Whiskers, also known as vibrissae (; sing. vibrissa; ), are specialized sensory hairs that help most species of mammals sense their environment. Whiskers function as highly sensitive feelers that help mammals navigate the world in many different ways, including the detection of air currents and physical objects to aid in navigation, hunting, and spatial awareness. Whiskers can trigger reflexes to protect the eyes. Whiskers also play a significant role in the communication between individual animals in different species of mammals. They convey an animal's emotional state, essentially sending a message to other members of their species about what emotion they are feeling at the time. Whisker movement can be thought of as a form of language that animals use to silently communicate with the other animals around them. Whiskers grow in clusters at various places around the body, including above the eyes, on the chin, on the forelegs, near the ears, and above the upper lip. Most mammals have whiskers, including all non-human primates, marsupials, and especially nocturnal mammals. Monotremes, however, lack them. Whiskers are sensitive tactile hairs that aid navigation, locomotion, exploration, hunting, social touch and perform other functions. While mammals are the only animals that grow whiskers, other classes of animal species are known to have similar structures that are also used to sense the environment. Species with structures that function similarly to whiskers include different species of birds, fish, insects, crustaceans, and other arthropods.
Etymology Vibrissae (from Latin vibrāre '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. The species of mammals that have the greatest amount of whiskers, as well as the longest whiskers, are typically species that are social, arboreal (live in trees), nocturnal, and physically smaller than other mammals. Whiskers of aquatic mammals are the most sensitive. During foraging in complex, dark habitats, whiskers are rapidly moved in a cyclic way, tracing small circles at their tips. This motion, called "whisking" can occur at speeds of 25 Hz in mice, which is one of the fastest movements that mammals can make. Small animals use whisking to position their front paws during locomotion.
Vibrissal groups
Vibrissae typically grow in clusters. These groups vary somewhat in form and function, but they are relatively consistent among land mammals. Between land and marine mammals, there is less consistency (though commonalities are certainly present). Many land mammals, like rats 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). Not only are these two types hard to distinguish on an animal's face (see for example the image of a rat here), there are similarly weak distinctions in how they are used, though the distinction is nonetheless referred to ubiquitously in scientific literature and is considered useful in analysis. Many land mammals, including domestic cats, also have vibrissae on the underside of the leg just above the paws (called carpal vibrissae). 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.
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, whereas every single one of the body hairs of the Florida manatee may be a vibrissa (see image). 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.
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