Fore Eyes

Virtually all of the wild critters I encounter around here have their eyes located on either side of their head—birds, reptiles, fish, deer, mice, rabbits… even our dog. We humans are once again the oddball, with eyes in the front of our heads. While this brings us binocular vision, all the others have panoramic vision. I got to wondering about the advantages and disadvantages of each.

It seems that evolution saw to it that creatures who live in daylight do need eyes (a good starting point), but some of us evolved to have our eyes point in the same direction, while others have side-pointing eyes. Why? Evolution rewards advantageous developments (which often spontaneously occur and then get passed on, while the disadvantageous ones die out). Thus, for panoramic vision, the advantage is being able to see pretty much in all directions at once. Small animals need to evade numerous predators, and need to see them coming from any direction. So nature gave them eyes on either side of their head. In contrast, large animals are usually the predators, so they need to be able to have good depth perception—so they got forward-looking binocular vision. That’s their advantage. It allows them to pursue little critters and know when and how to pounce, so as not to miss.

Recent research has shed more light on the issue. It seems that large animals who evolved in cluttered environments—like primates in the jungle (that’s us)—have forward-facing eyes that can see “through” the clutter. Hold a finger up a foot or so in front of your face. You are able to look past or “through” the finger, without any important object in the distance being blocked. Our eyes, spaced a few inches apart, give us binocular vision; which may be limited in its field of view, but any dangling leaves in our face don’t bother us. We can still see our prey (or foe).

Critters who evolved in open environments, however, don’t need binocular vision—nothing is in their face. They get an advantage with panoramic vision. What’s more, small animals (insects and mice) have their eyes much closer together than we do. Even a fat blade of grass—let alone a leaf—gets in their way. (Place your whole hand in front of your face—like a leaf to a mouse. Can you see things behind it? Watch out! You’ve just missed seeing an attacker.)

So now I can understand the advantage my bird friends have, with eyes on each side of their head. It’s hard to sneak up on them. But I bet that if I tossed one of them a ball, he’d do a poor job of catching it.