Of the many questions one is left with after listening to the
nursery rhyme Three Blind Mice, none is more vexing than how three
blind rodents were able to chase anything, let alone a farmer's
wife. As the three mice in question died in 1805, we'll probably
never know the full answer. There are some clues in the scientific
record, though. The fact is, mice and other nocturnal rodents can
take in sophisticated three-dimensional information about their
surroundings without using their eyes.
Rodent eyes don't function like our eyes. Ours are on the front
of our head and we see in stereo, their eyes are on the side of
their head and their field of vision doesn't overlap. Our eyes
always move together, a rodent's eyes can move in opposite
directions. If a rat points its snout downward, its eyes look up,
rather than where its nose is pointing. If its head tilts down to
the right, the right eye looks up while the left eye looks
There is a purpose to these odd movements - to keep the space
above in view at all times. To a rodent, constant scanning overhead
for birds of prey is more important for survival than clearly
seeing the path ahead.
So how do rats sense what's in front of their noses? Instead of
relying on sight, they pass their whiskers over all surfaces
repeatedly from different angles to get three-dimensional images of
their surroundings. This process, called "whisking," occurs
constantly and rapidly - up to 12 times per second, among the
fastest movements measured in mammals. Each whisker has its own
muscle and can move independently. There are also muscle arrays
that move all the whiskers together.
If you were to examine a rat's face, you would see that it has
two different varieties of facial hair: long stiff whiskers that
stick out around the snout and above the eyes, and shorter, finer
whiskers below the nostrils that point down. The whiskers are
arranged in a defined grid pattern. If you could peer into the
rat's head, you would discover that each whisker is connected to a
discrete bundle of brain cells. These bundles are arranged in
exactly the same grid pattern as the whiskers to which they are
connected. This organization of sensors provides the rat with a
detailed "snapshot" of the space in front of it.
Whiskers themselves are not living tissue. Their exquisite
sensitivity lies at the base, in the follicle where the whisker
sprouts from the skin. When a whisker touches a surface, it bends.
The tension from bending is sensed by nerves in the follicle that
send a signal to the brain.
While the bending of whiskers transmits spatial information, the
sensitivity of whiskers to vibration allows the rat to feel
differences in texture with astonishing accuracy. Whiskers are far
more sensitive than the human fingertip - for instance, a rat can
tell the difference between a totally smooth surface and one scored
with microscopic grooves 30 microns deep (a micron being one
thousandth of a millimeter).
Different whiskers resonate with different frequencies of sound.
The thicker, longer whiskers away from the nose vibrate at lower
frequencies, while the shorter, finer whiskers closer to the nose
respond to higher frequencies. Like strings on a piano, the
whiskers form an orderly array of filaments tuned to a gradation of
sounds, or vibrations, from low to high. It is also likely that
whiskers sense air currents in burrows and narrow spaces for
additional directional clues.
Whiskers, of course, do not operate in isolation. The rat senses
its environment though hearing and smell too. In fact whisking and
smelling, or sniffing, are locked into the same rhythm, so that
every time a rat whisks it also sniffs. Thus a shape, texture and
smell 'snapshot' is transmitted simultaneously to the brain.
All of which is to say that three blind mice, rats, or other
whisking creatures wouldn't be nearly as helpless as one might
expect. While out in the open they might be easy prey for a hawk,
or the farmer's wife, in other contexts they could navigate well
without sight. For a life often lived in shadows, scurrying beneath
vegetation and navigating burrows, the touchy-feely sense of
whisking offers a rich way to "see" that isn't dependent on
Li Shen is an adjunct professor at the Dartmouth Medical School
and the chair of the Thetford, Vermont, Conservation Commission.
The illustration for this column was drawn by Adelaide Tyrol. The
Outside Story is assigned and edited by Northern Woodlands magazine
and sponsored by the Wellborn Ecology Fund of New Hampshire