The Nile crocodile is a member of the animal class Reptilia, which contains the Orders Squamata (snakes and lizards), Testudines (tortoises, terrapins and turtles), Rhynchocephalia (the two tuatara species of New Zealand) and Crocodylia (crocodiles, including alligators). These are ancient reptile groups that are distantly related, although the Crocodylia are more closely related to birds. This sounds unlikely but birds and reptiles both originated from an ancestral reptile.
The reptiles are sometimes erroneously regarded as ancient vertebrates but they developed long after the fishes and amphibians. They developed some 320 to 310 million years ago from an ancient group of Amphibia in the Carboniferous Period when vast swamp-forests covered much of the earth’s land masses. Reptiles were in turn the ancestors of the early crocodiles and later the mammals. The earliest crocodile-like reptiles lived some 250 million years ago, feeding on vegetation and fishes.
The crocodile is depicted in the hieroglyphs of ancient Egypt as the Egyptian god Sabek, who had the body of a man and the head of a crocodile. He was the god of the Nile, the Lord of the Waters, a symbol of power, fertility and military prowess and was believed to have developed from the ancient marshes on the shores of Lakes Quarun and Moeris of the El Faiyum oasis in the Nile Delta.
Laurenti first described the crocodile scientifically as a member of the family Crocodilia in 1768, based on a specimen from the Nile River; today there are some 16 species throughout the world. The names ‘crocodile’ and Crocodylus are based on the Greek words kroko for ‘pebble’ and delios for ‘worm’. This name literally means ‘the worm of the stones’ while the specific epithet niloticus means ‘from the Nile River’, referring to its habitat.
Description
The crocodile is the largest living lizard-like reptile in Africa. It is responsible for hundreds of human deaths every year and attacks prey in and some distance away from water. It is a quadruped that can run at some speed for short distances on land on its four short, splayed legs. However, the forelegs are shorter than the hind legs. There is no webbing between the five toes of the front feet but the four toes of the hind feet are webbed.
Among the reptiles, the Nile crocodile is exceeded in length only by the saltwater crocodile of the Indo-Pacific region. In South Africa, the mean length at maturity varies from 3,4 to 3,7 m. However, the mean weight of a mature male is 400 kg, as opposed to 150 kg in a female. The largest known crocodile in Southern Africa, which was collected in the Okavango Delta in Botswana, was 5,87 m long and weighed an estimated 816 kg. However, a specimen with a length of 6,45 m and a weight of 1 095 kg was shot near Mwanza in Tanzania in 1905. In comparison, the massive leatherback turtle Dermochelys coriacea has a mean weight of up to 700 kg. Under favourable conditions, a crocodile can increase its length by 300 mm per year until it reaches maturity.
The head and upper jaw consist of solid bone but the bone of the lower jaw is fragile. The skin is attached directly to the bone and the head is used as a weapon in attack and defence. There are 64 to 68 conical teeth that are razor-sharp in young crocodiles and are replaced some 45 times during a crocodile’s lifetime. The enlarged fourth tooth in the lower jaw fits into a notch on the upper jaw and remains visible when the mouth is closed. The teeth are not used to chew food but only to secure prey. The bite is extremely powerful and is unique among all animals. A Nile crocodile can bite with a force of 22 kN but the muscles that open the mouth are weak. Once the jaw muscles have been locked, it is almost impossible to pry the mouth open again.
Nile crocodile hunting blue wildebeest, Masai Mara, Kenya
‘When fighting or capturing prey, the eyeball can be drawn into the skull to protect it.’
The fixed tongue contains glands that excrete excess salt from the system of a crocodile and allows it to live in salt water. There also is a flap at the back of the tongue, which seals off the throat and is water- and airtight when pressed against the upper jaw. This allows the crocodile to stay submerged for up to two hours and when drowning its prey. When the flap is closed, the crocodile breathes by pulling air through the nostrils directly into the throat. Buoyancy is adjusted by compressing expended air in the lungs. The body temperature is controlled by cooling off in water or basking in the open sun. The nostrils are on top of the head, which facilitates submerging the body. Several other adaptations assist crocodiles to remain submerged for long periods. These reptiles are efficient swimmers, holding their legs against the body while swimming and using powerful thrusts of the tail to propel them forward.
The green eyes lie laterally on the top of the head and protrude above the water. The pupils are slits during the day but open to a circle at night. The upper and lower eyelids close to protect the eyes of a sleeping crocodile and transparent, nictitating membranes cover the eyes when the animal is submerged. When fighting or capturing prey, the eyeball can be drawn into the skull to protect it. The eyes reflect in a spotlight at night. The ears behind the eyes have slit-like openings and are sealed with flaps when submerging.
The thick, leathery skin of especially the tail is remarkably sensitive to touch. The upper side of the body is covered in squares known as scutes, which contain bone plates on the back and sides. The overall colour of the skin is dark, olive green to pale brown above, with darker markings on the sides and tail. The skin of the pale yellow to cream belly is smooth and is used extensively as leather for fashion goods. Juveniles are greenish to pale brown with vivid markings.
Nile crocodile hatchlings in nest with eggs, Madagascar
Distribution
Crocodiles mainly occur in the warm waters of tropical regions with deep pools and sandy banks but they also inhabit cooler regions at times. The Nile crocodile Crocodylus niloticus is common in Africa and some isolated islands such as Madagascar, Zanzibar and the Comoro Islands. It once roamed the swamps and waters of the Nile River in Egypt but following the construction of the Aswan Dam in the Nile in 1960, there are few left in southern Egypt. In South Africa, crocodiles occurred historically as far south as East London but they currently occur only in Mpumalanga, North-West, Limpopo and north of the Tugela River in KwaZulu-Natal. In the neighbouring countries they are found in Zimbabwe, Mozambique, northern Namibia and eastern and northern Botswana.
Characteristics
Typical crocodile habitat includes rivers, lakes, swamps, estuaries and mangroves, where they are active by day and night. They spend most of the day basking in the sun, and when doing so, there is a strict hierarchy of dominance based on the length of the crocodile. These reptiles are shy and will retire quickly to the safety of water when disturbed.
Diet and hunting
A crocodile’s night vision, the time when it mainly hunts for food, is excellent. When it is submerged, a crocodile can see below and above the water surface, which allows it to judge its distance from prey. It has sensitive hearing aids to detect prey and will be attracted by any disturbance in the water. The sense of smell is equally acute and allows a crocodile to detect prey over long distances on land and in the water. Different types of food are recognised by their taste. The bulk of the food consists of fish, which is augmented by any available food, from insects to mammals. However, the larger the meal, the more cost-effective it is in terms of energy.
Crocodiles become sexually mature at the age of 12 to 15 years, when their body length exceeds 2,3 m. Mating occurs from July to September when males occupy territories and defend a harem of females; fighting among males can end in death. A female who is ready to mate will lift her head partially above the water to expose her throat. The male then rubs his chin on her back, neck and head and twists his tail under her to copulate for a few minutes at a time, which is repeated several times a day over many days.
The female selects a suitable nest site on a dry, sunny patch of sand well above the flood line before laying her eggs from 16:00 to 22:00. She excavates a cavity of 300 to 350 mm deep with her hind feet and lays up to 75 round, hard-shelled eggs over a period of one to four hours. She covers the cavity with sand and compresses it with her hind feet. The eggs incubate for 75 to 95 days, depending on the temperature of the sand and factors such as humidity and depth of the eggs under the sand.
Eggs that incubate at a temperature of 31 to 34 °C develop into males, and those incubating at temperatures lower and higher than that develop into females. The mother stays in the vicinity of the nest to protect it against monitor lizards, baboons and even spotted hyenas. Despite this, up to 90 per cent of the nests are destroyed by predators, mainly by monitor lizards. When a hatchling is ready to hatch it gives a sharp call from within its egg to alert the mother. The mother then opens the nest, picks up a few hatchlings at a time in her mouth, and carries them to the water. She will also pick up intact eggs and gently crack them to assist hatching. The hatchlings are 250 to 320 mm long when they hatch. Their parents protect them for a while but they forage alone for their own food.
Management and utilisation
A well-developed, intensive crocodile-production industry exists that has become a quick and lucrative form of income. The belly skins are tanned in four grades of quality for the fashion industry, and are sold to the local as well as export market. (For an extensive and complete review of the intensive production of Nile crocodiles, please see Blake [2005] under the references at the end of this article.)
Captivity
Crocodiles have been produced intensively in captivity with relative ease since the early 1970s, but the eggs are collected in the wild and are then incubated in commercial incubators.
Meat and live sales
The tail of a Nile crocodile produces some 1,5 kg and the body 2,5 kg of meat per adult crocodile but the meat of the tail is in greater demand. The meat is low in fat, high in protein and has a mild flavour with a firm texture. The taste is akin to a mixture between chicken and crab. Crocodiles are not sold on live-wildlife auctions.
Trophy hunting
The length of a crocodile has been recognised as a trophy since the 20th edition of Rowland Ward’s book of records. Some of the recent record trophies based on two international systems include the following:
• Rowland Ward:
Minimum length – 14,000 ft (4,30 m);
record length – 17,740 ft (5,46 m)
• Safari Club International:
Minimum – 14 points; record – 18.583 points
• South African Measuring Method
(skull length is used):
Minimum skull length to qualify – 29″ (73,660 cm); record – 40,240″ (1,022 m) ASM
References
Alexander, G & Marais, J. 2010. A guide to the reptiles of southern Africa, third impression. Cape Town: Random House Struik, pages 350–355. Blake, D. K. 2005. The Nile crocodile. In J du P Bothma & N van Rooyen (eds), Intensive wildlife production in southern Africa. Pretoria: Van Schaik, pages 268–300. Bothma, J du P & Du Toit, J. G. 2016. Game ranch management, sixth edition. Pretoria: Van Schaik, pages 719–753.
