By Dr. David Wartinbee, for the Redoubt Reporter
At this time of the year in Alaska, a drive of more than a couple hundred miles will involve some time during darkness. While heading north to visit friends during the holidays, I spotted a small, bright spot in the roadway ahead.
As my lights got closer, a huge, brown ungulate appeared surrounding that tiny light spot. I slowed appropriately. A few miles farther down the road, a pair of bright spots appeared on one side of the road. A second later I was able to see the faintly lit image of a lynx crossing the road. I had first seen both of these animals because of their tapetum lucidum reflecting my headlight illumination.
While many are familiar with the phrase, “A deer-in-the-headlights look,” not as many may realize there is an interesting anatomical basis for this situation. Most nocturnal animals, like dogs, cats, deer, etc., will demonstrate “eye shine” when a bright light is shown on them at night. What happens is the light entering the animal’s eyeball is being reflected right back at us as if there were a mirror in there. The mirror analogy is actually pretty close to what is happening.
In order to understand how the tapetum lucidum works, we have to know a little about the layers inside the eyeball itself. First, the retina is the thin, innermost layer of the eye and it contains the light-sensitive cells called rods and cones, and lots of blood vessels. In very close proximity to the rods and cones is a black layer called the choroid.
The important choroid layer absorbs light that has just passed by the light-sensitive
rods and cones. In humans that do not have a heavily pigmented choroids, like albinos, to absorb the passing rays, light gets reflected and scattered inside the eyeball. These individuals suffer with visual difficulties and even small amounts of light are blindingly bright.
In nocturnal animals with a tapetum lucidum, the choroid, or a special portion of the retina, will act as a slight reflector. The actual tapetum lucidum can vary in its composition depending on the specific animal. Since there are so many different animals that exhibit “eye shine,” it is easy to understand that there are many different kinds of reflective layers.
In some, it is composed of a special layer of iridescent crystals like guanine, or in others it might be a layer of cells with reflective fibers.
No matter how the reflection by the tapetum is accomplished, the incoming light is reflected right back past the very rods and cones that it originally passed. This essentially gives the rods and cones a second chance to detect the same light rays. That simple reflection greatly enhances night vision.
Also, by reflecting the light exactly back from where it came, the animal is able to create a crisp visual image rather than getting a blurry image that would occur if the light scattered randomly within the eyeball. It is believed that cats can see about nine times better at night than we can, since they have a tapetum and we do not.
The color of the reflective “eye shine” will vary according to the animal. Many reptiles
show bright red eye shine, while mammals vary from yellow, white, blue and green. There are variations even within the same species and there are lots of examples of one eye reflecting back one color while the other eye reflects back a different color. Along with birds, some fish, like walleye, have a tapetum.
Most primates — like humans — pigs, kangaroos and other day-active animals do not have a tapetum lucidum. Accordingly, our night vision is mediocre compared to those animals that are active at night.
Humans can produce a confusing situation known in the photography arena as “red eye.” This occurs when a flash of light illuminates the blood vessel-rich retina in the back of the eye and the eyes appear to be red. There is no tapetum involved here, just illumination of blood vessels. If the source of the flash were somewhere other than on the camera, there would be no “red eye.”
Imagine how our flash pictures might look if we had chartreuse-colored blood?
Another unusual situation in humans and some animals is the white glow that can appear in the eyes. This “eye glow” occurs when there are cataracts. The glow is caused by illumination of the crystals embedded within the lens of the eye and, again, is not caused by a tapetum.
Moose certainly have a tapetum lucidum, but I have wondered why we don’t often see the same bright headlight reflection from moose eyes that can be seen from smaller animals like dogs or cats. The answer probably comes from several different reasons.
First, moose are large animals and their eyes are often held above the level where our headlights are aimed. Because a moose’s eyes are on extreme sides of its head, it is difficult to see both of their eyes at the same time. So we most commonly get a reflection from only one eye at a time.
I have not been able to find much documentation on the size of a moose tapetum compared with other animals. However, after shining a bright flashlight beam at a moose from various angles, I have discovered that the tapetum in a moose’s eye is mostly in the very back of the eyeball.
When the moose is at right angles to you, your lights are not striking the tapetum, and you only get a slight “red-eye” reflection.
Only when the moose is looking directly at you, and they don’t do this all that often, will you see the bright, two-eyed reflection.
So, as a moose crosses the road, you and I get a single, slight reddish glow from an eye that is not reflecting back much light from our headlights. No wonder we don’t easily see that moose crossing up ahead.
The next time your lights fall on an animal and you see the distinctive “deer-in-the-headlights” reflection, you will know that particular animal is normally active at night. You’ll also know they have a microscopically thin tapetum lucidum in the back of their eyeball that aids in their night vision.
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Cook Inlet watershed.