Whiskers | Gentle Bedtime Reading For Sleep

Relax with calm, bedtime reading to help you drift into a peaceful sleep and ease insomnia. In this episode we explore whiskers—special tactile hairs (vibrissae) many mammals have—and how they work to let animals feel their surroundings, hunt, and avoid danger. You’ll learn about the anatomy of whiskers, how they move, and why they’re different from regular fur. Benjamin’s voice will guide you softly as you discover facts without any whispering or hypnosis. Great for restless nights, curious minds, and finding calm. Press play and let yourself drift off. Happy sleeping!

Transcript

Welcome to the I Can't Sleep Podcast,

Where I help you drift off one fact at a time.

I'm your host Benjamin Boster,

And today's episode is about whiskers.

Thanks to Nick Saunders for sponsoring today's episode.

Whiskers,

Also known as vibrisi,

Are a type of stiff,

Functional hair used by most therian mammals to sense their environment.

These hairs are finely specialized for this purpose,

Whereas other types of hair are coarser as tactile sensors.

Although whiskers are specifically those found around the face,

Vibrisi are known to grow in clusters at various places around the body.

Most mammals have them,

Including all non-human primates,

Marsupials,

And especially nocturnal mammals.

Monotremes,

However,

Lack them.

Whiskers are sensitive,

Tactile hairs that aid navigation,

Locomotion,

Exploration,

Hunting,

Social touch,

And perform other functions.

This article is primarily about the specialized sensing hairs of mammals,

But some birds,

Fish,

Insects,

Crustaceans,

And other arthropods are known to have similar structures,

Also used to sense the environment.

Vibrisi,

From Latin vibrare,

To vibrate,

From the characteristic motion seen in a small rodent that is otherwise sitting still.

In medicine,

The term also refers to the thick hairs found inside human nostrils.

The last common ancestor of all extant mammals had vibrisi.

All other extant mammal species,

Besides great apes,

Retain the same ancestral layout of the whiskers,

Along with the special facial muscles that move them.

Vibrisi are anatomically distinguished from other hair.

They are easily visually identified since they are longer,

Stiffer,

Significantly larger in diameter,

And stand above the surrounding fur by a considerable amount.

In addition,

They have well-intervented follicles,

And an identifiable representation in the somatosensory cortex of the brain.

The largest number,

And the longest,

Are found among the small,

Social,

Arboreal,

And nocturnal mammals.

Whiskers of aquatic mammals are the most sensitive.

During foraging in complex,

Dark habitats,

Whiskers are rapidly moved in a cyclic way,

Tracing small circles at their tips.

This motion,

Called whisking,

Can occur at speeds of 25 Hz in mice,

Which is one of the fastest movements that mammals can make.

Small animals use whisking to position their front paws during locomotion.

Vibrisi typically grow in clusters.

These groups vary somewhat in form and function,

But they are relatively consistent among land mammals.

Between land and marine mammals,

There is less consistency,

Though commonalities are certainly present.

Many land mammals,

Like rats and hamsters,

Have four typical whisker groups on their heads,

Called a craniovibrisi,

Which might vary among animals due to different lifestyles.

These cranial groups include above the eyes,

Superorbital,

On the cheeks,

Geno,

Where the mustache would be,

Mustachial,

Under the snout,

Mandibular.

The mustachial whiskers can be roughly identified as macrovibrisi,

Long whiskers for feeling the space around the head,

And microvibrisi,

Small down-pointing whiskers for identifying objects.

Not only are these two types hard to distinguish on an animal's face,

There are similarly weak distinctions on how they are used.

Though the distinction is nonetheless referred to ubiquitously in scientific literature,

And is considered useful in analysis.

Many land mammals,

Including domestic cats,

Also have vibrisi on the underside of the leg,

Just above the paws,

Called carpovibrisi.

While these five major groups are often reported in studies of land mammals,

Several other groups have been reported more occasionally,

For instance,

Nasal,

Angular,

And submental whiskers.

Marine mammals can have substantially different arrangements of their vibrisi.

For instance,

Whales and dolphins have lost their snout whiskers and gained vibrisi around their blowholes,

Whereas every single one of the body hairs of the Florida manatee may be avibrisa.

Other marine mammals,

Like seals and sea lions,

Have head vibrisi just like those on land mammals,

Although these groups function quite differently.

Vibrisal follicles have evolved other functions in dolphins,

Such as electroreception.

The vibrisal hair is usually sicker and stiffer than other types of pelagic hair,

But like other hairs,

The shaft consists of an inner material,

Keratin,

And contains no nerves.

However,

Vibrisi are different from other hair structures because they grow from a special hair follicle incorporating a capsule of blood,

Called a blood sinus,

Which is heavily interwoven by sensory nerves.

Vibrisi are symmetrically arranged in groups on the face and supply the trigeminal nerve.

The mustachial macrovibrisi are shared by a large group of land and marine mammals,

And it is this group that has received by far the most scientific study.

The arrangement of these whiskers is not random.

They form an ordered grid of arcs,

Columns,

And rows,

With shorter whiskers at the front and longer whiskers at the rear.

In the mouse,

Gerbil,

Hamster,

Rat,

Guinea pig,

Rabbit,

And cat,

Each individual follicle is innervated by 100 to 200 primary afferent nerve cells.

These cells serve an even larger number of mechanoreceptors of at least 8 distinct types.

Accordingly,

Even small deflections of the vibrisal hair can evoke a sensory response in the animal.

Rats and mice typically have approximately 30 microvibrisi on each side of the face,

With whisker lengths up to around 50 mm in laboratory rats,

30 mm in laboratory mice,

And a slightly larger number of microvibrisi.

Thus,

An estimate for the total number of sensory nerve cells serving the mustachial vibrisal array on the face of a rat or mouse might be 25,

000.

Natural shapes of rats' mustachial pad vibrisi are well approximated by pieces of the Euler spiral.

When all these pieces for a single rat are assembled together,

They span an interval extending from one coiled domain of the Euler spiral to the other.

Marine mammals may make even greater investment in their vibrisal sensory system than rats and mice.

Seal whiskers,

Which are similarly arrayed across the mustachial region,

Are each served by around 10 times as many nerve fibers as those in rats and mice,

So that the total number of nerve cells innervating the mustachial vibrisi of a seal has been estimated to be in excess of 300,

000.

Manatees,

Remarkably,

Have around 600 vibrisi on or around their lips.

Whiskers can be very long in some species.

The length of a chinchilla's whiskers can be more than a third of its body length.

Even in species with shorter whiskers,

They can be very prominent appendages.

Thus,

Whilst whiskers certainly could be described as proximal sensors,

In contrast to,

Say,

Eyes,

They offer a tactile sense with a sensing range that is functionally very significant.

The follicles of some groups of vibrisi in some species are motile.

Generally,

The supraorbital,

Genal,

And macrovibrisi are motile,

Whereas the microvibrisi are not.

This is reflected in anatomical reports that have identified musculature associated with the macrovibrisi that is absent for the microvibrisi.

A small muscle sling is attached to each microvibrisa and can move it more or less independently of the others,

Whilst larger muscles in the surrounding tissue move many or all of the macrovibrisi together.

Amongst those species with motile macrovibrisi,

Some – rats,

Mice,

Flying squirrels,

Gerbils,

Chinchillas,

Hamsters,

Shrews,

Porcupines,

Opossums – move them back and forth periodically in a movement known as whisking,

While other species – cats,

Dogs,

Raccoons,

Pandas – do not appear to.

The distribution of mechanoreceptor types in the whisk or follicle differs between rats and cats,

Which may correspond to this difference in the way they are used.

Whisking movements are amongst the fastest produced by mammals,

And all whisking animals in which it has so far been measured.

These whisking movements are rapidly controlled in response to behavioral and environmental conditions.

The whisking movements occur in bouts of variable duration and at rates between 3 and 25 whisks per second.

Movements of the whiskers are closely coordinated with those of the head and body.

Generally,

Vibrisi are considered to mediate a tactile sense,

Complementary to that of skin.

This is presumed to be advantageous in particular to animals that cannot always rely on sight to navigate or to find food,

For example,

Nocturnal animals or animals which forage in muddy waters.

Whiskers can also function as wind-detecting antennae,

Such as the supraorbital ones in rats.

Sensory function aside,

Movements of the vibrisi may also indicate something of the state of mind of the animal,

And the whiskers play a role in social behavior of rats.

The sensory function of vibrisi is an active research area.

Experiments to establish the capabilities of whiskers use a variety of techniques,

Including temporary deprivation either of the whisker sense or of other senses.

Animals can be deprived of their whisker sense for a period of weeks by whisker trimming they soon grow back,

Or for the duration of an experimental trial by restraining the whiskers with a flexible cover,

Like a mask.

The latter technique is used in particular in studies of marine mammals.

Such experiments have shown that whiskers are required for or contribute to object localization,

Orientation of the snout,

Detection of movement,

Texture discrimination,

Shape discrimination,

Exploration,

Sigmataxis,

Locomotion,

Maintenance of equilibrium,

Maze learning,

Swimming,

Locating food pellets,

Locating food animals,

And fighting.

Whisking,

The periodic movement of the whiskers,

Is also presumed to serve a tactile sensing in some way.

However,

Exactly why an animal might be driven to beat the night with sticks,

As one researcher once put it,

Is a matter of debate,

And the answer is probably multifaceted.

Scholarpedia offers,

Since rapid movement of the vibrisi consumes energy and has required the evolution of specialized musculature,

It can be assumed that whisking must convey some sensory advantages to the animal.

Likely benefits are that it provides more degrees of freedom for sensor positioning,

That it allows the animal to sample a large volume of space with a given density of whiskers,

And that it allows control over the velocity with which the whiskers contact surfaces.

Animals that do not whisk but have motile whiskers presumably also gain some advantage from the investment in musculature.

Anecdotally,

It is often stated that cats use their whiskers to gauge whether an opening is wide enough for their body to pass through.

This is sometimes supported by the statement that the whiskers of individual cats extend out to about the same width as the cat's body.

But at least two informal reports indicate that whiskers' length is genetically determined and does not vary as the cat grows thinner or fatter.

In the laboratory,

Rats are able to accurately discriminate the size of an opening,

So it seems likely that cats can use their whiskers for this purpose.

However,

Reports of cats,

Particularly kittens,

With their heads firmly stuck in some discarded receptacle are commonplace,

Indicating that if a cat has this information available,

It does not always make best use of it.

Pinnipeds have well-developed tactile senses.

Their mustachial vibrissae have ten times the innervation of terrestrial mammals,

Allowing them to effectively detect vibrations in the water.

These vibrations are generated,

For example,

When a fish swims through water.

Detecting vibrations is useful when the animals are foraging and may add to or even replace vision,

Particularly in darkness.

Barber seals have been observed following varying paths of other organisms that swam ahead several minutes before,

Similar to a dog following a scent trail.

And even to discriminate the species and the size of the fish responsible for the trail.

Blind seals have even been observed successfully hunting on their own in Lake Saima,

Likely relying on their vibrissae to gain sensory information and catch prey.

Unlike terrestrial mammals,

Such as rodents,

Pinnipeds do not move their vibrissae over an object when examining it,

But instead extend their moveable whiskers and keep them in the same position.

By holding their vibrissae steady,

Pinnipeds are able to maximize their detection ability.

The vibrissae of seals are undulated and wavy,

While sea lion and walrus vibrissae are smooth.

Research is ongoing to determine the function,

If any,

Of these shapes on detection ability.

The vibrissae's angle relative to the flow,

And not the fiber's shape,

However,

Seems to be the most important factor.

Most cetaceans have whiskers at birth,

But they are typically lost during maturation.

The follicles in any vestigial hair sometimes function as touch or electrical sense organs.

A large part of the brain of whisker specialist mammals is involved in the processing of nerve impulses from vibrissae,

A fact that presumably corresponds to the important position the sense occupies for the animal.

Information from the vibrissae arrives in the brain via the trigeminal nerve and is delivered first into the trigeminal sensory complex of brainstem.

From there,

The most studied pathways are those leading up through parts of thalamus and into barrel cortex,

Though other major pathways through the superior colliculus and midbrain,

A major visual structure in visual animals,

And the cerebellum,

To name but a couple,

Are increasingly coming under scrutiny.

Neuroscientists and other researchers studying sensory systems favor the whisker system for a number of reasons,

Not least the simple fact that laboratory rats and mice are whisker rather than visual specialists.

The presence of mastacial vibrissae in distinct lineages with remarkable conservation of operation suggests that they may be an old feature present in a common ancestor of all Therian mammals.

Indeed,

Some humans even still develop vestigial vibrissal muscles in the upper lip,

Consistent with the hypothesis that previous members of the human lineage had mastacial vibrissae.

Thus,

It is possible that the development of the whisker sensory system played an important role in mammalian development more generally.

Researchers have begun to build artificial whiskers of a variety of types,

Both to help them understand how biological whiskers work and as a tactile sense for robots.

These efforts range from the abstract through feature-specific models to attempts to reproduce complete whiskered animals in robot form,

Scratchbot and Shrewbot,

Both robots by Bristol Robotics Laboratory.

A range of non-mammals possess structures which resemble or function similarly to mammalian whiskers.

Some birds possess specialized hairlike feathers called rectal bristles around the base of the beak which are sometimes referred to as whiskers.

The whiskered oclet has striking,

Stiff,

White feathers protruding from above and below the eyes of the otherwise slate-gray bird and a dark plume which swoops forward from the top of its head.

Whiskered oclets sent through a maze of tunnels with their feathers taped back bumped their heads more than twice as often as they did when their feathers were free,

Indicating they used their feathers in a similar way to cats.

Other birds that have obvious whiskers are kiwis,

Flycatchers,

Swallows,

Nightjars,

Whip-poor-wills,

The kakapo,

And the long-whiskered owlet.

Some fish have slender,

Pendulous,

Tactile organs near the mouth.

These are often referred to as whiskers although they are more correctly termed barbels.

Fish that have barbels include the catfish,

Carp,

Goatfish,

Hagfish,

Sturgeon,

Zebrafish,

And some species of shark.

The pymelodidae are a family of catfish commonly known as the long-whiskered catfishes.

Anurognosid pterosaurs had a rugose,

Wrinkled jaw texture that has been interpreted as the attachment sites for vibrissae,

Though actual vibrissae have not been recorded.

More recently,

A specific type of feathers has been found around anurognosid mouths.

A sense is a biological system used by an organism for sensation,

The process of gathering information about the surroundings through the detection of stimuli.

Although in some cultures five human senses were traditionally identified as such,

Namely sight,

Smell,

Touch,

Taste,

And hearing,

Many more are now recognized.

Senses used by non-human organisms are even greater in variety and number.

During sensation,

Sense organs collect various stimuli,

Such as the sound or smell,

For transduction,

Meaning transformation into a form that can be understood by the brain.

Sensation and perception are fundamental to nearly every aspect of an organism's cognition,

Behavior,

And thought.

In organisms,

A sensory organ consists of a group of interrelated sensory cells that respond to a specific type of physical stimulus.

Via cranial and spinal nerves,

Nerves of the central and peripheral nervous systems that relay sensory information to and from the brain and body,

The different types of sensory receptor cells and sensory organs transduct sensory information from these organs towards the central nervous system,

Finally arriving at the sensory cortices in the brain,

Where sensory signals are processed and interpreted or perceived.

Sensory systems,

Or senses,

Are often divided into external,

Extraception,

And internal,

Interoception sensory systems.

Human external senses are based on the sensory organs of the eyes,

Ears,

Skin,

Nose,

And mouth.

Internal sensation detects stimuli from internal organs and tissues.

Internal senses possessed by humans include spatial orientation,

Proprioception,

Body position,

Both perceived by the vestibular system located inside the ears,

And nociception,

Pain.

Further internal senses lead to signals such as hunger,

Thirst,

Suffocation,

And nausea,

Or different involuntary behaviors,

Such as vomiting.

Some animals are able to detect electrical and magnetic fields,

Air moisture,

Or polarized light,

While others sense and perceive through alternative systems,

Such as echolocation.

Sensory modalities,

Or submodalities,

Are different ways sensory information is encoded or transduced.

Multimodality integrates different senses into one unified perceptual experience.

For example,

Information from one sense has the potential to influence how information from another is perceived.

Sensation and perception are studied by a variety of related fields,

Most notably psychophysics,

Neurobiology,

Cognitive psychology,

And cognitive science.

Non-human animals experience sensation and perception with varying levels of similarity to and difference from humans and other animal species.

For example,

Other mammals in general have a stronger sense of smell than humans.

Some animal species lack one or more human sensory system analogues,

And some have sensory systems that are not found in humans,

While others process and interpret the same sensory information in very different ways.

For example,

Some animals are able to detect electrical fields and magnetic fields,

Air moisture,

Or polarized light.

Others sense and perceive through alternative systems,

Such as echolocation.

Recent information theory suggests that plants and artificial agents,

Such as robots,

May be able to detect and interpret environmental information in an analogous manner to animals.

Sensory modality refers to the way that information is encoded,

Which is similar to the idea of transduction.

The main sensory modalities can be described on the basis of how each is transduced.

Listing all the different sensory modalities,

Which can number as many as 17,

Involves separating the major senses into more specific categories or submodalities of the larger sense.

An individual's sensory modality represents the sensation of a specific type of stimulus.

For example,

The general sensation and perception of touch,

Which is known as somatosensation,

Can be separated into light pressure,

Deep pressure,

Vibration,

Itch,

Pain,

Temperature,

Or hair movement,

While the general sensation and perception of taste can be separated into submodalities of sweet,

Salty,

Sour,

Bitter,

Spicy,

And umami,

All of which are based on different chemical binding to sensory neurons.

Sensory receptors are the cells or structures that detect sensations.

Stimuli in the environment activate specialized receptor cells in the peripheral nervous system.

During transduction,

The physical stimulus is converted into action potential by receptors and transmitted towards the central nervous system for processing.

Different types of stimuli are sensed by different types of receptor cells.

Receptor cells can be classified into types on the basis of three different criteria,

Cell type,

Position,

And function.

Receptors can be classified structurally on the basis of cell type and their position in relation to stimuli they sense.

Receptors can further be classified functionally on the basis of the transduction of stimuli,

Or how the mechanical stimulus,

Light,

Or chemical change the cell membrane potential.

One way to classify receptors is based on their location relative to the stimuli.

An exoreceptor is a receptor that is located near a stimulus of the external environment,

Such as the somatosensory receptors that are located in the skin.

An interreceptor is one that interprets stimuli from internal organs and tissues,

Such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus.

The cells that interpret information about the environment can be either 1.

A neuron that has a free nerve ending with dendrites embedded in tissue that would receive a sensation.

2.

A neuron that has an encapsulated ending in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity.

Or 3.

A specialized receptor cell which has distinct structural components that interpret a specific type of stimulus.

The pain and temperature receptors in the dermis of the skin are examples of neurons that have free nerve endings.

Also located in the dermis of the skin are laminated corpuscles,

Neurons with encapsulated nerve endings that respond to pressure and touch.

The cells in the retina that respond to light stimuli are an example of a specialized receptor,

A photoreceptor.

A transmembrane protein receptor is a protein in the cell membrane that mediates a physiological change in a neuron.

Most often through the opening of ion channels or changes in the cell signaling processes.

Transmembrane receptors are activated by chemicals called ligands.

For example,

A molecule in a food can serve as a ligand for taste receptors.

Other transmembrane proteins which are not accurately called receptors are sensitive to mechanical or thermal changes.

Physical changes in these proteins increase ion flow across the membrane and can generate an action potential or a graded potential in the sensory neurons.

A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes.

Stimuli are of three general types.

Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane.

Some stimuli are physical variations in the environment that affect receptor cell membrane potentials.

Other stimuli include the electromagnetic radiation from visible light.

For humans,

The only electromagnetic energy that is perceived by our eyes is visible light.

Some other organisms have receptors that humans lack,

Such as the heat sensors of snakes,

The ultraviolet light sensors of bees,

Or magnetic receptors in migratory birds.

Receptor cells can be further categorized on the basis of the type of stimuli they transduce.

The different types of functional receptor cell types are mechanoreceptors,

Photoreceptors,

Chemoreceptors,

Thermoreceptors,

Electroreceptors in certain mammals and fish,

And nociceptors.

Physical stimuli,

Such as pressure and vibration,

As well as the sensation of sound and body position,

Balance,

Are interpreted through a mechanoreceptor.

Photoreceptors convert light,

Visible electromagnetic radiation,

Into signals.

Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli,

Such as an object's state or smell,

While osmoreceptors respond to a chemical solute concentrations of body fluids.

Nociception,

Pain,

Interprets the presence of tissue damage from sensory information from mechano,

Chemo,

And thermoreceptors.

Another physical stimulus that has its own type of receptor is temperature,

Which is sensed through a thermoreceptor that is either sensitive to temperatures above heat,

Or below cold,

Normal body temperature.

Reviews

I love words. Thank you for gifting me "vibrissae".

Whiskers! Whooda thought there’d be so much to talk about!? Thanks, Ben. It did the trick! (I had listened to this quite a while ago but evidently didn’t leave a review.)

Thank you, Benjamin. Somewhat interesting topic since I’m obsessed with my cat’s whiskers! He has the most adorable whiskers, they are SO long. I’ve noticed that tuxedo cats have the most amazing whiskers. 😻😻😻

I was ALMOST asleep until I heard the bit about kittens getting their heads stuck in containers, "so if they do have access to this information, they don't always make good use of it."

I started laughing out loud, waking myself🙋‍♀️ my bed partner 🛌 and the cat 😽 🐈‍⬛️ 🤣