Whiskers or vibrissae (
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 and hamsters, have an arrangement of
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. 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. 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 ).
Marine mammals can have substantially different vibrissal arrangements. For instance, cetaceans have lost the vibrissae around the snout and gained vibrissae around their blowholes, whereas every single one of the body hairs of the Florida manatee (see image) may be a vibrissa. 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
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 (
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). In the mouse, gerbil, hamster, rat, guinea pig, rabbit, and cat, each individual follicle is innervated by 100–200 primary
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. Manatees, remarkably, have around 600 vibrissae on or around their lips.
Whiskers can be very long in some species; the length of a