Whiskers

[3] Whiskers are sensitive tactile hairs that aid navigation, locomotion, exploration, hunting, social touch and perform other functions.

[6] 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.

They are easily visually identified since they are longer, stiffer, significantly larger in diameter, and stand above the surrounding fur by a considerable amount.

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.

Many land mammals, like rats[8] and hamsters,[9] have four typical whisker groups on their heads (called cranial vibrissae), which might vary among animals due to different lifestyles.

[11] In the mouse, gerbil, hamster, rat, guinea pig, rabbit, and cat, each individual follicle is innervated by 100–200 primary afferent nerve cells.

[11] 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.

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.

[28] Sensory function aside, movements of the vibrissae may also indicate something of the state of mind of the animal,[29] and the whiskers play a role in social behaviour of rats.

Such experiments have shown that whiskers are required for, or contribute to: object localization,[32][33] 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.

However, exactly why an animal might be driven "to beat the night with sticks", as one researcher once put it,[34] is a matter of debate, and the answer is probably multi-faceted.

Scholarpedia[7] 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.

Their mystacial vibrissae have ten times the innervation of terrestrial mammals, allowing them to effectively detect vibrations in the water.

[42] 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.

[43] 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.

[46] 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.

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.

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.

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

Fish that have barbels include the catfish, carp, goatfish, hagfish, sturgeon, zebrafish and some species of shark.