Crocodilian Biology Database - Integumentary Sense Organs

Integumentary Sense Organs

General Biology / Sense Organs: Ways in which the crocodile can increase the amount of information it receives from its surroundings

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GENERAL BIOLOGY

Integumentary Sense Organs

Head of alligator, sensory pits highlighted

LOCATION:	Upper jaw, lower jaw and head of all crocodilians, extending over most surfaces of neck, body, tail and inside legs of subfamily Crocodylinae and Gavialinae.
TYPE:	Dermal sensory receptors
FUNCTION:	Largely unknown - pressure changes, sensory role, underwater prey detection around head, possibly sensitive to changes in salinity in other areas

All crocodilians possess 'integumentary sense organs' (ISOs)- that is, sensory cells present in the integumentary layer (skin). These ISOs are often called a number of different names, primarily because their exact function still hasn't been conclusively determined. The image above shows the head of an alligator with an enlarged view of the sensory pits on the outer edge of the lower jaw. In the Alligatorinae family (alligators and caimans) these are the only place that ISOs are found - the upper jaw, nose, around the eyes, lower jaw, even the upper palate. There are none elsewhere on the animal.

GULAR REGION (NECK) OF AMERICAN ALLIGATOR

GULAR REGION (NECK) OF AMERICAN CROCODILE

ISOs in members of the Crocodylinae and Gavialinae family are not restricted to the head, but are distributed all over the body. Distribution varies between species, and this is possibly related to their function. On some areas (e.g. around the jaws) there is at least one ISO per scale.

Some scales possess three or four ISOs. More usually, only one ISO is present per scale (e.g. belly in crocodiles). Except on the head, ISOs are normally positioned around the outer edge of a scale, and are visible as a pigmented spot.

Function and structure of ISOs varies depending on their position around the body. Those around the jaws are mechanoreceptors which can detect pressure changes. Almost certainly, these sense organs are used when the crocodile is underwater to sense the proximity of prey items. A fish swimming past the head of an alligator, for example, will fire the sense organs as pressure waves from its movement through water impinge upon the ISOs. It is likely that the wave of firing across the head of the alligator tells it where the pressure wave is coming from (i.e. a directional sense), and hence where the prey is. Some of these ISOs may also play a sensory role while searching for prey. If an alligator is searching for food on a substrate, rocks and pebbles do not elicit a biting response. A piece of meat, however, is recognised as food. Vision is poor underwater, and a prominent role of the ISOs in prey capture under such circumstances is at least implied.

Elsewhere on the body of true crocodiles, the function of the ISOs is less clear. They are normally present on most scales (dorsal and ventral), including those on the belly, the legs, the tail and even around the cloaca (as shown in the image to the left). It has been suggested that they may be involved in salinity assessment (a possible link to their absence in alligatorines, which also lack lingual salt glands), but there is currently no evidence to support this or any other function. At present, the only significance of the ISOs on the main body of true crocodiles is for identification purposes. Leather fashioned from the skin of crocodiles contains the telltale ISO spots, unlike that which originates from alligators or caimans.