Whiskers

Vibrissae (from Latin '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. ==Evolution==

The last common ancestor of all extant mammals had vibrissae, and it has been suggested that the trait evolved in the non-mammalian cynodont ancestors of mammals belonging to the clade Prozostrodontia during the Triassic period based on the presence of a infraorbital canal similar to those of modern mammals in members of this group. 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. == Anatomy ==

Whiskers are anatomically different from other types of hair that mammals grow. In comparison to other types of mammalian hair, whiskers are stiffer, significantly larger in diameter, and are significantly longer than the surrounding fur. Whiskers have well-innervated follicles, and an identifiable representation in the somatosensory cortex of the brain. and hamsters, have four typical whisker groups on their heads (called cranial vibrissae), which might vary among animals due to different lifestyles. These cranial groups include: • above the eyes (supraorbital) • on the cheeks (genal) • where a moustache would be (mystacial) • under the snout (mandibular). The mystacial whiskers can be roughly identified as macrovibrissae (long whiskers for feeling the space around the head) and microvibrissae (small, down-pointing whiskers for identifying objects). Whilst 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. may be vibrissae. (Phoca vitulina) Marine mammals can have substantially different arrangements of their vibrissae. For instance, whales and dolphins have lost their snout whiskers and gained vibrissae around their blowholes, Other marine mammals, like seals and sea-lions, have head vibrissae just like those on land mammals (see image), although these groups function quite differently. Vibrissal follicles have evolved other functions in dolphins, such as electroreception. Vibrissae 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. Vibrissae are symmetrically arranged in groups on the face and supply the trigeminal nerve. 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). 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. Natural shapes of rat's mystacial pad vibrissae 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 vibrissal sensory system than rats and mice. 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. == Operation ==

Movement shows how the mystacial macrovibrissae can be swept forward. The follicles of whiskers that grow on some species of animals are motile, meaning that they are capable of motion. Tiny muscles attached to the base of each whisker allow mammals to move their whiskers and change their position. Generally, the supraorbital, genal and macrovibrissae are motile, 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, 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. 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 they occur at rates between 3 and 25 whisks per second. Movements of the whiskers are closely coordinated with those of the head and body. In addition to their sensory functions, the various movements of whiskers can also indicate an animal's state of mind. 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 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. is a matter of debate, and the answer is probably multi-faceted. Scholarpedia reports some whisker movement during prey capture (in cats, in this case): 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 does not always make best use of it. Marine mammals Pinnipeds (commonly known as seals) have well-developed tactile senses. Their mystacial 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. . The lower undulated whisker belongs to a harbor seal. Harbor 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 ringed seals have even been observed successfully hunting on their own in Lake Saimaa, 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. 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 vibrissa's angle relative to the flow, and not the fiber shape, however, seems to be the most important factor. == Lines of research ==

Neuroscience 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 in 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 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. Evolutionary biology 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. through feature-specific models, to attempts to reproduce complete whiskered animals in robot form (ScratchBot and ShrewBot, both robots by Bristol Robotics Laboratory). ==In non-mammals==

Although they are not technically whiskers, a range of non-mammals possess structures which resemble or function similarly to mammalian whiskers. In birds 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 kiwis, flycatchers, swallows, nightjars, whip-poor-wills, the kākāpō and the long-whiskered owlet (Xenoglaux loweryi). In fish 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 Pimelodidae are a family of catfishes (order Siluriformes) commonly known as the long-whiskered catfishes. In pterosaurs Anurognathid 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 anurognathid mouths. == Gallery ==

File:Will do tricks for fish...O).jpg|An otter with facial whiskers. File:Whiskers of the Hooded Lister Rat ATLAS-070713-0016.jpg|Macrovibrissae of a Lister hooded laboratory rat. File:Cats whiskers.jpg|A cat's prominent macrovibrissae. File:EquineVibrissaeCrossSection.JPG|Micrograph cross section of an equine vibrissa. File:Vibrissae of a Tiger at Chester Zoo.jpg|Macrovibrissae of a tiger. File:Black 6 mouse eating.jpg|Laboratory mouse (C57BL/6) showing macrovibrissae. File:HorseNose2.jpg|Prominent immotile vibrissae on a horse's muzzle. File:Closeup portrait of cat and its whiskers.jpg|Closeup picture of the vibrissae of a black and white house cat. File:Battlecat 2432.JPG|Supraorbital vibrissae and mystacial macrovibrissae of a house cat. File:Eastern Gray Squirrel, Louisiana, US imported from iNaturalist photo 411198365.jpg|Supraorbital and mystacial macrovibrissae of an eastern gray squirrel. File:Brown Thrasher.jpg|Whiskers of the brown thrasher near the head. == References ==