Snake
Perception

Some of the most highly developed sensory systems are found in the Crotalidae, or pit vipers—the rattlesnakes and their associates. Pit vipers have all the sense organs of other snakes, as well as additional aids. Pit refers to special infrared-sensitive receptors located on either side of the head, between the nostrils and the eyes. In fact the pit looks like an extra pair of nostrils. All snakes have the ability to sense warmth with touch and heat receptors like other animals;however, the highly developed pit of the pit vipers is distinctive. Each pit is made of a pit cavity and an inner cavity, the larger one lies just behind and generally below the level of the nostril, and opens forward. Behind this larger cavity is a finer one, barely visible; the cavities are connected internally, separated only by a membrane with nerves that are extraordinarily attuned to detecting temperature changes between. As in the overlapping vision fields of human eyes, the forward-facing pit on either side of the face combined produces a field of vision: a pit viper can distinguish between objects and their environments, as well as accurately judge the distance between objects and itself. The heat sensing ability of a pit viper is so great that it can react to a difference as small as one third of a degree Fahrenheit. Other infrared-sensitive snakes have multiple, smaller labial pits lining the upper lip, just below the nostrils.

A snake tracks its prey using smell, collecting airborne particles with its forked tongue, then passing them to the vomeronasal organ or Jacobson's organ in the mouth for examination. The fork in the tongue provides a sort of directional sense of smell and taste simultaneously. The snake's tongue is constantly in motion, sampling particles from the air, ground, and water, analyzing the chemicals found, and determining the presence of prey or predators in the local environment. In water-dwelling snakes, such as the anaconda, the tongue functions efficiently underwater.

To pick up particles in the air, the tongue is flicked out. Like a hand getting the weight of something, the fork in the tongue simultaneously provides a sort of directional sense. Snakes have a good sense of smell, but this sense is greatly enhanced in the window of a special organ, the Jacobson's organ. As the tongue is peeled back into the mouth, the forked tip is pressed into the cavities of the Jacobson's organ. Withdrawn to a point, the tongue and the Jacobson's organ work in concert for a taste-smell analysis. The organ itself gives the snake an extrasensory conduit. Quite literally, the snake gets a taste of the neighborhood, capable of slithering in rooms of information like the doors are open.

Up until as late as the mid 20th century it was assumed snakes could not hear. In fact snakes have two distinct and wholly independent systems. One of these systems, the somatic, involves transmission of frequencies through ventral skin receptors via the spine. The other system involves vibrations that are transmitted through the snake's attenuated lung to the brain via cranial nerve. A snake's sensitivity to vibration is extremely high. In a quiet room, a snake can hear someone speaking softly.

Snake vision varies greatly between species. Some have keen eyesight and others are only able to distinguish light from dark, but the important trend is that a snake's visual perception is adequate enough to track movements. Generally, vision is best in tree-dwelling snakes and weakest in burrowing snakes. Some have binocular vision, where both eyes are capable of focusing on the same point, an example of this being the Asian vine snake. Most snakes focus by moving the lens back and forth in relation to the retina. Diurnal snakes have round pupils and many nocturnal snakes have slit pupils. Most species possess three visual pigments and are probably able to see two primary colors in daylight. The annulated sea snake and the genus Helicops appears to have regained much of their color vision as an adaption to the marine environment they live in. It has been concluded that the last common ancestors of all snakes had UV -sensitive vision, but most snakes that depend on their eyesight to hunt in daylight have evolved lenses that act like sunglasses for filtering out the UV-light, which probably also sharpens their vision by improving the contrast.
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