Window on Nature
By Lowell and Kaye Christie, F47246
November 2007
Physical adaptations allow nocturnal animals to see through the darkness.
Off in the distance, under a moonless midnight sky, a meadow mouse rustles nearly unseen in the dry stubble of last season’s grass. The mouse feels a sense of security in the dark surroundings, but on the other side of the meadow, perched on a dead branch, an owl watches it intently.
Many creatures are more active at night, and most see quite well in the dark. An owl, for example, has oversized eyes that are capable of spotting a mouse from 100 yards away. In some owls, the eyes occupy more than half the skull and are too large to turn from side to side. To compensate, owls can rotate their heads 270 degrees.
Most nocturnal animals have proportionally larger eyes than humans, and the pupils open wider to admit more light. But more light is not enough. It takes other differences in these eyes that watch from the shadows. What is required is a more efficient use of the light that exists.
First a little background on how eyes work. After light passes through the pupil, it is focused on the retina that lines the back of the eye. The retina passes visual information to the brain via the optic nerve. On the retina are two types of light receptors, rods and cones, named after their physical shapes.
Cones operate in bright light, allowing us to distinguish different colors and to examine the fine details of an object. Humans have three different types of cones that respond to different wavelengths, letting us view the world in color. But cones do not work well in dim light.
Rods, on the other hand, work in low-light conditions and help detect motion. As you might expect, night-active animals have more rods than cones. In fact, some bats and nocturnal snakes and lizards have no cones at all.
On the rods of the retina, molecules of rhodopsin absorb light photons. These protein molecules are ultra-sensitive to low light levels, but they become less effective as it becomes brighter. In fact, intense light bleaches them out, and it can take up to 30 minutes for rhodopsin to recover. You experience this effect every time you enter a dark room after being outside on a sunny day and must wait for your eyes to adjust.
By giving up all or most of the cones in their eyes, night creatures exchange detailed color vision for a less precise, but more useful, ability to detect motion in low light. But this ability comes at a cost. In addition to losing color sensitivity, the eye must now be protected from the bright glare of daylight.
Many animals have a round pupil that allows light to enter the eye. Depending upon the amount of light available, this pupil will open or close to adjust to the quantity of light. However, this round opening, while good at letting light in, is not very efficient at either closing quickly or completely blocking all light. Because of this, many night critters have slit pupils.
Instead of a circular opening, light enters through a narrow slit, often vertical, but sometimes horizontal or diagonal. This slit can quickly close like a sliding door and, when combined with upper and lower eyelids, is extremely effective in excluding light. Many reptiles and even small domestic cats have slit pupils. (Large cat breeds, such as lions, have round pupils.)
Another night adaptation in certain species acts to intensify light that already exists. Just behind the retina the animals have an additional layer called the tapetum lucidum, Latin for bright carpet. This acts as a mirror, reflecting light that has already passed through the retina, giving the rhodopsin on the rods a second chance to capture light that wasn’t absorbed the first time. Any light that isn’t captured on this second trip exits back through the eye.
This creates what is called “eye shine.” On dark nights, a flashlight beam or car headlight will bounce back from the eyes of certain animals. This is the reflection from the tapetum layer. Depending on the animal, the reflection can be red, yellow, blue, green, or white. Since only certain critters have this layer, it is possible to identify the species by a combination of eye shine color and animal size.
From your personal experience, you realize that it’s not possible to step out of your motorhome door on a moonless night and be able to see the owl on that dead snag, and certainly not the small mouse in the meadow. Human eyes haven’t adapted to the night as well as many other creatures. But we can take some simple steps to improve our night sight.
First, let your eyes adjust to the dim light. In five or 10 minutes you can begin to see better, but it takes 20 to 30 minutes before your eyes fully adapt to the dark. And remember, any exposure to intense light will “bleach” the rhodopsin, and your eyes will have to start the recovery process all over again.
If you must have some light for safety (or to read a map or sky chart), use a low-intensity red light. Rhodopsin in humans is insensitive to the color red, so cover the lens of a small flashlight with red cellophane or plastic and use it sparingly.
Of course, there is always the problem of random headlights from passing cars. A partial solution is to quickly cover one eye with your hand. Only the exposed eye will lose its night vision, and as the dark returns, the protected eye will still be adjusted to the night.
Because the cones in our retinas are much more concentrated at the center, try using your peripheral vision in the dark. Instead of looking directly at something, try focusing near the object and you’ll put more rods to work. And don’t stare “” keep your eyes moving. By doing this, the individual rods will not get acclimated to a particular light source, however dim, and will remain more sensitive.
The last tip is to look for shapes rather than colors. We are so accustomed to a multicolored world that we do much of our identification by color. At night you lose that advantage, so learn to watch for shapes and movement “” the specialty of the rods in the eye.
Human eyes may not be specifically adapted for night vision, but much of the animal world is active after the sun sets. Dress warm, get outside, and let your eyes adjust. You might not have the night sight of an owl, but you certainly can see enough to watch as it decides whether to investigate that mouse on the other side of the meadow.