Which Lizards Can Move Their Eyes in Two Different Directions?

Chameleons are the only lizards who can see in two different directions at once. A chameleon's eyes protrude from his head on what looks like tiny turrets. The cone-shaped eyelids swivel with the eyes, exposing only the pupil. This lets him keep a fairly narrow field of vision with each eye as the eyes move independently of each other.

Chameleons have monocular vision, which means their brains can process the images from both eyes separately. By itself, this isn't unusual in the animal world; many prey species, such as deer, have eyes on the sides of their head that see two different images at once. But chameleons add a new element to the monocular vision, as the only animals who can move their eyes in different directions while capturing two images at once. When a chameleon spots his prey, he can swivel both eyes forward for instant binocular vision, giving him the spot-on depth perception necessary to accurately shoot out his tongue to grab his next meal.

Most predator species have eyes in the front of their faces to focus sharply on prey using binocular vision. Chameleons have another trick that helps them become successful hunters while using monocular vision, allowing them to spot prey while their eyes move in different directions. Their eyes have uniquely shaped lenses and retinas that allow them to focus extremely fast and zoom in on potential prey, making images appear larger than they actually are so the chameleons can quickly differentiate between a tasty insect and a yucky leaf. Their eyes work basically like telephoto camera lenses.

Although a chameleon is the only lizard that can move his eyes in two directions at once, he's not the only lizard with interesting eyes. Some types of horned lizards squirt blood from their eyes, sometimes as far as several feet, to confuse predators and give the lizards time to escape. Lizards such as the green iguana have a third eye on the top of their heads, used to help them determine when there's enough light to bask in the sun and help regulate hormone production. These extra eyes can distinguish green and blue colors, but they're not advanced enough to see shapes. Most lizards, including chameleons, have some sort of eyelid. But some, such as a gecko, don't have eyelids in the traditional sense. They use a clear membrane to keep the dirt out, but they don't have skin-covered lids to close.