Whiskers

"Whisker" redirects here. For other uses, see Whisker (disambiguation).

A Patagonian fox. Supraorbital vibrissae (large whiskers above the eye), macrovibrissae (large whiskers on the side of the snout) and microvibrissae (small whiskers under the snout) are visible.

Macrovibrissae of a Hooded Lister Laboratory rat are very prominent appendages, and generally the first part of the rat to come into contact with whatever it encounters.

An Otter in Eagle Heights Wildlife Park, proudly displaying its impressive facial whiskers.

A Manatee showing its hundreds of short facial whiskers.

A Chinchilla with large macrovibrissae.

Vibrissae (singular: vibrissa), or whiskers, are specialized hairs (or, in certain bird species, specialized feathers) usually employed for tactile sensation. The term may also refer to the thick hairs found inside human nostrils, but these have no sensorial function and only operate as an airborne particulate barrier. Vibrissae hair grow around the nostrils, above the lips, and on other parts of the face of most mammals, and all primates except man, as well as on the forelegs and feet of some animals. The presence of mystacial (where a moustache would be) vibrissae in distinct lineages (Rodentia, Afrotheria, Marsupials) 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 our lineage had mystacial vibrissae in the past.

Anatomy

Vibrissae are usually thicker and stiffer than other types of hair but consist of inert material and contain no nerves, like other hairs. However, vibrissae are different from other hairs because they are implanted in a special Hair follicle incorporating a capsule of blood called a blood sinus and heavily innervated by sensory nerves.

A wide range of species have a similar arrangement of mystacial vibrissae (see images). The arrangement of 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 (see images). In mouse, gerbil, hamster, rat, guinea pig, rabbit, and cat, each individual follicle is innervated by 100–200 primary afferent nerve cells. These cells serve an even larger number of Mechanoreceptors of at least eight distinct types. Accordingly, even small deflections of the vibrissal hair can evoke a sensory response in the animal. Seal whiskers, which are similarly arrayed across the mystacial region, are served by as many as 1,500 nerve cells each.

Rats and mice typically sport around 30 whiskers on each side of the face, with whisker lengths up to around 50 mm in (laboratory) rats and 30 mm in (laboratory) mice. Thus, a rough estimate for the total number of sensory nerve cells serving the vibrissal array on the face of a rat or mouse might be over 9000. Manatees, remarkably, have around 600 vibrissae on or around their lips - indeed, it seems that all of their hairs, all over their body, are vibrissae rather than Fur (pelagic hairs).

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 (see image). Even in species with shorter whiskers, they can be very prominent appendages (see images).

Whisker movement

In some mammals, some vibrissa follicles are motile. Typically, these are the large vibrissae (macrovibrissae) towards the rear of the mystacial area, whilst the supraorbital (above the eye) vibrissae and the much shorter vibrissae arrayed around the mouth or on the lips (microvibrissae) are immotile (see the whiskers held to the side, and out in front, in the two images of cats, below). A small muscle 'sling' is attached to each macrovibrissa and can move it more-or-less independently of the others, whilst larger muscles in the surrounding tissue move many or all of the whiskers together.

Dorothy Souza, in her book "Look What Whiskers Can Do" has:

"Whiskers bend forward as the cat pounces. Teeth grasp the mouse tightly around its neck. The cat holds on until the prey stops wriggling."

Amongst those species with motile whiskers, some (rats, mice, flying squirrels, gerbils, chincillas, hamsters, shrews, porcupines, opossums) palpate their vibrissae, a movement known as 'whisking' (Video of rat whisking), while other species (cats, dogs, racoons, pandas) do not appear to. The distribution of mechanoreceptor types in the whisker 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. In all whisking animals in which it has so far been measured, these whisking movements are precisely and rapidly controlled in response to behavioural and environmental conditions.

The purpose of whisking is presumed to be to serve tactile sensing in some way, though the exact answer to the question 'Why whisk?' is a matter of debate, and is probably multi-faceted. Scholarpedia offers:

"Since rapid movement of the vibrissae consumes energy, and has required the evolution of specialised 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 larger volume of space with a given density of whiskers, and that it allows control over the velocity with which the whiskers contact surfaces."

Function

Why might an animal be driven "to beat the night with sticks", as one researcher once put it? Generally, vibrissae are considered to mediate a tactile sense, complementary to the tactile sense offered by skin. This is advantageous in particular to animals that cannot always rely on sight to navigate or to find food (for example, nocturnal animals).

Vibrissae have been shown to be required for, or to contribute to: object localization, orienting of the snout, detection of movement, texture discrimination, shape discrimination. Deprived of their vibrissal sense, rats and shrews have shown deficits in exploration, thigmotaxis, locomotion, maintenance of equilibrium, maze learning, swimming, locating food pellets, and fighting. The function of vibrissae is a very active research area. Sensing function aside, movements of the vibrissae may indicate something of the state of mind of the animal, and the whiskers play a role in social behaviour in rats.

Whilst contact with the whiskers is the most obvious stimulus to evoke a behavioural response, air currents are also effective. Indeed, some aquatic mammals (such as seals) probably make the most use of their whiskers in detecting water currents, which enables them to follow the path of an object that 'swam' ahead several minutes past and even to discriminate the species and/or size of the fish responsible for the trail.

Anecdotally, it is often stated that cats use their whiskers to gauge whether an opening is wide enough for their body to pass through (e.g. Why Do Cats Have Whiskers?, Cat Behaviour Explained). 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 one report indicates that whisker length is genetically determined and does not vary as the cat grows thinner or fatter. Certainly, rats have been shown in the laboratory to be able to accurately (within 5-10%) discriminate the size of an opening, so it seems likely that cats can use their whiskers for this task. Mother cats often bite off their kittens' whiskers, lending further support to this theory, the suggested reason being that young kittens would be less likely to roam and explore if their tactile senses were blunted in this way^{[citation needed]}. However, reports of cats (particularly, kittens) with their heads firmly stuck in some discarded receptacle are commonplace (e.g. Cops save kitten with head stuck in can) indicating that, if a cat has this information available, it doesn't always make best use of it.

Neuroanatomy

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 Superior colliculus in midbrain (a major visual structure in visual animals) and Cerebellum, to name but a couple, are increasingly coming under scrutiny. Aside from its use as a model sensory system, neuroscientists studying many aspects of brain function favour the system for a number of reasons (see Barrel cortex).

Artificial whiskers

Recently, researchers have begun to build artificial whiskers of a variety of types, both to help them to understand how biological whiskers work and as a possibly useful tactile sense for robots. These efforts range from the fairly abstract, through feature-specific models (Sculpted Face, AMouse), to attempts to reproduce complete whiskered animals in robot form (ScratchBot [1] and ShrewBot [2] [3] [4], both robots by Bristol Robotics Laboratory). An upcoming article at Scholarpedia will discuss the history of Whiskered Robots in detail.

Additional images

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  A house cat's prominent vibrissae are known as whiskers (these are the macrovibrissae).

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  Micrograph cross section of an equine vibrissa taken at Loughborough University.

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  Macrovibrissae and supraorbital vibrissae of Phoca vitulina.

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  Macrovibrissae of a tiger (Chester Zoo).

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  Lab mouse (C57BL/6) showing macrovibrissae on either side of the snout.

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  A yawning cat shows how the macrovibrissae can be swept forward.

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  Prominent non-actuated vibrissae at the end of a horse's muzzle.

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  Albertina, a pet rat, clearly showing the grid-like arrangement of the macrovibrissae on the face.

Further reading

• A night in the life of a rat, Annals of the New York Academy of Sciences, 1225:110–118, April 2011.