Backyard Biology

Nature stories from my backyard and beyond

Nature Activities

Night Vision

Friday, October 30, 2009

A hunter sits motionless in the woods trying to blend in with the scenery, hoping to go unnoticed by his prey. He is wearing bright orange clothing.
A skunk is wandering boldly through the woods hoping to be noticed by his predators. He wears no bright colors – just black and white.

To understand why this is so, one must understand something about night vision. Us humans, and most other daytime animals, see the world in a wonderful array of colors. The colors we see come from light. White light from the sun contains all of the colors of the rainbow. When this white light strikes an object, some of the colors are absorbed by the object and the rest of the light bounces off. The colors we see are the colors that bounce off. So in the picture below, blue light is bounced off or reflected from the paper strip to the right, red light is reflected from the next strip and so on.colored paper image








However, if no light – or very little light – is striking an object, then no colors will be reflected. Objects will look either black – total absence of light – or gray. No light – no color. Here is a simple experiment to test this:

  • Collect some colored paper strips (like the ones in the picture above)
  • Show them to a friend on a dark night and ask your friend what color they are.
  • Now shine a light on them to show your friend what colors they really are.

It shouldn’t matter what colors your paper strips are. In dim light, they should all appear some shade of gray. Dark colors should appear the same dark shade of gray and light colors should appear the same light shade of gray, like the picture and white paper image








In our eyes, we have two kinds of light receptors - two kinds of cells that can detect light. We have cones that allow us to see color, and rods that can only detect shades of gray. The cones allow us to see colors in the daytime and the rods allow us to see in low light situations.
Since night time animals do not need to see color - there is no color at night - they have no need for cone cells in their eyes. Instead their eyes consist mostly or totally of rod cells. Their vision in low light situations is excellent but they can’t see color.

Hunters that hunt during the daytime for night time animals, like deer, want to be seen by other hunters. So they wear bright orange. To other hunters, the orange is very visible and warns them that other hunters are in the vicinity. However, to deer who can’t see color, the orange appears as a shade of gray along with all of the other shades of gray in the woods. If the hunter does not move, the deer may not see him until it is too late.
A skunk wants to be seen by all; to see a skunk is to leave it alone. No one wants to go unnoticed by a skunk until it is too late. So why isn’t a skunk bright orange like the hunter? After all, a bright orange skunk would stand out like a sore thumb in the woods. In the day time. But skunks are creatures of the night. And color disappears at night. Therefore a bright orange skunk at night would appear gray. Blending in with all the other grays in the woods at night. The color that is most easily seen at night is white. Therefore it is to the benefit of the skunk to be black and white rather than black and orange. Look at the two pictures below. While the orange skunk may be quite obvious in the light, the white skunk is definitely more visible at night.skunks image








black and white skunks image











Answer to the Nature Story riddle:
Eye shine. Many night creatures have a shiny layer in the back of the eye that redirects the light back through the retina - the part of the eye with the light-sensitive cells. This layer is called the tapetum. The structure of the tapetum may be different in different animals, but the end result is the same. Light enters the eye and stimulates the cells in the retina. Then the light hits the tapetum and is reflected back through the tapetum, stimulating the retinal cells a second time. The animal sees when the light comes in and it sees again when the light goes out. This enhances an animal’s night vision.
Some of the light is reflected back out the eye, and this is what we see as eye shine.