On The Trails: Whiskers on mammals

A contented cat shows whiskers curving forward. Note the upright whiskers above the eyes. (Photo by Pam Bergeson)

By Mary F. Willson

The word "whiskers" covers a variety of structures, stiff or soft, and most mammals have them. What we often call "whiskers" in humans are facial hairs grown chiefly for adornment, although they may sometimes have minor ancillary functions, and this essay does not deal with them.

Most mammalian whiskers are sense organs, expanding the sense of touch beyond the skin. These sensory whiskers are often called vibrissae, although that term may include a diversity of structures, some of which are mentioned below.

I will use the term vibrissae here, in that broad sense. Whiskers with a tactile sensory function seem to have evolved long ago and were present in the reptiles ancestral to mammals. Vibrissae and hairs (as well as hooves and nails) are made of keratin, a structural protein, and they develop from follicles in the skin. Typical vibrissae are stiffer and thicker than hair. Their follicles are enclosed in blood-filled sinuses and well-served by nerve endings. The blood sinuses are reported to be transducers of movement in the vibrissae to the nerves, and then sending messages to the brain. Some vibrissae are surrounded by small muscles connecting to bone or skin or to other vibrissae.

Our companion cats give us a great opportunity to observe whiskers in action. Vibrissae in cats typically develop on the side of the face, above the eyes, and on the back of the forelegs. They are sensitive to air currents from nearby objects, allowing the cat to pass safely near things. They help assess spatial obstructions, allowing the cat to fit safely through narrow spaces and carefully poke its nose into little nooks. Vibrissae can detect the presence of little bits of debris, so the cat can flick them off its face. They sense movement of prey, useful to a hunter, and movement of nearby cats, important in social interactions. Facial vibrissae express emotion: moving and curving forward when the cat is contented and interested and back when disgusted or angry. In short, vibrissae are essential to all aspects of a feline life.

Another cat shows interest in something. (Photo by KM Hocker)

Elephants have lots of whiskers (sometimes called vibrissae). Located all along the trunk, these help the animals explore their surroundings. Like typical vibrissae, trunk whiskers have a tactile function. Their structure is unusual, in that they are bigger and stiffer at the base, but softer and more flexible at the tip. Unlike the whiskers of cats and rats, they are a bit flattened and are not uniformly dense throughout their length. But they ARE connected directly to the brain, sending separate messages, so the animal can tell which whiskers were stimulated.  Unlike cats’ whiskers, however, they do not move independently. The flexible whiskers detect ground vibrations as well as the shapes of objects to be examined.  

Seals have whiskers (or vibrissae) on the muzzle, where they help navigating in turbid waters. They are not round (as in cats) but have uneven diameters (called "wavy" in some accounts). That structure reportedly helps hold the whisker steady as the seal swims, but still be responsive to the water currents created by fish prey.

Manatees have lots of facial vibrissae, used for exploring food possibilities, etc., but they also have them all over their bodies (instead of normal hair). Body vibrissae are touch receptors, each one connected separately to the brain, used for detecting water currents and movement of other animals — very convenient in murky waters.

Whales have facial bristles (sometimes called vibrissae) that have a tactile function. Baleen whales are reported to have them on the face, where they detect water movement and prey density. Toothed whales such as dolphins have tactile facial bristles when they are babies, used for finding the source of milk on the mothers. But baby dolphins drop those bristles, leaving behind (!)electro-sensitive pits that are sensitive to low-energy electrical signals such as those from prey (these are different from the electro-sensitive organs of sharks, rays, and some bony fishes).

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent. For a complete archive of Mary Willson’s “On the Trails” essays, go to https://onthetrailsjuneau.wordpress.com.