The eyes of crocodilians are very close together, and only 7 cm separates them in a 5 m long animal. Most crocodilian hunting takes place at night, using the "eyeshine" to detect the animal. This "eyeshine" is a reflection of light from the retinal tapetum, and it can be seen from quite a distance away. When a spotlight or torch is shone on a crocodilian at night, a red reflection from the eyes results. Alligators and caimans have colour vision, and it is likely that all crocodilians have it. This image intensifying device, in combination with at least two different types of receptors in the retina, allow crocodilians to see better in low light situations. Light passing through the retina is reflected back through it by these crystals. At the back of the eyeball, behind the retina, is a thin layer of guanine crystals (retinal tapetum).
The eyes of crocodilians are focused for aerial distance viewing, and it is unlikely that their vision underwater is good.Īs in many other nocturnal animals, the pupils close to a vertical slit in bright light and open to a full circle in the dark.
The eyeballs themselves can be drawn into the eye sockets, presumably to avoid injury during attacks on prey or when fighting with other crocodiles. Above and below the eye are the conventional eyelids, which cover the eye completely. Firstly, they are protected by a transparent eyelid that moves sideways across the eye when the animal submerges or attacks prey. The eyes of crocodilians are specialised in a number of ways. When prey are being held in the water, the mouth may be open, but the palatal valve is closed, preventing water going down the throat - and breathing takes place through the nostrils. When in water, the mouth is usually closed and they breathe mostly through their nostrils. When basking on land with the mouth open, crocodilians breathe mostly through their mouth (the throat/palatal valve is open). At the back of the throat is a palatal valve that can be opened or closed. A second route of breathing is through the mouth. Along these canals are chambers in which "smell" is sensed - crocodilians have a very good sense of smell. These lead into canals that pass through the bone of the snout, and open into the back of the throat. The nasal disc on the tip of the snout contains two nostrils, each with a protective valve or flap at their opening. The changes in snout shape have not compromised this basic crocodilian posture, even though the two groups have been separated from each other for some 60 million years. Alligators, for example, have a broader snout than crocodiles, but, when in their "minimum exposure" posture in water, the two appear identical - the increased snout width is under the water. This "minimum exposure" posture has been important to crocodilians throughout their evolution. To potential prey, the exposed areas of the head give little indication of the real size of the predator's body. All the sensory apparatus is thus exposed while the most of the snout length, and the bulk of the body, is hidden. The shape of the head is intimately associated with the way crocodilians position themselves in water.Ĭrocodilians have a "minimum exposure" posture in water, in which only the eyes, the cranial platform (overlying the brain), ears and nostrils lie above the waters' surface. The head is typically one seventh of the total body length, regardless of whether the species has a narrow or broad snout. The elongated snout of crocodilians is probably one of their most distinctive features. They have a long tail and the limbs are short and straddled sideways from the body rather than being erect beneath it, as in mammals.
In general, the body form of crocodilians is "lizard-like".