Authorities are not in complete agreement as to the exact classification of crocodiles but at least 21 species are recognised. They include the largest living reptiles and have changed little since the time of the dinosaurs - more than 150 million years ago. But they represent an advanced group of reptiles from that period and they have certain features which are not found in other reptiles today. For example, the heart has four chambers and, like a mammal's heart, can pump blood to and from the lungs without mixing. In crocodiles there is also a by-pass arrangement so that when the animal is under water, and its lungs have given up their oxygen, most of the blood can by-pass them and just keep circulating rand the rest of the body (Pooley & Gans, 1976)

But despite their special features crocodiles are reptiles and this determines many of the requirements for captive rearing. Generalisations can be made under the following headings:

2.1 Body Temperature

Reptiles can not generate much body heat of their own so they must depend upon warmth from outside. They deliberately warm themselves by basking in the sun or on a warm surface. When they need to cool off crocodiles choose to lie in the shade or enter the water. Even the orientation of their bodies in relation to the sun and wind can be important. When crocodiles are warming-up the heart rate increases and more blood flows to the surface. This speeds up heat intake and distribution through the body (Grigg & Alchin, 1976).

Reptiles can not sweat or lose heat rapidly by any other means if they are forced to remain in the sun. Consequently crocodiles can be killed in a very short time by overheating. This can easily happen by accident if, for example, pools are emptied while the animals have no shade.

At low body temperatures crocodiles cease to feed and become torpid. In South Africa young Nile crocodiles in captivity generally show no interest in food when air or water temperatures are below 15.6 C (60 F). At temperatures below 7.2 C (45 F) they are unable to move properly and can not keep their balance in the water. In this state they may drown (Pooley, 1971).

2.2 Breathing

Reptiles must breathe air and crocodiles drown when they become entangled in nests or otherwise held under water for long periods. In one incident, known to the writer, crocodiles were drowned because they were being kept temporarily in a pool with a wire netting cover. During the night a storm raised the water level above the netting.

The nostrils of crocodiles are located on a raised pad at the tip of the snout but the internal openings are not directly underneath; they are far back in the throat. In front of these openings a flap can be raised from the floor to a ridge on the roof of the mouth. This closes the throat so that a crocodile can sieze food under water without any water entering the windpipe or even being swallowed. Similarly, a crocodile needs only the tip of its snout above water to be able to breathe normally whether its mouth is open or not.

2.3 Feeding and Growing

Crocodiles are carnivorous. The teeth are set deep in the jawbone and are conical or peg-like and slightly curved. Throughout life new teeth repeatedly form, like small white cones, beneath the old ones which they replace. The teeth can not therefore be used for estimating the age of a crocodile. The powerful jaws are well equipped for holding and crushing but they can not chew or grind food into smaller pieces. Food which is too big to be swallowed whole has to be shaken, torn or twisted apart by one or more crocodiles. In general, very small crocodiles feed on tiny animals - especially invertebrates.

Digestion is efficient and passage through the digestive system is quite rapid for a reptile. In the Nile crocodile it has been estimated at somewhat more than 72 hours (Pooley & Gans, 1976). It is quite usual to find stones in the stomachs of wild crocodiles. Whatever other functions they may have they must certainly aid digestion by their grinding action while the stomach is churning . Stones in the crop of some birds have a similar function.

Irrespective of body temperature, crocodiles will not accept food if they are frightened or in a state of stress because of previous handling or other disturbance. They can survive for months without food but gradually lose weight, weaken and become even less interested in feeding. Obviously, it is important to maintain a routine of regular, frequent feeding.
Needless to say, crocodiles do not grow while they are fasting but neither do they grow very much if they are only given enough food for survival. In short, growth and size are related much more to food intake than to age. For this reason well-fed captive crocodiles will reach commercial skin size in far less time than they would need in the wild.

2.4 Reproduction

In general it is not possible to distinguish the sex of crocodiles by the external appearance except for old males of a species which may be much bigger than the largest females. Sexual maturity is attained after several years. Estimates of 10 to 15 years have been made for age at first nesting in wild females of the Nile crocodile in different parts of Africa. The average age at first nesting for wild American alligators was found to be 9 years 10 months (Joanes & McNease, 1975, 1982). In general, smaller species mature earlier than the larger ones. Captive crocodiles, because of faster growth rates, can mature much earlier than wild ones. Crocodiles live for many years and probably continue breeding into old age. Males usually mate with more than one female.

All crocodiles lay hard-shelled eggs in clutches. Most types build nests from vegetation and plant litter, sometimes with earth or mud which the female scrapes and presses together in a mound. Others dig holes in sand or earth and bury the eggs as do turtles. Two species may dig holes or else pile the sand or earth into a mound nest.

One clutch a year, seasonally, is usual. Incubation takes about 9 to 13 weeks depending on species and temperature. A nest temperature of 31-32C is probably safe for all species and a few degrees higher or lower can be fatal - at least in the types most studied.

During incubation the mother may guard the nest very closely and then dig out the eggs and young when they are ready to hatch and can call out. She may then carry the hatchlings very gently to the water and stay with them for some weeks until they disperse. Such parental care has been recorded in both parents and was first documented and photographed in the Nile crocodile (Pooley & Gans 1976) but has since been observed in several species from South America, Asia and the South Pacific.

2.5 The Senses

More is known about the sensory equipment of crocodiles than about how they use it. The small brain is much like that of other reptiles but has been found to be more complex. The sense of smell must be very well developed and crocodiles can evidently detect odours in the air by a special sniffing action (Pooley & Gans 1976). There is a pair of musk glands beneath the throat and another pair just inside the cloaca. They presumably have a special use in communication by scent.

Hearing seems to be good. The ear openings are immediately behind the eyes, each protected by a moveable flap. Sound is obviously important for parents respond to the calls of their young, and adults may roar or bellow at breeding time and when they are caught by man.

The structure of the eye suggests that vision is good both in daylight and at night. The eyes are positioned so as to give wide fields of view both to the sides and front where the fields overlap to produce binocular vision. There is a transparent "third eyelid" as in birds. The pupil constricts to a vertical slit in bright light.

Other sensory organs include taste buds in the roof of the mouth and touch sensors on the jaws (Bellairs, 1971). Alone among reptiles, crocodiles also have pressure sensors beneath the teeth (Pooley & Grans 1976).

2.6 The Skin

The skin of crocodiles (as in other vertebrates) consists of two distinct layers. The outer layer (epidermis) has a thin, living base from which new skin grows but to the outside of this the skin becomes horny and shaped into scales or scutes. The inner layer (dermis) is thick and very tough, consisting of interlaced fibres. This layer also has nerves, blood vessels and plates of bone called osteoderms in association with the scales.

On a crocodile's head the skin is actually fused to the bones of the skull.

The horny scutes of a crocodile's back are raised into ridges or keels. Two rows of raised scutes continue as a double crest down the tail, joining about half way along the tail to form a single median row which continues to the tail tip.

The large, keeled scutes of the back are each strengthened by an osteoderm and in some types of crocodile there are osteoderms also beneath the belly scales where they are known in the leather trade as 'buttons'. This is of major significance because only skin which is not ossified (i.e. bony) can be brought to the high lustre which in crocodile leather is known as a 'classic' finish (Fuchs 1975). Partly for this reason the most valued skin comes from the belly, flanks, throat and neck where bone is least likely to be present.

Another factor which influences the skin value of different crocodiles is the size of the belly scales. A small scale pattern is preferred. Fortunately, for the future of crocodile farming, the most widespread species, geographically, has the most valuable skin with small belly scales free of bone. This is the saltwater or estuarine crocodile (Crocodylus porosus) which extends from Sri Lanka, eastern India and Bangladesh eastwards through coastal southeast Asia and south to northern Australia. Brief notes on other crocodiles are presented in section 3.