Reptiles Eyesight: The class Reptilia includes crocodiles, alligators, snakes, lizards, worm lizards, tortoises, turtles, and tuataras.  A reptile needs to recognize the shape, of prey. It also needs to monitor the movements of mates, offspring, and predators. Reptiles that live primarily underground or in muddy water have very small eye. Day active reptiles that live aboveground often rely on sight ore than on their other senses.

A reptile's eye work much like the human eye. Light waves that are reflected from objects pass through a "window" in the eye called the cornea and enter a slot in the iris (the colored part) called the pupil.  The light waves then pass through the lens, which focuses the image. They finally reach the back of the eye, or retina, which is made up of two types of cells that send messages to the brain: rod shaped cells that convey objects shapes and cone shaped cells that convey their colony. The brain interprets these messages as an image, which is what the reptile sees.

A tree snake focuses by aligning its pupils with the grooves on its snout.  The caimans eyes are high on its head.  The snapping turtle has binocular vision. The tuatara has a tiny "third eye" on its head.

Pupil Shape: Some reptiles, including crocodiles, pythons, vipers, tuataras, and geckos, are active at night but also spend time basking in the sun.  Their pupils are unusually vertical slits, which can be closed more completely in bright light that round pupils.  The gecko's pupils close in daylight, leaving four tiny pinholes in each one.  Only a small amount of light enters through these holes, but the gecko's view is sharpened because it is composed of four superimposed images.

Eye Protection: Certain reptiles have eyelids to protect their eyes. But the eye lids differ from those of mammals as the reptiles lower eye lid is usually larger and more movable than its upper one. Snakes do not blink because they keep dust out of their eyes with fixed, transparent "eyeglasses" that ae called brilles. Geckos also have brilles, which they sometimes lick clean with their tongues.

Other reptiles have a third eyelid called a nictitating (winking) membrane. This transparent fold of thin skin is regularly drawn across the cornea to clean and lubricate its surface. Crocodiles draw this membrane over their eyes when they swim underwater. Some reptiles use tear glands for protection, but sea turtles use their tears to get rid of excess salt.  The tears are most noticeable when a sea turtle comes ashore to dig a nest on a sandy beach. The tears cause grains of sand to stick to the turtle's eyes and face.

Color Vision: Many lizards seem able to distinguish between colors. For example, some can recognize the red and black markings on insects that are harmful to eat.  Giant tortoises can see in color, and some terrapins can see in color, and some terrapins seem sensitive to red light.  They also appear to see infrared rays, which are invisible to human.  Crocodiles and snakes, however, are color blind. In low light, American night lizards she shape rather then color. Their retinas contain more shape sensitive rod cells than color sensitive cone cells.  In poor light the granite night lizards eyes distinguish shades rather than color.

Key Facts:

Monocular and Binocular Vision: A reptiles vision depends on the position of its eyes. A reptile with monocular vision has eyes on either side of the head.  The area that one eye can see does not over lap the area the other eye can see.  Land tortoises, crocodiles, and many lizards have monocular vision.  A reptile with binocular vision has both eyes facing forward. The area that each eye can see overlaps. Reptiles with binocular vision, like the snapping turtle, focus both eyes on their prey and can accurately judge its distance.  A chameleons eyes swivel independently, back and forward through 180 degrees, also up and down. This allows it to scan a wide area for prey. When it finds a cictum close enough to kill, it focuses both eyes on its prey.

Heat Detection in Snakes: Some snakes detect prey by sending their body heat. Boids (members of the boa family), including boa constrictors and pythons, have a row of up to 12 heat sensitive organs.  Pit vipers, including rattle snakes and moccasins, are named for the heat sensitive pit organs between the eye and the nostril on either side of the head. The tiny it detects infrared heat radiated by its potential prey. This information is focused onto a surface of several thousand nerve ending and passed to the brain, where it is seen like signals coming from the eyes.

The result is a heat picture that reveals the lactation of any warm blooded creature nearby. A pit viper can detect temperature changes as small as half a degree.  Pythons and pit vipers also use heat sensors in the mouth to guide them when prey is near enough to kill.

Heat Sensors in Pit Viper: The North America water moccasin is a pit viper. Its well defined, highly sophisticated pit organ (enables it to detect the presence of warm blooded animals and sense minute temperature changes.

Heat Sensors in Boid Species: The reticulate python is more primitive than a pit viper, having evolved from a different origin. It has a row of several heat sensitive organs but they are less sophisticated than those of the pit viper.

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