Do Cat's Eyes Glow in the Dark?
you've ever wondered why your cat's eyes seem to glow in the dark,
or pondered how a hummingbird finds its way to certain nectar-filled
flowers, what you are really asking is, do animals see the way we
do? Though we might like to think that animals see the world with
the same colors and shadows as human beings-although maybe from
a different angle, perhaps high in the sky like an eagle, or under
the sea like a shark-the truth is that animal vision is not like
our own and differs greatly among animal species.
in the Dark
beings are not like tigers, or housecats for that matter, whose
eyes are superbly well adapted to seeing in the dark. One reason
is that cats have more rods than cones in their retinas, unlike
humans, making kitty's night and motion vision superior. (Rods are
the receptors that the eye uses for nighttime viewing and sudden
movement; cones are used during the daytime and process color information.)
cats' pupils are shaped differently than those of humans (they are
elliptical rather than round), which allows for a much larger pupil
size. In fact, the most notable feature of nocturnal animals is
the size of their eyes. The reason for this is that large eyes can
collect more ambient light.
fact: An owl's eyes fill over half of its skull!
well, cats' eyes open and close much faster than do ours. And cats
have a special membrane on the back of their eyes (called the tapetum
lucidum, literally meaning "bright carpet") that increases
the quantity of light caught by the retina. The tapetum collects
and re-emits light back to the retina, giving the rods a second
chance to absorb the image, thus maximizing their sensitivity to
low light levels. As this light is reflected off the tapetum, the
animal's eyes appear to glow.
You Thought "Four-Eyes" Was Insulting
about "hundred eyes"? That's the insult you'd have to
hurl at a scallop. The scallop's mantle is lined with small blue
eyes. Each eye has a lens and a retina which is attached to a branch
of the optic nerve. Behind the retina is a tapetum, perhaps the
only thing a cat and a sea creature have in common! The hundred
or so eyes work together to alert the mollusk to changes in light
it comes to light, we humans are limited. The light we see with
our eyes is really a very small portion of the electromagnetic spectrum.
and butterflies may be small, but they can see better than us. In
fact, their range of vision extends into the ultraviolet part of
the spectrum, which is invisible to us with the naked eye. The leaves
of the flowers they pollinate have special ultraviolet patterns
which guide the insects deep into the flower to locate the plant's
nectar. So, next time you admire a pretty flower, just think about
all the beauty you're not seeing!
snakes have an extra pair of "eyes," or sensory organs,
located on their foreheads that can detect infrared radiation. They
can "see" the heat of a mouse from a meter away, even
in conditions where our eyes would detect only pitch black.
those common urban pests, pigeons, have one up on us: Pigeons can
see patterns of polarized light in the daytime sky which are invisible
to us, providing yet another clue to the remarkable homing abilities
of these birds.
Judge a Creature by Its Cones
couldn't forget to tell you about shark vision. Sharks do not possess
the same variety of photoreceptors as humans. They have few retinal
cones, and as a result, for years it was thought that their vision
was much less acute than ours. However, though different from our
eyes, shark eyes work just as well, relying on visual pigments such
as rhodopsin to supply the color vision we achieve with cones.
course, sharks have other advantages as well: They have a sharp
sense of hearing and can hear prey many miles away. They also possess
a superior sense of smell and can smell even the smallest drop of
blood over long distances. Once they are close to their prey, they
employ special sensory pores on their heads to detect electrical
fields created by prey. Getting scared yet?
you go out and rent Jaws, know that you are much more likely to
get hit by lightning than eaten by a shark. And here's another cool
shark fact: Like cats and scallops, sharks, too, have a tapetum,
which gives them about 10 times the light sensitivity we have. This
works well in the dark, but the light-enhancing mechanism can be
a bother during daylight. So sharks have evolved a fun solution:
Many species of sharks have migratory pigment cells that can close
up the tapetum under bright-light conditions-basically, a pair of
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