Vibrissae (; singular: vibrissa; ), more generally called Whiskers, are a type of stiff, functional hair used by mammals to sense their environment. These hairs are finely specialised for this purpose, whereas other types of hair are coarser as tactile sensors. Although whiskers are specifically those found around the face, vibrissae are known to grow in clusters at various places around the body. Most mammals have them, including all non-human primates and especially nocturnal mammals. Whiskers are sensitive tactile hairs that aid navigation, locomotion, exploration, hunting, social touch and perform other functions. This article is primarily about the specialised sensing hairs of mammals, but some birds, fish, insects, crustaceans and other arthropods are known to have similar structures also used to sense the environment. Etymology 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 universal common ancestor of all extant mammals had vibrissae. Great apes are the only extant mammal species which do not have vibrissae. All extant mammal species retain the ancestral layout of the whiskers along with the special facial muscles that move them. Anatomy Vibrissae are anatomically distinguished from other hair. They are easily visually identified since they are longer, stiffer, significantly larger in diameter, and stand above the surrounding fur by a considerable amount. In addition they have well-innervated follicles, and an identifiable representation in the somatosensory cortex of the brain. The largest number and the longest are found among the small, social, arboreal, and nocturnal mammals. Whiskers of aquatic mammals 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. It is clear that these 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