Whiskers or vibrissae (singular: vibrissa) are a type of mammalian hair that are typically characterised, anatomically, by their large size, large and well-innervated hair follicle and by having an identifiable representation in the somatosensory cortex of the brain.
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 and hamsters, 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. 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. 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. 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 ).
Marine mammals can have substantially different vibrissal arrangements. For instance, cetaceans have lost the vibrissae around the snout and gained vibrissae around their blowholes, whereas every single one of the body hairs of the Florida Manatee (see image) may be a vibrissa. 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 the groups may function quite differently.
The vibrissal hair is usually thicker and stiffer than other types of (pelagic) hair but, like other hairs, the shaft consists of an inert material (keratin) and contains no nerves. 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.
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). In the 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. 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. 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. Manatees, remarkably, have around 600 vibrissae 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 (see image). 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.
The follicles of some groups of vibrissae in some species are motile. Generally, the supraorbital, genal, and macrovibrissae are reported to be motile, whilst the microvibrissae are not (the image of the yawning cat, to the right, shows how the macrovibrissae can be swept forward). This is reflected in anatomical reports that have identified musculature associated with the macrovibrissae that is absent for the microvibrissae. 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 macrovibrissae together.
Amongst those species with motile macrovibrissae, some (rats, mice, flying squirrels, gerbils, chincillas, hamsters, shrews, porcupines, opossums) move them back and forth periodically in a movement known as 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 rapidly controlled in response to behavioural and environmental conditions. The whisking movements occur in bouts of variable duration, and at rates between 3 to 25 whisks/second. Movements of the whiskers are closely co-ordinated with those of the head and body.
Generally, vibrissae 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. Sensory function aside, movements of the vibrissae may also indicate something of the state of mind of the animal, and the whiskers play a role in social behaviour of rats.
The sensory function of vibrissae 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, orienting of the snout, detection of movement, texture discrimination, shape discrimination, exploration, thigmotaxis, locomotion, maintenance of equilibrium, maze learning, swimming, locating food pellets, locating food animals, and fighting, as well as nipple attachment and huddling in rat pups.
Whilst contact between the whiskers and solid objects is the most obvious stimulus to evoke a behavioural response, air and water currents are also effective. Indeed, some aquatic mammals probably make the most use of their whiskers in detecting water currents. In seals, this enables them to follow the varying path of an object that 'swam' ahead several minutes before, similar to a dog following a scent trail, and even to discriminate the species and/or size of the fish responsible for the trail.
Whisking—the periodic movement of the whiskers—is also presumed to serve 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 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."
Animals that do not whisk, but have motile whiskers, presumably also gain some advantage from the investment in musculature. Dorothy Souza, in her book "Look What Whiskers Can Do" reports some whisker movement during prey capture (in cats, in this case):
"Whiskers bend forward as the cat pounces. Teeth grasp the mouse tightly around its neck. The cat holds on until the prey stops wriggling."
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 whisker length is genetically determined and does not vary as the cat grows thinner or fatter. In the laboratory, rats are able to accurately (within 5-10%) 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 doesn't always make best use of it.
Lines of research
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. Neuroscientists, and other researchers, studying sensory systems favour the whisker system for a number of reasons (see Barrel cortex), not least the simple fact that laboratory rats and mice are whisker, rather than visual, specialists.
The presence of mystacial 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 previous members of the human lineage had mystacial 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). An upcoming article at Scholarpedia will discuss the history of whiskered Robots in detail.
In non-mammalian animals
A range of non-mammalian animals possess structures which resemble or function similarly to mammalian whiskers.
Some birds possess specialized hair-like feathers called rictal bristles around the base of the beak which are sometimes referred to as whiskers.
The Whiskered Auklet (Aethia pygmaea) has striking, stiff white feathers protruding from above and below the eyes of the otherwise slate-grey bird, and a dark plume which swoops forward from the top of its head. Whiskered auklets 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 use their feathers in a similar way to cats.
Other birds that have obvious "whiskers" are flycatchers, swallows, goatsuckers, nightjars, whip-poor-wills and the Long-whiskered Owlet (Xenoglaux loweryi).
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.
An otter with facial whiskers.
Macrovibrissae of a Hooded Lister laboratory rat.
Micrograph cross section of an equine vibrissa.
Macrovibrissae of a tiger.
Laboratory mouse (C57BL/6) showing macrovibrissae.
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