Whiskers

A Patagonian fox showing four major cranial groups of vibrissae: supraorbital (above the eye), mystacial (where a moustache would be), genal (on the cheek, far left), and mandibular (pointing down, under the snout).
A pet rat clearly showing the grid-like arrangement of the macrovibrissae on the face, and the microvibrissae under the nostrils. The supraorbital vibrissae above the right eye are also visible.
All the hairs of the manatee may be vibrissae.
Macrovibrissae and supraorbital vibrissae of Phoca vitulina.
A chinchilla with large macrovibrissae.

Whiskers or vibrissae (i/; singular: vibrissa; ə/) are a type of mammalian hair that are typically characterised, anatomically, by their long length, large and well-innervated hair follicle, and by having an identifiable representation in the somatosensory cortex of the brain.[1]

They are specialised for tactile sensing (other types of hair operate as more crude tactile sensors). Vibrissae grow in various places on most mammals, including all primates except humans.[2]

In medicine, the term vibrissae also refers to the thick hairs found inside human nostrils.[3]

Anatomy

Vibrissal groups

Vibrissae (derived from the Latin "vibrio" meaning to vibrate) typically grow in groups in different locations on an animal. These groups are relatively well conserved across land mammals, and somewhat less well conserved between land and marine mammals (though commonalities are certainly present). Species-specific differences are also found. Vibrissae of different groups may vary in their anatomical parameters and in their operation, and it is generally assumed that they serve different purposes in accordance with their different locations on the body.

Many land mammals, for example rats[4] and hamsters,[5] have an arrangement of cranial (of the skull) vibrissae that includes the supraorbital (above the eyes), genal (of the cheeks), and mystacial (where a moustache would be) vibrissae, as well as mandibular (of the jaw) vibrissae under the snout.[6] These groups, all of which are visible in the accompanying image of the Patagonian fox, are well conserved across land mammals though anatomical and functional details vary with the animal's lifestyle.

Mystacial vibrissae are generally described as being further divided into two sub-groups: the large macrovibrissae that protrude to the sides and the small microvibrissae below the nostrils that mostly point downwards.[7] Most simply described, macrovibrissae are large, motile and used for spatial sensing, whereas microvibrissae are small, immotile and used for object identification. These two sub-groups can be identified in the accompanying image of the rat, but it can also be seen that there is no clear physical boundary between them. This difficulty in delineating the sub-groups visually is reflected by similarly weak boundaries between them in anatomical and functional parameters, though the distinction is nonetheless referred to ubiquitously in scientific literature and is considered useful in analysis.

Apart from cranial vibrissae, other groups are found elsewhere on the body. Many land mammals, including domestic cats, also have carpal (of the wrist) vibrissae on the underside of the leg just above the paws.[8] Whilst these five major groups (supraorbital, genal, mystacial, mandibular, carpal) are often reported in studies of land mammals, several other groups have been reported more occasionally (for instance, see [9]).

Marine mammals can have substantially different vibrissal arrangements. For instance, cetaceans have lost the vibrissae around the snout and gained vibrissae around their blowholes,[10] whereas every single one of the body hairs of the Florida manatee (see image) may be a vibrissa.[11] Other marine mammals (such as seals and sea-lions) have cranial vibrissal groups that appear to correspond closely to those described for land mammals (see the accompanying image of a seal), although these groups function quite differently.

Vibrissae

The vibrissal hair is usually thicker and stiffer than other types of (pelagic) hair[12] but, like other hairs, the shaft consists of an inert material (keratin) and contains no nerves.[12] However, vibrissae 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 innervated by sensory nerves.[13][14]

The mystacial macrovibrissae are shared by a large group of land and marine mammals (see images), 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 (see images).[7] In the mouse, gerbil, hamster, rat, guinea pig, rabbit, and cat, each individual follicle is innervated by 100–200 primary afferent nerve cells.[13] These cells serve an even larger number of mechanoreceptors of at least eight distinct types.[14] Accordingly, even small deflections of the vibrissal hair can evoke a sensory response in the animal.[15] Rats and mice typically have approximately 30 macrovibrissae 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 microvibrissae.[7] Thus, an estimate for the total number of sensory nerve cells serving the mystacial vibrissal array on the face of a rat or mouse might be 25,000.

Rats and mice are considered to be "whisker specialists", but marine mammals may make even greater investment in their vibrissal sensory system. Seal whiskers, which are similarly arrayed across the mystacial region, are each served by around 10 times as many nerve fibres as those in rats and mice, so that the total number of nerve cells innervating the mystacial vibrissae of a seal has been estimated to be in excess of 300,000.[16] Manatees, remarkably, have around 600 vibrissae on or around their lips.[10]

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

Other Languages
беларуская: Вібрысы
bosanski: Vibrise
català: Vibrissa
dansk: Knurhår
Deutsch: Vibrisse
español: Vibrisas
فارسی: شارب
français: Vibrisse
galego: Vibrisas
한국어: 동모
italiano: Vibrissa
ქართული: ვიბრისი
Latina: Vibrissae
Lëtzebuergesch: Vibriss
Limburgs: Snorhaor
მარგალური: ვიბრისი
Nederlands: Snorhaar
日本語: 洞毛
norsk: Værhår
português: Vibrissa
русский: Вибриссы
Simple English: Whisker
suomi: Tuntokarva
svenska: Morrhår
українська: Вібриси
中文: 感覺毛