By Lilian Shen
posted
Jul 11, 2012
Illustration by Adelaide Tyrol
I'll always remember the time I ran into a wire fence at dusk. I
was taking a shortcut through some woods, and the impact sent me
tumbling. Even when I looked carefully, I could barely discern the
thin strands of wire in the gloomy evening light.
All vertebrates share the same basic eye structure: a pupil that
dilates or constricts to control how much light enters the eye, a
lens to focus the image onto a light-sensitive retina, and nerves
that relay the information to the brain. Yet humans flounder with
the departure of daylight, while many animals are able to forage
and hunt by night.
How do animals see in the dark? For one, they have big eyes.
Nocturnal hunters like owls and cats have pupils that, when open
wide, cover the entire front of the eye. So do tree frogs, which
have to be able to jump from branch to branch. In owls, eye size
approaches the extreme: their eyes occupy over half the volume of
their skulls.
There are also physiological differences between the eyes of
nocturnal and diurnal animals. Owls' eyes are tubular, rather than
spherical, with a very large lens positioned close to the retina.
This structure allows a lot of light to register on the retina, but
at the expense of flexible focusing. Owls are thought to be
somewhat far-sighted. Tubular eyes cannot rotate in their sockets
like the spherical eyes you and I have, so owls compensate with
incredibly flexible necks that allow them to turn their heads 270
degrees.
Many nocturnal animals have a mirror-like layer, called the
tapetum, behind the retina, which helps them make the most of small
amounts of light. Light that passes through the retina is reflected
off the tapetum, giving the retinal cells a second chance to sense
it. This makes some animals' eyes shine in the glare of car
headlights. The color you see is the pigment on the inner layer of
the retina.
At the heart of all vision is the retina, which contains two
types of light-sensing cells: rods and cones. Cones account for
color vision but require bright, focused light, whereas rods can
sense very dim, scattered light, but don't produce a color image.
While each cone has its own brain connection, multiple rods are
wired to a single brain connector. This pools the information
collected from the rods and creates a stronger signal, but the
image is less defined.
As you might expect, the retinas of nocturnal animals are packed
with rods and have few cones. However, because their large eyes
create a big image that is focused on a big retina, they capture
some detail despite the shortage of cones.
In our eyes, the cones connect to circuits that send either
"light" or "dark" signals to the brain, which increases sensitivity
to movement and the edges of objects. Nocturnal animals possess a
pathway through which rods connect to the same "dark" circuits used
by cones, which allows them better perception of edges, movement,
and silhouettes in dim light.
Even the nuclei of the rod cells are adapted for night vision.
In diurnal animals, the chromosomes in the nucleus are densest
around the edges, which means that any absorbed light is scattered
around the edges. In nocturnal animals, the densest materialis in
the center of the nucleus, effectively focusing all of the
available light in one area.
One can only guess at what nocturnal animals see. It's likely to
be shades of gray, sensitive to movement but maybe lacking fine
detail. Most nocturnal animals also have a highly developed sense
of hearing, touch (e.g., whiskers), or smell, to complement their
vision. One should not get the impression that an animal's night
vision is perfect - even nocturnal animals aren't active in the
darkest hours of a moonless night.
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
Charitable Foundation: wellborn@nhcf.org.
Tagged:
Animals, Night Vision
