Nile crocodile

For the crocodile inhabiting west and northwest Africa, see Desert crocodile.

Nile crocodile
Conservation status
Least Concern  (IUCN 3.1)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Archosauromorpha
Clade:	Archosauriformes
Order:	Crocodilia
Family:	Crocodylidae
Genus:	Crocodylus
Species:	C. niloticus
Binomial name
Crocodylus niloticus Laurenti, 1768
Range map from before the West African crocodile was considered separate
Synonyms
Crocodylus vulgaris Cuvier, 1802

The Nile crocodile (Crocodylus niloticus) is an African crocodile and the second largest extant reptile in the world, after the saltwater crocodile. The Nile crocodile is quite widespread throughout Sub-Saharan Africa, occurring mostly in the central, eastern, southern and some western parts of the continent and lives in different types of aquatic environments such as lakes, rivers and marshlands. Although capable of living in saline environments, this species is rarely found in saltwater, but occasionally inhabits deltas and brackish lakes. The range of this species once stretched northward throughout the Nile, as far north as the Nile delta. On average the Nile crocodile is between 4.1 metres (13 ft) to 5 metres (16 ft), weighing around 410 kg (900 lb). However, specimens measuring 6.1 metres (20 ft) in length and weighing 900 kg (2,000 lb) are not uncommon. They have thick scaly skin that is heavily armored.

The Nile crocodile is an opportunistic apex predator and a very aggressive species of crocodile that is capable of taking almost any animal within its range. They are generalists, taking a variety of prey. Their diet consists mostly of different species of fish, reptiles, birds and mammals. The Nile crocodile is an ambush predator and can wait for hours, days and even weeks for the suitable moment to attack. They are quite agile predators and wait for the opportunity for the prey item to come close within the range of attack. Even swift prey are not immune to attack. Like other crocodiles, Nile crocodiles have an extremely powerful bite that is unique amongst all animals and sharp conical teeth that sink into flesh allowing for a grip that is almost impossible to loosen. They can apply high levels of force for extended periods of time, a great advantage for holding down large prey underwater to drown.

Nile crocodiles are very social crocodiles. They share basking spots and large food sources such as schools of fish and big carcasses. There is a strict hierarchy, that is determined by size. Large, old males are at the top of this hierarchy and have primary access to food and best basking spots. Crocodiles know their place in the hierarchical order and rarely act against it, but when they do, the results are very bloody and sometimes even fatal. Like other reptiles, Nile crocodiles lay eggs to reproduce, which are guarded by the female. The hatchlings are also protected for a period of time, but hunt by themselves and are not fed by the parents. The Nile crocodile is one of the most dangerous species of crocodile and is responsible for hundreds of deaths of humans every year. It is a rather common species of crocodile and is not endangered.

Taxonomy and etymology

The binomial name Crocodylus niloticus is derived from the Greek kroko ("pebble"), deilos ("worm", or "man"), referring to its rough skin; and niloticus, meaning "from the Nile River". The Nile crocodile is called timsah al-nil in Arabic, mamba in Swahili, garwe in Shona, ngwenya in Ndebele, ngwena in Venda, kwena in Sotho and Tswana.

Characteristics

View of a Nile crocodile from the side

Nile crocodiles have a dark bronze colouration above, with black spots on the back and a dirty purple on the belly. The flanks, which are yellowish-green in colour, have dark patches arranged in oblique stripes. There is some variation relative to environment; specimens from swift-flowing waters tend to be lighter in colour than those dwelling in lakes or swamps. They have green eyes.

Like all crocodiles, the Nile crocodile is a quadruped with four short, splayed legs, a long, powerful tail, a scaly hide with rows of ossified scutes running down its back and tail, and powerful jaws. It has nictitating membranes to protect the eyes and lachrymal glands to cleanse its eyes with tears. The nostrils, eyes, and ears are situated on the top of the head, so the rest of the body can remain concealed underwater. The coloration also helps to camouflage it; juveniles are grey, multicoloured, or brown, with dark cross-bands on the tail and body. As it matures, it becomes darker and the cross-bands fade, especially those on the body. The underbelly is yellowish green.

Size

Healthy subadult

The Nile crocodile is the largest crocodilian in Africa and is the second-largest crocodilian after the saltwater crocodile. The male crocodile usually measure from 3.5 to 5 m (11 ft 6 in to 16 ft 5 in) long, but very old, mature ones can grow to 5.5 m (18 ft 1 in) or more. Mature female Nile crocodiles measure 2.4 to 3.8 m (7 ft 10 in to 12 ft 6 in). Like all crocodiles they are sexually dimorphic, with the males up to 30% larger than the females, though the difference is less compared to some species, like the saltwater crocodile.

Average Nile crocodile size (green) along with size of Gustave (grey) compared to a human

Typical Nile crocodile weight is from 225 to 550 kg (496 to 1,213 lb), though exceptionally large males can range up to 900 kg (2,000 lb) or more, in weight. The largest accurately measured male, shot near Mwanza, Tanzania, measured 6.47 m (21 ft 3 in) and weighed about 1,090 kg (2,400 lb). Another notable giant, caught alive by J.G. Kulmann in Venda, South Africa, measured 5.5 m (18 ft 1 in) in length and weighed 905.7 kg (1,997 lb). Seven-meter specimens and larger have been reported, but since gross overestimation of size is common, these reports are suspect. The largest living specimen is purported to be a man-eater from Burundi named Gustave; he is believed to be more than 6.1 m (20 ft 0 in) long. Such giants are rare today; before the heavy hunting of the 1940s and 1950s, a larger population base and more extensive wetland habitats meant more giants.

Evidence exists of Nile crocodiles from cooler climates like the southern tip of Africa being smaller, and may reach lengths of only 4 m (13 ft). A smaller population from Mali, the Sahara Desert and elsewhere in West Africa reaches only 2 to 3 m (6 ft 7 in to 9 ft 10 in) in length, but it is now recognized as a separate species, the West African crocodile.

Distribution and habitat

Nile crocodile in Ethiopia

The Nile crocodile is the most common crocodilian found in Africa today. They are common throughout much of the continent. Their historic range however, was even wider. They were found as north as the Mediterranean coast in the Nile delta. Today they are common in Somalia, Ethiopia, Uganda, Kenya, Egypt, Central African Republic, Democratic Republic of the Congo, Burkina Faso, Equatorial Guinea, Tanzania, Rwanda, Burundi, Zambia, Zimbabwe, Gabon, Angola, South Africa, Malawi, Mozambique, Sudan, South Sudan, Botswana, and Cameroon. Isolated populations also exist in Madagascar. They are recorded by Herodotus to have inhabited Lake Moeris. They are thought to have become extinct in the Seychelles in the early 19th century. They are known from fossil remains to have once inhabited Lake Edward. The Nile crocodile's current range of distribution extends from the Sudan to the Cunene and the Okavango Delta. In Madagascar, crocodiles occur in the western and southern parts from Sembirano to Port Dauphin. They have occasionally been spotted in Zanzibar and the Comoros.

In East Africa, they are found mostly in rivers, lakes, marshes, and dams. They have been known to enter the sea in some areas, with one specimen having been seen 11 kilometres (6.8 mi) off St Lucia Bay in 1917. In Madagascar, they have adapted to living in caves.

The species was previously thought to extend in the whole of West Africa, but these populations are now recognized as a distinct species, the West Africa (or desert) crocodile. Despite this confusion, the Nile crocodile is more closely related to crocodiles of the Americas (American, Cuban, Morelet's and Orinoco) than it is to the West African crocodile.

Biology and behavior

Adult Nile crocodile basking

They have a four-chambered heart, although modified for their ectothermic nature due to an elongated cardiac septum, physiologically similar to the heart of a bird, which is especially efficient at oxygenating their blood. They normally dive for only a few minutes, but will stay underwater for up to 30 minutes if threatened, and if they remain inactive they can hold their breath for up to two hours. They have an ectothermic metabolism, so can survive for long periods between meals—though when they do eat, they can eat up to half their body weight at a time.

They normally crawl along on their bellies, but they can also "high walk" with their trunks raised above the ground. Smaller specimens can gallop, and even larger crocodiles are capable of surprising bursts of speed, briefly reaching up to 12 to 14 km/h (7.5 to 8.7 mph). They can swim much faster by moving their bodies and tails in a sinuous fashion, and they can sustain this form of movement much longer at about 30 to 35 km/h (19 to 22 mph).

They have a rich vocal range, and good hearing. Their skin has a number of poorly understood integumentary sense organs (ISOs) that may react to changes in water pressure.

The bite force exerted by an adult Nile crocodile has been shown by Dr. Brady Barr to measure 5,000 lbf (22 kN). However, the muscles responsible for opening the mouth are exceptionally weak, allowing a man to easily hold them shut with a small amount of force. Their mouths are filled with a total of 64 to 68 cone-shaped teeth. On each side of the mouth, there are five teeth in the front of the upper jaw (premaxilla), 13 or 14 in the rest of the upper jaw (maxilla), and 14 or 15 on either side of the lower jaw (mandible). Hatchlings quickly lose a hardened piece of skin on the top of their mouths called the egg tooth, which they use to break through their eggshells at birth.

Crocodile longevity is not well established, but larger species like the Nile crocodile live longer, and may have an average life span of 70 to 100 years.

Hunting and diet

The Nile crocodile possesses unique predation behavior characterized by the ability of preying both within its natural habitat and out of it, which often results in unpredicted attacks on almost any other animal up to a couple of times of its size. Nile crocodiles are apex predators throughout its range. In the water, it is an agile and rapid hunter relying on both movement and pressure sensors to catch any prey unfortunate enough to present itself inside or near the waterfront. Out of water, however, the Nile crocodile can only rely on its limbs, as it gallops on solid ground, to chase prey. Most hunting on land is done at night by lying in ambush near forest trails or roadsides, up to 50 m (170 feet) from the water's edge.

Drawing depicting the mythical relationship between plovers and crocodiles – no reliable observations exist of this purported symbiosis.

Therefore most terrestrial prey are caught by ambush attacks when the animal approaches water to drink. The crocodile slowly comes closer, most of its body underwater, sometimes only its eyes and nostrils visible. On other occasions more of its head and upper body visible. The attack is sudden and unpredictable. The crocodile lunges its body out of water in the blink of an eye and grasps on to its prey. The teeth of a crocodile are not for tearing up flesh but to sink deep in to it and holding on to the prey item. The immense bite force, which may be as high as 5,000 lbf (22,000 N) in large adults, ensures the prey item can't escape through the grip. The rest depends on the crocodile's body power and weight to pull the prey item back into the water where it is either drowned to death or killed by sudden thrashes of the head or by tearing it up into pieces by other crocodiles.

The size of the prey depends on mostly the size of the crocodile. Young hatchlings generally feed on smaller prey, preferring small fish, frogs, insects and small aquatic invertebrates before taking on larger fish, amphibians and small reptiles. Juveniles and subadults take a wider variety of prey with additions such as birds, turtles, snakes, Nile monitors and small to mid-sized mammals, such as various monkeys, duikers, rodents, mongoose, hares, pangolins, porcupines, bats, dik-dik, and other small ungulates up to the size of a Thomson's gazelle. Various birds including, storks, small wading birds, waterfowl, fish eagles and even small swift flying birds may be snatched. Throughout their lives, crocodiles can feed on fish and other small vertebrates on occasion, when large food is absent, as a side diet. Larger fish like catfish and freshwater bass are preferred by adults. Very small fish are likely to be eaten only in case of sudden encounter, mostly in shallow dry season ponds where much effort is not needed to catch the small prey. Almost 70% of a young adult's diet is still fish. This will change drastically, however, as full grown adults.

Adults are apex predators and prey upon various birds, reptiles and mammals, in addition to prey consumed also by the younger specimens. Large birds such as ostrich, and large snakes such pythons are among non-mammalian prey. Among the mammals, the bulk of the prey are antelopes. In general, gazelles, waterbuck, bushbuck, impala, sitatunga, lechwe, eland, kudu, gemsbok, sable antelope and wildebeest are among the most common prey. Zebras, warthogs and baboons are also readily taken. When crocodiles grow they prefer larger prey for energy efficiency. Therefore large adults rarely tackle small prey. Their prey consists almost exclusively of mammals. Large adults sometimes take on larger prey such as giraffe, Cape buffalo, young hippos, and young elephants. In several instances, large crocodiles have been observed to take down much larger prey such as the black rhinoceros and hippopotamus. However, other than rare instances, adults of these species are not considered as regular prey and will not be attacked. Nile crocodiles are also known to prey on other large predators such as hyenas, cheetah, African wild dogs, jackals, leopards, and even lions on occasion. However predators are usually more intelligent and wary of their surroundings than prey species and are more difficult to catch since they usually avoid waters infested with crocodiles. In order to save energy, crocodiles do not prefer such agile animals as most strikes will be empty handed. They usually only attack other predators in absence of regular prey items.

Crocodiles that live close to villages and other populous areas also prey on domestic animals. When given the chance, they are known to prey on domestic chickens, goats, sheep and cattle. Nile crocodiles also prey on humans frequently, far more often than other crocodilian species, although in parts of the Philippines and New Guinea, saltwater crocodile attacks can also be common.

Wildebeest crossing the Mara River, attacked by crocodiles

Sub-adult and smaller adult Nile crocodiles use their bodies and tails to herd groups of fish toward a bank, and eat them with quick sideways jerks of their heads. They also cooperate, blocking migrating fish by forming a semicircle across the river. The most dominant crocodile eats first. Their ability to lie concealed with most of their bodies underwater, combined with their speed over short distances, makes them effective opportunistic hunters of larger prey. They grab such prey in their powerful jaws, drag it into the water, and hold it underneath until it drowns. They will also scavenge or steal kills from other predators, such as lions and leopards. Groups of Nile crocodiles may travel hundreds of meters from a waterway to feast on a carcass. They also feed on dead hippos as a group, tolerating each other. Once their prey is dead, they rip off and swallow chunks of flesh. When groups are sharing a kill, they use each other for leverage, biting down hard and then twisting their bodies to tear off large pieces of meat in a "death roll". They may also get the necessary leverage by lodging their prey under branches or stones, before rolling and ripping.

Herodotus claimed Nile crocodiles have a symbiotic relationship with certain birds, such as the Egyptian plover, which enter the crocodile's mouth and pick leeches feeding on the crocodile's blood, but there is no evidence of this interaction actually occurring in any crocodile species, and it is most likely mythical or allegorical fiction.

Reproduction

A float of crocodiles at Disney's Animal Kingdom

For males, the onset of sexual maturity occurs when they are about 3 metres (9.8 ft) long, while for females, it occurs when they reach 2 to 2.5 metres (6 ft 7 in to 8 ft 2 in) in length. This takes about 10 years for either sex, under normal conditions.

During the mating season, males attract females by bellowing, slapping their snouts in the water, blowing water out of their noses, and making a variety of other noises. The larger males of a population tend to be more successful. Once a female has been attracted, the pair warble and rub the undersides of their jaws together. Females lay their eggs about two months after mating.

Nesting is in November or December, which is the dry season in the north of Africa, and the rainy season in the south. Preferred nesting locations are sandy shores, dry stream beds, or riverbanks. The female then digs a hole a few metres from the bank and up to 500 mm (20 in) deep, and lays between 25 and 80 eggs. The number of eggs varies, but averages around 50. Multiple females may nest close together.

The eggs resemble chicken eggs, but have a much thinner shell.

Unlike most other crocodilians, female Nile crocodiles bury their eggs in sand rather than incubate them in rotting vegetation. After burying the eggs, the female then guards them for the three-month incubation period. The sire will often stay nearby, and both parents will fiercely attack anything approaching their eggs. The female will only leave the nest if she needs to cool off (thermoregulation) by taking a quick dip or seeking out a patch of shade. Despite the attentive care of both parents, the nests are often raided by humans and monitor lizards or other animals while she is temporarily absent.

Nile crocodile eggs

The hatchlings start to make a high-pitched chirping noise before hatching, which is the signal for the mother to rip open the nest. The parents may pick up the eggs in their mouths, and roll them between their tongue and the upper palate to help crack the shell and release their offspring. Once the eggs hatch, the female may lead the hatchlings to water, or even carry them there in her mouth, as female American alligators have been observed doing.

Nile crocodiles have temperature-dependent sex determination (TSD), which means the sex of their hatchlings is determined not by genetics, but by the average temperature during the middle third of their incubation period. If the temperature inside the nest is below 31.7°C (89.1°F), or above 34.5°C (94.1°F), the offspring will be female. Males can only be born if the temperature is within that narrow range.

Hatchlings are about 300 mm (12 in) long at birth, and grow that much each year. The new mother will protect her offspring for up to two years, and if there are multiple nests in the same area, the mothers may form a crèche. During this time, the mothers may pick up their offspring either in their mouths or gular fold (throat pouch), to keep the babies safe. The mother will sometimes carry her young on her back to avoid their being eaten by turtles or water snakes. At the end of the two years, the hatchlings will be about 1.2 m (4 ft) long, and will naturally depart the nest area, avoiding the territories of older and larger crocodiles.

Interspecific predatory relationships

Outside water, crocodiles can meet competition from other dominant savannah predators, notably felines such as lions and leopards. Occasionally, if regular food becomes scarce, both the big cats and the crocodile will steal kills from each other and depending on size, will be dominant over one another. Usually both sides avoid physical contact on such confrontations and use intimidation to resolve these conflicts. However when size differences are prominent, the predators may prey on each other.

Environmental status

From the 1940s to the 1960s, the Nile crocodile was hunted, primarily for high-quality leather, though also for meat with its purported curative properties. The population was severely depleted, and the species faced extinction. National laws, and international trade regulations have resulted in a resurgence in many areas, and the species as a whole is no longer threatened with extinction. Crocodile 'protection programs' are artificial environments where crocodiles exist safely and without the threat of extermination from hunters.

Large adults in captivity, Djerba

An estimated 250,000 to 500,000 individuals occur in the wild. The Nile crocodile is also widely distributed, with strong, documented populations in many countries in eastern and southern Africa, including Somalia, Ethiopia, Kenya, Zambia and Zimbabwe. Successful sustainable-yield programs focused on ranching crocodiles for their skins have been successfully implemented in this area, and even countries with quotas are moving toward ranching. In 1993, 80,000 Nile crocodile skins were produced, the majority from ranches in Zimbabwe and South Africa.

The situation is more grim in central and west Africa, which make up about two-thirds of the Nile crocodile's habitat. The crocodile population in this area is much more sparse, and has not been adequately surveyed. While the natural population in these areas may be lower due to a less-than-ideal environment and competition with sympatric slender-snouted and dwarf crocodiles, extirpation may be a serious threat in some of these areas. Additional factors are a loss of wetland habitats, and hunting in the 1970s. Additional ecological surveys and establishing management programs are necessary to resolve this.

The Nile crocodile is the top predator in its environment, and is responsible for checking the population of species such as the barbel catfish, a predator that can overeat fish populations on which other species, like birds, depend. The Nile crocodile also consumes dead animals that would otherwise pollute the waters. The main threats to them, in turn, are pollution, loss of habitat, hunting, and human activities such as accidental entanglement in fishing nets.

Much of the hunting stems from their reputation as a man-eater, which is not entirely unjustified. Unlike other "man-eating" crocodiles, such as the saltwater crocodile, the Nile crocodile lives in close proximity to human populations, so contact is more frequent. Although most attacks do not get reported, the Nile crocodile is estimated to kill hundreds (possibly thousands) of people each year, which is more than all other crocodilian species combined. One study posited the number of attacks by Nile crocodiles per year as 275 to 745, of which 63% are fatal, as opposed to an estimated 30 attacks per year by saltwater crocodiles, of which 50% are fatal. In both species, the mean size of crocodiles involved in nonfatal attacks was about 3 m (9.8 ft) as opposed to a reported range of 2.5–5 m (8.2–16.4 ft) or larger for crocodiles responsible for fatal attacks. The average estimated size of crocodiles involved in fatal attacks is 3.5 m (11 ft) Since a majority of fatal attacks are believed to be predatory in nature, the Nile crocodile can be considered the most prolific predator of humans among wild animals.

The IUCN Red List assesses the Nile crocodile as "Least Concern (LR/lc)". The CITES lists the Nile crocodile under Appendix I (threatened with extinction) in most of its range; and under Appendix II (not threatened, but trade must be controlled) in the remainder, which either allows ranching or sets an annual quota of skins taken from the wild. This species is farmed for its meat and leather in some parts of Africa.

See also

• Crocodile attack

Further reading

• Britton, Adam. (n.d.). Crocodylus niloticus (Laurenti, 1768).
• El-Noshokaty, Amira. (January 17–23, 2002). Lord of the Nile. Al-Ahram Weekly On-line, 569. Retrieved December 16, 2004.
• Nile crocodiles: Temperature dependent sex determination. (February 2000). Pulse of the Planet, 2075. Retrieved December 16, 2004 from Pulse of the Planet.
• Ross, James Perran (ed.). (n.d.). Species Accounts: Crocodylus niloticus.

External links

• Media related to Crocodylus niloticus at Wikimedia Commons
• "Nile crocodile" at the Encyclopedia of Life
• Multimedia information from National Geographic Kids site