Becky Haynie, age 13, of Reedville, Virginia, for her question:
Do our eyes reflect light like those of certain animals?
If humans could reflect that glowing eyeshine, they could prowl around after dark hunting for mice and such. Actually, people do not care for this type of occupation. But cats and other night prowling animals think it's just fine, perhaps because their hunting forays are successful. Eyeshine is the main reason why they succeed. Humans do not have this kind of night vision because it does not suit their particular life style.
The different senses gather various impressions from outside and inside our bodies and convey them to the brain to be interpreted. For humans and for many animals, by far the most useful of the senses is sight. We depend on vision to gather perhaps 90 per cent of our impressions of the outside world. The survival of most familiar wild mammals depends mainly on sight though the dogs also have fantastic sniffers. Though the gift of sight is bestowed on humans and numerous animals, nature did not bestow a sort of standard utility type of vision on all species. In order to maintain the give and take necessary in a balanced ecology, every species, including man, has a different life style: The eyes and vision of each species are designed and adapted to its particular survival needs.
Cats, dogs and opossums are night prowlers by nature. The cats and dogs are meat eating carnivores, the opossum thrives on a mixed diet of meat, fruit and vegetables. All these night hunters need special vision to detect their small shadowy victims in the shadowy darkness. Eyeshine provides a special reflection that makes this job easier.
When hunting by starlight, a cat, for example, opens her slit pupils to the full. They fill almost her whole eyes and gather a maximum of the dim light available. Our pupils also open wide when the light is dim and a shadowy image of the scenery falls on the retina screens in back of the eyeballs. This is the best we can do. Puss can do better because there are layers of glassy cells behind her retinas.
These act as reflectors and mirrors. Dim light from the scenery passes through the retinas and reflects back through them a second time. This gives a double exposure, which darkens the shadows and lightens the brighter patches. We see the reflected light from behind the retinas as eyeshine.
It enables nocturnal animals to see better in a dim light than we do. But they cannot see in total darkness. And, as usual, nature plays no favorites. The eyeshine that helps a cat to spot a mouse in the shadows also may signal a warning to the mouse. Let's not wish to trade .for the eyeshine eyes of a dog or cat. For one thing, their mammalian eyes do not have the glorious gift of color vision.