There is only one species of crocodile in Southern Africa, the Nile crocodile (Crocodylus niloticus). Individuals can weigh up to 1000 kg, can reach a length of nearly 6 m (although most tend to be less than 3 m long) and can live to be over 100 years old. They have five toes on the front feet, four toes on the back feet, which are partially webbed. The Nile crocodile has a long snout with teeth that are exposed even when the mouth is closed (unlike those of an alligator). The snout of an alligator tends to be shorter and rounder than that of a crocodile. The eyes, ears and nostrils lie high on the head so that they can keep them above the surface of the water while the rest of the body is under the surface. This allows them to remain fairly hidden and they can thus approach prey easier or hide from enemies.
Nile Crocodiles tend to be an olive colour when adult, but juveniles are more brightly coloured with more contrasting colouration.
When a crocodile catches prey, like catfish, it has to open its mouth underwater. When this happens it has valves in the nostrils, which close to stop water from entering. It also has a gular flap at the back of its mouth, which closes and stops water from entering the lungs. This means that a crocodile cannot swallow its prey underwater and has to come to the surface to swallow.
Nictitating membranes protect the crocodile’s eyes while underwater and the ear slits close when going below the water surface. It is estimated that a large crocodile can remain under the water for up to 2
hours, if it remains inactive. Crocodiles show many adaptations which enable them to remain underwater for long periods. They have a unique circulatory system that allows for greater control of blood flow, as opposed to other reptiles. The heart can also be slowed down during dives, and in the process oxygenated blood is directed only to the most important organs such as the heart and brain.
It is believed that when crocodiles are lying in the sun with their mouths wide open they are regulating their body temperature by using the moist surface inside their mouths in the manner of a radiator (thereby losing heat). They may also use the mouth to absorb heat from the sun in cooler weather.
Crocodile teeth are designed for gripping rather than chewing and if prey is too large to be swallowed, they will tear off smaller chunks by rolling in the water. As they cannot chew their prey they need to swallow chunks whole. They do this by throwing the food up into the air and catching it in their mouths.
A crocodile’s diet changes with age. Hatchlings prey mainly on insects, frogs and snails. When they get over a metre long, they feed mainly on fish, especially catfish. Crocodiles over 3 m eat anything that they can overpower, including fish, terrapins, birds, snakes, other crocodiles and even larger mammals. They take live prey as well as carrion.
Crocodiles become sexually mature at 12 to 15 years old (about 2 to 3 metres long).
Males may fight to establish dominance. Mating usually happens in late winter (July and August). Females will usually use sandy banks as nest sites. These sites need to lie above the flood levels and the females will often make use of the same sites for many successive seasons. They usually lay their eggs in early summer (October and November). The gravid female will dig a hole, often in the late afternoon or at night, with her back feet, and lay between 16 to 80 hard-shelled eggs. She will defend the nest site against predators like water monitors, mongooses and other crocodiles. The female does not feed during this period but will drink. She will also chase the male away from the nest site. After 73 – 95 days the baby crocodiles hatch. The sex of the hatchlings is dependent on the temperature of incubation in the nest (females develop at temperatures between 26 and 30 degrees C and males at between 31 to 34 degrees). The hatchlings make a “cheeping” sound just before and during the time that they are hatching, and the female then opens the nest and carries all the hatchlings to the water in her mouth, where they are released. They remain close to the area, and together, for about 6 weeks before dispersing. They are preyed on by numerous different creatures including monitor lizards, eagles, herons, marabou storks, ground hornbills, mongooses, otters and baboons. On average only one crocodile out of a nest of 50 survives to adulthood. As juveniles are more vulnerable to predation than adults they tend to favour more heavily vegetated backwaters, particularly where the larger crocodiles do not usually venture.
In southern Africa Nile crocodiles are largely restricted to game reserves and large wild areas. It is estimated that there are fewer than 12 000 individual crocodiles remaining in the wild. The Nile crocodile is listed in Appendix II of CITES, and thus trade is regulated through a government permit system. Crocodiles are, however, not considered by the IUCN to be “threatened”.
Nile crocodiles can be extremely dangerous to humans. They are one of the very few animals that consider humans to be prey. Many people are killed each year by crocodiles (possibly more people are possibly killed by crocodiles than even by hippos or any other large animal). People most at risk are those people who live near rivers, lakes and dams (particularly those who depend on the water for drinking, recreation, fishing etc.). Those people who come down to the water’s edge at a regular time each day are even more at risk, as crocodiles often patiently watch what is happening and soon establish whether there are routines.