Megan's Nature Nook
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We’ve all had that moment when you’re driving in the dark and suddenly you see eyes glowing on the side of the road! But why does that happen? Why do animals’ eyes seem to glow in the dark but not humans’ eyes? Many animals, especially ones that are more active at night, have a special reflective surface behind the retina called the tapetum (Ta-PEA-tum) lucidum or “tapestry of light.” This reflective surface helps them to see better in low light conditions, but how exactly does that work? Light such as moonlight or starlight enters the eye and hits photoreceptors that transmit information to the brain. Sometimes there is not enough light for the brain to process an image, so the tapetum lucidum works like a mirror and bounces light back for a second chance for the photoreceptors to capture the information for the brain to process. Not all animals have a tapetum lucidum. Animals that do have it are usually most active at night. The tapetum lucidum boosts their night vision and helps them to see in low light conditions. There are many mammals, reptiles, invertebrates (like spiders), amphibians, and some nocturnal birds that have a tapetum lucidum. Owls which are often active at night do not have a tapetum lucidum but have other adaptations that help them to see at night. Although humans’ eyes may “glow” red in photographs we do not have a tapetum lucidum. What you are seeing when that happens is the flash reflecting off red blood cells behind the retina.
When you’ve seen an animal’s eyes glow, have you ever noticed that the color of the eyes can vary? Glow or eyeshine color can vary by species so you can sometimes use it to help identify the animal in the dark. Different eyeshine colors are caused by the mineral content (like Riboflavin or Zinc) in the tapetum lucidum and pigment amounts in the retina. Age of the animal can also affect the color so color isn’t always a dependable way to identify an animal. Although color can vary for each individual, even animals of the same species, there are a few generalizations that can help you identify whose eyes are glowing even if you can’t see the rest of the animal. Deer generally have a white glow, raccoons have yellow, and house cats are green. While researching I found many lists with more animals and colors, but with cross referencing not many of them added up. This could be because of how people perceive colors, yellow to one person may look more orange to another. Or the mineral content, eye pigment, and animal ages cause enough of a variation that you can’t easily categorize all individuals of a species into one color category. Luckily there are more things we can look at the help identify an animal in the depths of night. Besides the eyeshine color, you can look at the shape and size of the eye, placement (side vs front) on the head, pupil slit orientations (think cat eyes), and the height of the eyes (small animal would be closer to the ground than a larger animal). Now the next time you see eyes shining back at you in the depths of the night, you’ll know why and you’ll be able to identify who those eyes belong to!
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