Vision in nature

Eyes are such a useful piece of survival equipment that many taxonomists (biologists who categorise animal species) believe they have evolved independently over 40 times. Because of this, not all animals see in the same way. In fact, only our closest animal relatives are thought to be able to see in a similar way to humans.

Positioning of the eye

A good clue to how any particular animal sees is in the positioning of the eyes. Monkeys and apes, including our own species, for example, have eyes in the front of their heads. Where each eye’s field of vision overlaps with the other’s, we have stereoscopic, or 3D, vision.

Some other animals, like rabbits and horses, have eyes that are further round to the side of their heads. This gives them much more peripheral vision, which allows them to see any lurking predators coming up from behind. The same is true of animals, such as ducks, that have eyes near the top of their heads.

Some creatures, such as squirrels, have eyes that are near the top and side of a narrow face, giving them good 3-D and peripheral vision - excellent for judging the distance of tree-top leaps and watching out for predators.

Compound and other eye types

Many insects have compound eyes. These are clusters of eyes arranged in two bubble-like mounds on their heads. The dragonfly, for example, has around 30,000 of these lenses. These give insects superb all-round vision, which is one reason why flies can be so tricky to swat!

Some creatures have eyes on the ends of stalks. A snail’s eyes, for example, are mounted at the end of tentacles which it can bend and twist to get a closer look at things. A crab’s eyes are on the ends of stalks that it can stick up like periscopes when it’s buried in the sand.

You may have noticed that some animals’ eyes reflect light at night. Deer, cats, crocodiles and even moths have eyes that do this and it’s because they have a tapetum. A tapetum is a reflector at the back of the eye that effectively doubles the amount of light the eye can use, helping an animal to see at night. Humans don’t have a tapetum, the only time our eyes seem to reflect light is when they look red in some pictures. This happens because the flash of a camera is too fast for the iris to close the pupil, so the light is reflected by the retina. The blood around the retina being lit up is what causes the "redeye" effect in photos.

Seeing in colour

Not all animals see in colour and even those that do often can’t see the full spectrum of colours that we can. A dog, for example, can distinguish between several different colours, but cannot tell red from green.

Some animals can see parts of the spectrum that we can’t. Penguins and bees, for example, can see ultraviolet (UV). Bees use this sight to pick up special patterns on flower petals that act as signposts to the nectar. It also means they can find their way back to the hive even in cloudy weather.

Sea birds such as gulls and terns have developed something else that we haven’t. Their retinas contain minute droplets of reddish oil, which screen out much of the blue light, cutting down on glare from the sea and sky - much like inbuilt sunglasses!

Compound eye image courtesy of BioMEDIA ASSOCIATES-

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