Many animals have two eyes so they can see things in 3 dimensions.
The STEREO satellites use the same idea.
Ever wondered why we have two eyes?
Try this. Find a partner, move apart, both close one eye, then throw a ball to your partner. How easy it is for your partner to catch the ball? You use both eyes to judge distances and speeds. This ability of ours is called stereoscopic (binocular) vision and probably evolved as a means of survival.
Not all living creatures have stereoscopic vision. Some animals (eg horses, rabbits and cows) have an eye on either side of their heads. This increases the area they can see, particularly around the sides. These animals tend to live on plants, but need to watch out for predators. Carnivorous animals (eg cats, lions) hunt and eat meat . They need to move fast and accurately judge the location and speed of their prey.
Many birds (eg hawks and owls) also have binocular vision, so that they can swoop down on their prey and judge distances precisely. Some birds (eg chickens and pigeons) do not have binocular vision. They judge distance by moving their heads about. You have probably judged distances in this way when travelling in a car or on the train. The closest objects seem to speed by, while the more distant objects can be seen for longer.
Do fishes have binocular vision?
Most do not. They have eyes on either side of their head. Some flatfish have two eyes on one side of their head. They look a bit odd!!
Flounder Illustration by Dr Tony Ayling
How does 3D vision work?
Binocular vision relies on the fact that we have two eyes, about 3 inches apart. Each eye sees the world from a slightly different perspective. The brain fuses these two views together. It understands the differences and uses these to estimate distance, creating our sense of depth and 3D vision.