A whisker is a long hair like all others on the body: a column of dead cells that grows from a follicle in the skin. All follicles are supplied with blood, a few nerve endings, and some tiny muscles (responsible for goosebumps in humans, called piloerection in animals). But that's where the similarity ends, because whiskers are far more sophisticated than regular fur.
Each whisker grows from a follicle that is sealed by a capsule of blood called a blood sinus. When a whisker is touched it bends and pushes the blood against the opposite side of the sinus. The liquid blood amplifies the movement, triggering nerves that send a message to the brain (for more, see Kathleen Wong's Whisker page at the California Academy of Sciences). The rat can tell what direction and how far each whisker is moved.
Rats have long whiskers that grow in rows on either side of their nose, long whiskers sprouting from their eyebrows and cheeks, and very short whiskers around the mouth.
Photo courtesy of R. A. of the Dapper Rat
Baby rats are born with whiskers. Fine whiskers appear in the embryo when it is 12 days old (Yamakado and Yohro 1979), and the newborn rat gains the ability to whisk its whiskers back and forth 12 days after birth (Welker 1964).
Like all hairs, whiskers grow from a follicle in the skin. Follicles go through repeated cycles of active growth and rest. Over the course of this growth and rest cycle, the hair grows, stays in place for a while, then falls out and a new hair grows in its place. During most of the growth phase, whiskers grow about 1 mm per day (Ibrahim and Wright 1975). If a loses a whisker, a new one starts growing to replace it within 8-11 days of the loss (Ibrahim and Wright 1975).
The electrical message from the whisker follicle shoots up a pathway to an area of the brain called the barrel cortex. The rat's barrel cortex is responsible for processing all the messages that come in through the whiskers. The barrel cortex takes up a large part of the somatosensory area of the rat's brain, the area that deals with touch. (In contrast, large parts of the human somatosensory cortex are devoted to our sensitive areas like hands, fingertips, and lips).
The barrel cortex is arranged like a map: each whisker's nerve goes to one spot in the cortex (called a barrel, because of its shape), and the spatial layout of the barrel cortex reflects the spatial layout of the whiskers (Woosley and Van der Loos 1970). A diagram shows the one-to-one correspondence between whiskers and barrels (Carol Welker 1976), and a photo shows the similar layout of a rat's whiskers and barrel cortex.
The cortex whisker map is shaped by experience during development and continues to be modified in adults. If an adult's whisker is cut off, within a few hours or days the brain will adjust: the corresponding barrel in the cortex will atrophy, and neighboring barrels will grow larger to compensate (Karel Svoboda). If a rats' whiskers are trimmed so that some whiskers are longer than others, the rat's brain will adjust after about a week (Armstrong-James et al. 1994).
If a newborn rat's whisker follicle is removed, the corresponding barrel won't grow, but neighboring barrels will grow larger to compensate (Woosley and Van der Loos 1973). A rat that grows up with no whiskers ends up with abnormal brain wiring (Erchova et al. 2003), and as an adult, its brain does not have as much plasticity (ability to adjust to changes) as one who grew up with normal whiskers (Rema et al. 2003). A rat that grows up with no whiskers does not orient (Symons and Tees 1990) or discriminate surface textures as well as a normal rat (Carvell and Simons 1996).
Bobbin extends his whiskers in front of his face to investigate a finger.
Rats whisk their whiskers along surfaces of objects, sometimes hitting the same object several times in different places as they pass, which enables them to glean a three dimensional picture of their surroundings. Whiskers are extremely sensitive, more so than a human's fingertips. Trained rats can distinguish between a smooth surface and one with microscopic grooves 30 microns deep and 90 microns apart (Carvell and Simons1990), between sandpaper with 200 grains/cm2 or 25 grains/cm2 (Guic-Robles, Valdivieso, and Guajardo 1989).
Rat whiskers also resonate at different frequencies, like the strings of a harp. Low sound frequencies in the air and coarse textures on the ground cause the long whiskers to vibrate, while high air frequencies and fine textures vibrate the short whiskers. These vibrations may help the rat make fine distinctions between textures, and may even enable them to 'hear' sounds transmitted through the air (Andermann et al. 2004, Neimark et al. 2003, Hartman et al. 2003, Andermann et al. 2004). For more, see hearing with whiskers.
Rats use their whiskers to navigate in their world, orient themselves, and balance. Whiskers help the rat find and discriminate food, court a mate, determine whether he can fit through an opening or not. At short distances rats rely more on their whiskers than on vision to perceive depth (Schiffaman 1970). Whiskers can detect wind and slight breezes too, which helps rats orient themselves underground. Whiskers are used when swimming, especially in turbulent water. Whiskers help the rat feel when its nose is above the surface. Without whiskers, the rat will drown (Ahl 1986, Biay 1993, Bugbee 1972, Carvell 1990, Guic-Robles 1989, Gustafson 1977, Vincent 1912).
Whiskers are used socially, too. During nose-offs and boxing with other rats, the defending rat attempts to keep whisker-to-whisker contact with the aggressive rat. Whiskers interfere with an attack on the head. As long as whisker-to-whisker contact is maintained, the dominant rat can't get around the whiskers to deliver a bite. However, if a rat's whisker pads are anesthetized, that rat boxes less and freezes more, and is more likely to be bitten (Blanchard et al. 1977).
A rats without whiskers may be more impaired than a rat who is blind!
Two of my rats (Cricket and Snip) have standard fur and straight whiskers; Widget is a rex and has curly whiskers.
To give you an idea of the whisker difference between standard and rex rats, Cricket's whiskers are long and nearly straight. They are arrayed in an orderly spray spreading up, sideways, forward, down and backward from his face. His whiskers constantly sweep back and forth. Cricket's whisker-span (distance between the furthest left tip and the furthest right tip) is 13 cm wide. His longest whiskers are about 5.9 cm long. Cricket has several eyebrow whiskers that project 4.5 cm above his head, and many short, straight whiskers around his mouth that are only a few millimeters long.
Widget, however, has curled whiskers, about half of which curl in a complete circle such that the tips touch his face. The tips of most of the rest point downward to the floor. He has only four or five whiskers that point upward. His whisker-span is only 6.9 cm, about half the span of Cricket's whiskers. Widget has only a few eyebrow whiskers and they are so strongly curled that they do not project upward at all. Most of his short mouth whiskers are curled as well.
Widget's whiskers are therefore effectively only half as long as Cricket's. There are areas around Widget's head, such as the space above his forehead, which have no whisker coverage at all and are therefore tactile "blind spots" for him.
How do curly whiskers affect the rat's ability to sense the world around him? Little research has been done on this topic, though a new mutation, curly vibrissae (cv), may open up this area of sensory research (Fujii et al. 2003).
We do know, however, that brains are largely able to adjust to whatever sensory input is available. For example, rats who grow up with shortened whiskers have the same somatosensory layout as normal-whiskered rats, albeit with a number of subtle differences -- the brain turns out to be quite resiliant to abnormal sensory input (Erchova et al. 2003). In addition, if a rat's whiskers are kept at an uneven length through trimming, the receptive field of the rat's brain adjusts within about a week, such that the brain pays more attention (specifically, allocates more of its receptive field) to the long whiskers than the short ones (Armstrong-James, 1994).
So, curly-whiskered rats' brains are probably largely similar to normal rat brains, and are probably fine-tuned to extract the most information from the messages sent by their curled, disordered whiskers. Rats with curly whiskers will do well with what they have.
However, nothing can replace an absence of whiskers, and no amount of brain re-organization will give Widget the ability to feel the space over his head, or the space beyond an inch to either side of his face, where his whiskers do not reach. The tactile world of the curly-whiskered rat is therefore more limited than that of a normal rat.
If a rat loses a whisker, his brain will adapt to the new configuration within a few hours or days: the space allocated in the brain to that whisker (the barrel) will shrink and the neighboring barrels will grow larger to compensate (Karel Svoboda). This isn't a permanent condition, however: a new whisker will grow to take the place of the lost one, and should appear within 8-11 days of the loss (Ibrahim and Wright 1975). The rat's brain will re-adjust to the new configuration.
If a rat's whiskers are shortened so that some are shorter than the others, the rat's brain will adjust after about a week. Over the 7-10 days following the trim, the rat's brain allocates more space to the areas corresponding to the long whiskers, and less space to the short ones (Armstrong-James et al. 1994). Short whiskers are still functional, but are less useful than full-length whiskers. So, a rat with shortened whiskers may be impaired, but this is temporary. These short whiskers will eventually be shed, and new ones will grow to take their place. The rat's brain will re-adjust as he grows new whiskers.