How Exactly Are Nocturnal Animals Able to See in the Dark?

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  • Topic: Eye, Photoreceptor cell, Retina
  • Pages : 2 (494 words )
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  • Published : September 29, 2012
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How exactly are nocturnal animals able to see in the dark?

Light entering our eyes is projected on the back surface of the eye (called the retina) where it stimulates light-sensitive photoreceptor cells. These cells convert the image of the world that lands on the retina into information the brain uses to create our perception of the world. But photoreceptors require a certain amount of light in order to respond. Nocturnal animals are active mainly at night so they face a challenge: They must gather enough light to stimulate the photoreceptors. One way to accomplish this is to develop very large eyes. Many nocturnal creatures do have large eyes but there are limits to how large they can be.

Why not have photoreceptors that are especially sensitive in low-light conditions? Nocturnal animals (and humans) use this strategy as well but there is a trade-off between the ability to see in near-darkness and the ability to see fine details. In the human retina there are two types of photoreceptor cells: cones and rods. Cones allow us to see fine details better than rods do, and cones give us color vision, but it takes more light to stimulate cones than rods. At night, there is not enough light to stimulate cones so we get around primarily using our rods—this is why the world looks blobby and colorless at night. Our rods are remarkably sensitive though: they can respond to the light of a candle 17 miles away. Some nocturnal animals—like the galago, a furry, squirrel-sized mammal that is related to monkeys—have a retina that is packed with rods. Galagos are able to be active at night in part because all of their photoreceptors are the more-sensitive rod variety (galagos also have large eyes). But without cones, galagos cannot see colors and they cannot make out fine details the way humans do.

Another way to improve night vision is to give light more than one opportunity to stimulate the photoreceptors. In humans, light that enters the eye can be absorbed by...
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