Whiskers

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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). Pseudalopex culpaeus.jpg
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. Rat whiskers 2.jpg
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. Hpim0279.jpg
All the hairs of the manatee may be vibrissae.
Macrovibrissae and supraorbital vibrissae of Phoca vitulina. Seehund.jpg
Macrovibrissae and supraorbital vibrissae of Phoca vitulina .
A chinchilla with large macrovibrissae. Chinchilla-Patchouli.jpg
A chinchilla with large macrovibrissae.

Whiskers or vibrissae ( /vˈbrɪsi/ ; singular: vibrissa; /vˈbrɪsə/ ) 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]

Contents

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.

Operation

Movement

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

The follicles of some groups of vibrissae in some species are motile. Generally, the supraorbital, genal and macrovibrissae are motile, [5] whereas the microvibrissae are not. This is reflected in anatomical reports that have identified musculature associated with the macrovibrissae that is absent for the microvibrissae. [18] 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. [18] [19]

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, [20] while other species (cats, dogs, racoons, pandas) do not appear to. [1] 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. [14] Whisking movements are amongst the fastest produced by mammals. [21] 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. [1] The whisking movements occur in bouts of variable duration, and at rates between 3 and 25 whisks/second. Movements of the whiskers are closely co-ordinated with those of the head and body. [1]

Function

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, [22] and the whiskers play a role in social behaviour of rats. [23]

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 [24] ). Such experiments have shown that whiskers are required for, or contribute to: object localization, [25] [26] 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. [1]

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, [27] is a matter of debate, and the answer is probably multi-faceted. Scholarpedia [1] 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 [28] 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. [29] [30] 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. [22] [31] In the laboratory, rats are able to accurately (within 5-10%) discriminate the size of an opening, [32] 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 [33] indicating that if a cat has this information available, it doesn't always make best use of it.

Marine mammals

Pinnipeds 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. [34] 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. [35]

The upper, smooth whisker belongs to a California sea lion. The lower undulated whisker belongs to a harbor seal. Sea-lion seal vibrissa.png
The upper, smooth whisker belongs to a California sea lion. 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, [24] [36] and even to discriminate the species and the size of the fish responsible for the trail. [37] 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. [38] 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. [35] By holding their vibrissae steady, pinnipeds are able to maximize their detection ability. [39] The vibrissae of seals are undulated and wavy while sea lion and walrus vibrissae are smooth. [40] 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. [39]

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, [41] 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. [42] 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. [43] Indeed, some humans even still develop vestigial vibrissal muscles in the upper lip, [44] 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. [43]

Artificial whiskers

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, [45] through feature-specific models, [46] [47] to attempts to reproduce complete whiskered animals in robot form (ScratchBot [48] and ShrewBot, [49] [50] [51] both robots by Bristol Robotics Laboratory).

In non-mammalian animals

"Whiskers" on a whiskered auklet Aethia pygmaea3.jpg
"Whiskers" on a whiskered auklet

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

In birds

The "whiskers" around the beak of a kakapo. Strigops habroptilus, face.jpg
The "whiskers" around the beak of a kakapo.

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. [52]

Other birds that have obvious "whiskers" are kiwis, flycatchers, swallows, nightjars, whip-poor-wills, the kakapo and the long-whiskered owlet (Xenoglaux loweryi).

In fish

"Whiskers" on a catfish Corydoras aeneus barbels.jpg
"Whiskers" on a catfish

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 have a rugose (wrinkled) jaw texture that has been interpreted as the attachment sites for vibrissae, [53] though actual vibrissae have not been recorded. [54]

More recently, a specific type of pycnofibers/feathers has been found around anurognathid mouths. [55]

Related Research Articles

Hair 2=2

Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. Most common interest in hair is focused on hair growth, hair types, and hair care, but hair is also an important biomaterial primarily composed of protein, notably alpha-keratin.

Pinniped Infraorder of mammals

Pinnipeds, commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic marine mammals. They comprise the extant families Odobenidae, Otariidae, and Phocidae. There are 33 extant species of pinnipeds, and more than 50 extinct species have been described from fossils. While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic lineage. Pinnipeds belong to the order Carnivora and their closest living relatives are believed to be bears and the superfamily of musteloids, having diverged about 50 million years ago.

Lambdopsalis bulla is an extinct multituberculate mammal from the Late Paleocene of China. It is placed within the suborder Cimolodonta and is a member of the superfamily Taeniolabidoidea. Fossil remains have been found in Upper Paleocene strata in Nao-mugen and Bayn Ulan of China.

Naked mole-rat Burrowing rodent; one of only two known eusocial mammals

The naked mole-rat, also known as the sand puppy, is a burrowing rodent native to parts of East Africa. It is closely related to the blesmols and is the only species in the genus Heterocephalus of the family Heterocephalidae. The naked mole-rat and the Damaraland mole-rat are the only known eusocial mammals, the highest classification of sociality. It has a highly unusual set of physical traits that allow it to thrive in a harsh underground environment and is the only mammalian thermoconformer, almost entirely ectothermic (cold-blooded) in how it regulates body temperature.

Northern fur seal The largest fur seal in the northern hemisphere

The northern fur seal is an eared seal found along the north Pacific Ocean, the Bering Sea, and the Sea of Okhotsk. It is the largest member of the fur seal subfamily (Arctocephalinae) and the only living species in the genus Callorhinus. A single fossil species, Callorhinus gilmorei, is known from the Pliocene of Japan and western North America.

Caniformia suborder of mammals

Caniformia is a suborder within the order Carnivora. They typically possess a long snout and nonretractile claws. The Pinnipedia are also assigned to this group. The center of diversification for Caniformia is North America and northern Eurasia. This contrasts with the feliforms, the center of diversification of which was in Africa and southern Asia.

Muridae family of mammals

The Muridae, or murids, are the largest family of rodents and of mammals, containing over 700 species found naturally throughout Eurasia, Africa, and Australia.

Barrel cortex

The barrel cortex refers to a region of somatosensory cortex that is identifiable in some species of rodents and species of at least two other orders and contains the barrel field. The 'barrels' of the barrel field are regions within cortical layer IV that are visibly darker when stained to reveal the presence of cytochrome c oxidase, and are separated from each other by lighter areas called septa. These dark-staining regions are a major target for somatosensory inputs from the thalamus, and each barrel corresponds to a region of the body. Due to this distinctive cellular structure, organisation, and functional significance, the barrel cortex is a useful tool to understand cortical processing and has played an important role in neuroscience. The majority of what is known about corticothalamic processing comes from studying the barrel cortex and researchers have intensively studied the barrel cortex as a model of neocortical column.

Mammaliaformes Clade of tetrapods

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Tritheledontidae genus of mammals (fossil)

The Tritheledontidae or tritheledontids, also known as ictidosaurs, is an extinct family of small to medium-sized cynodonts. They were extremely mammal-like, highly specialized cynodonts, although they still retained a very few reptilian anatomical traits. Tritheledontids were mainly carnivorous or insectivorous, though some species may have developed omnivorous traits. Their skeletons show that they had a close relationship to mammals. Tritheledontids or their closest relatives may have given rise to primitive mammals. The tritheledontids were one of the longest lived non-mammalian therapsid lineages, living from late Triassic to the Jurassic period. Tritheledontids became extinct in the Jurassic period, possibly due to competition with prehistoric mammals such as the triconodonts. They are known from finds in South America and South Africa, indicating that they may have lived only on the supercontinent of Gondwana. The family Tritheledontidae was named by South African paleontologist Robert Broom in 1912. The family is often misspelled "Trithelodontidae".

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The mountain viscacha rat or mountain vizcacha rat, historically viscacha rat or vizcacha rat, is a species of rodent in the family Octodontidae. It is endemic to Argentina.

Sleep in non-human animals sleep in non-human beings

Sleep in non-human animals refers to a behavioral and physiological state characterized by altered consciousness, reduced responsiveness to external stimuli, and homeostatic regulation. Sleep is observed in mammals, birds, reptiles, amphibians, and some fish, and, in some form, in insects and even in simpler animals such as nematodes. The internal circadian clock promotes sleep at night for diurnal organisms and in the day for nocturnal organisms. Sleep patterns vary widely among species. It appears to be a requirement for all mammals and most other animals.

Fur soft, thick, hairy coat of a mammal

Fur is a thick growth of hair that covers the skin of many animals. It is a defining characteristic of mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture and the underfur acts as an insulating blanket that keeps the animal warm.

Somatosensory system Widely distributed parts of the sensory nervous system

The somatosensory system is a part of the sensory nervous system. The somatosensory system is a complex system of sensory neurons and neural pathways that responds to changes at the surface or inside the body. The axons of sensory neurons connect with, or respond to, various receptor cells. These sensory receptor cells are activated by different stimuli such as heat and nociception, giving a functional name to the responding sensory neuron, such as a thermoreceptor which carries information about temperature changes. Other types include mechanoreceptors, chemoreceptors, and nociceptors which send signals along a sensory nerve to the spinal cord where they may be processed by other sensory neurons and then relayed to the brain for further processing. Sensory receptors are found all over the body including the skin, epithelial tissues, muscles, bones and joints, internal organs, and the cardiovascular system.

Sense Physiological capacity of organisms that provides data for perception

A sense is a physiological capacity of organisms that provides data for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology, and philosophy of perception. The nervous system has a specific sensory nervous system, and a sense organ, or sensor, dedicated to each sense.

Tactile sensor device that measures information arising from physical interaction with its environment

A tactile sensor is a device that measures information arising from physical interaction with its environment. Tactile sensors are generally modeled after the biological sense of cutaneous touch which is capable of detecting stimuli resulting from mechanical stimulation, temperature, and pain. Tactile sensors are used in robotics, computer hardware and security systems. A common application of tactile sensors is in touchscreen devices on mobile phones and computing.

Hydrodynamic reception

Hydrodynamic reception refers to the ability of some animals to sense water movements generated by biotic or abiotic sources. This form of mechanoreception is useful for orientation, hunting, predator avoidance, and schooling. Frequent encounters with conditions of low visibility can prevent vision from being a reliable information source for navigation and sensing objects or organisms in the environment. Sensing water movements is one resolution to this problem.

Whisking in animals Repetitive and rapid sweeping back and forth of the whiskers in animals

Whisking is a behaviour in which the facial whiskers (vibrissae) of an animal are repetitively and rapidly swept back and forth. This behaviour occurs particularly during locomotion and exploration. The whisking movements occur in bouts of variable duration, and at rates between 3 and 25 whisks/second. Movements of the whiskers are closely co-ordinated with those of the head and body, allowing the animal to locate interesting stimuli through whisker contact, then investigate them further using both the macrovibrissae and an array of shorter, non-actuated microvibrissae on the chin and lips. Whisking has been reported in a wide range of mammals, including two species of marsupial. Whisking contributes both to exploratory movements, which function to acquire sensory inputs, and to palpation movements, which are used in the discrimination of objects and in the control of spatial navigation.

Nocturnal bottleneck

The nocturnal bottleneck hypothesis is a hypothesis to explain several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively nocturnal through most of their evolutionary story, starting with their origin 225 million years ago, and only ending with the demise of the dinosaurs 66 million years ago. While some mammal groups have later evolved to fill diurnal niches, the approximately 160 million years spent as nocturnal animals has left a lasting legacy on basal anatomy and physiology, and most mammals are still nocturnal.

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