Vision in nature

You’ve probably already noticed some differences between your own eyes and the eyes of animals (even I see differently from you, I see more in pastel shades than you do). Maybe you’ve even wondered why they’re different. For example, why do cats’ pupils sometimes look like long slits? And why do pups eyes sometimes look like yellow discs of light in photos, when people’s eyes look red? And why don’t goldfish seem to blink?!

Animals’ eyes come in lots of different shapes and sizes and most of them are made differently from ours. For example, cats’ eyes have evolved to be very good at seeing in the dark. Because of this, their pupils can open very wide to let more light in when it’s dusk or nearly dark. When it’s bright, on the other hand, they need to be able to close their pupil down, so they’re not dazzled. Round pupils aren’t very good at becoming very small, so instead, the cat has pupils that become vertical slits when they get smaller - that way, they let in only a narrow band of light. When it’s very bright, the cat needs to stop even more light coming into its eyes, so it will narrow its eyes by lowering its eyelids. This way, a cat can make a very small hole indeed for light to get through.

Another thing that helps cats and other animals see better in the dark is something at the back of the eye called the tapetum lucidum (which is Latin for bright carpet!). This is more like a mirror than a carpet and it reflects light back into the eye. In this way, cats, dogs, deer and even crocodiles get a "second chance" to use the light that enters their eyes, making it easier to see in the dark. Clever, huh?! This is also the reason why camera flashes and car headlights can make an animal’s eyes seem to glow in the dark.

Some animals, like kangaroos and goats, have pupils that are like a horizontal slit. Animals that are totally nocturnal, like owls and bushbabies, usually have very large eyes, while those that live mostly in tunnels underground, like moles, often have very small, weak eyes. Some, like earthworms, are blind. Slugs and snails have eyes on the ends of tentacles, so they can see round corners.

Insects often have compound eyes, which are made up of hundreds or thousands of tiny eyes. These often give good all-round vision, which is one reason why flies can be so hard to surprise - they see you coming! Bees and butterflies can see ultraviolet light, which we can’t see at all. This helps them to find nectar in flowers that have special ultraviolet patterns on them, like invisible signposts!

The giant squid has eyes that are about 25cm across, and can even be as much as 40cm across. Which is bigger, your computer screen or a giant squid’s eye? Leeches, on the other hand, just have two "eye spots" - very basic eyes that don’t work very well.

The sharpest eyes of all belong to the birds of prey. American kestrels, for example, can see insects that are just 2mm long from 18 metres away, and hawks can see from 6 metres away what you can from 1.5 metres away. If you had eyes like that, you’d probably be able to read this from the other side of your classroom!