch is near the tip or near the base.
UniversityEuler–Bernoulli beam theory and the nerves of the sinus hair
The formula above is the solution from the "Euler–Bernoulli beam theory" of mechanics of materials, for a cantilever beam with a point load at its tip. Resistance to bending is set by the product EI (flexural rigidity) of Young's modulus E and the second moment of area I. In a whisker that tapers toward the tip, I changes from place to place, so real studies compute it numerically as a tapered beam. The sinus hair's base contains a blood-filled sac called the blood sinus and several kinds of mechanoreceptors, such as Merkel cells. In rats, signals are known to reach the "barrel cortex" of the brain, with a separate region for each whisker.
📖 Derivation of the formula and further reading: Cantilever beam ― Wikipedia (Japanese) ／ Sinus hair ― Wikipedia (Japanese)
ResearchWhat is still not clear
- Is the whisker span the same as the body width? It is often said that "the span between the left and right whisker tips equals the body width, so the cat can measure whether it fits through a gap". But few studies have properly tested this, and the link to growth and weight changes is unclear.
- How does a cat's brain combine whisker and eye information? Whisker research centres on rats, and much about how the cat brain processes it is said to be unknown.
- How well can whiskers read airflow? It has been shown that whiskers sense wind, but how much it helps cats avoid objects in the dark is still being studied.
So the content of this article, too, is "an explanation of what is known so far". Note in particular that the detailed numbers for cats vary a lot between individuals.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Action of forces", "Stimulus and response" | Levers, sense organs and nerves |
| High school | Physics: "Balance of rigid bodies"; Biology: "Receptors" | Moment of force, cells that receive mechanical stimuli |
| High school+ | Advanced physics | Reading the touched location from the moment |
| University | Mechanics of materials, neuroscience | Beam theory, flexural rigidity, barrel cortex |
| Research | Animal behaviour, neuroscience of touch | Whisker span vs body width, information processing in cats |
| ― | Link to daily life | Not cutting a cat's whiskers, the bending of a ruler |
- Wikipedia (Japanese), "Sinus hair" (洞毛)
- Wikipedia (Japanese), "Second moment of area" (断面二次モーメント)
- Ahl, A. S. (1986). The role of vibrissae in behavior: a status review. Veterinary Research Communications, 10, 245–268.
- Prescott, T. J., Mitchinson, B., & Grant, R. A. (2011). Vibrissal behavior and function. Scholarpedia, 6(10), 6642.
- Quist, B. W., & Hartmann, M. J. Z. (2012). Mechanical signals at the base of a rat vibrissa: the effect of intrinsic vibrissa curvature and implications for tactile exploration. Journal of Neurophysiology, 107, 2298–2312.
* This article is a general-audience science explainer. The numbers given are rough estimates to help you understand how things work. Whisker length and thickness vary between individual cats. If you have any concern about your cat's health, please consult a veterinarian.