The Nile crocodile (Crocodylus niloticus) is the largest of the four crocodile species found in Africa. They have a long tail and powerful jaws. Their back feet are webbed for swimming. They grow up to 6.45 m (21.2 ft) in length and can weigh up to 1,090 kg (2,400 lb). The males are usually bigger than the females.
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.
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.
Typical Nile crocodile weight is from 225 to 550 kg (496 to 1,213 lb), though exceptionally large males can range up to 907 kg (2,000 lb) or more, in weight. The largest accurately measured male, shot near Mwanza, Tanzania, measured 6.45 m (21 ft 2 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
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, 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 African (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
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.
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.
Most land prey is 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.
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.
The crocodiles mate in the water, and then the female lays her eggs in a hole dug in the ground. They can lay 60 eggs which take about 90 days to hatch. The female guards the nest during this time. Young crocodiles are able to reproduce when they get to about 12 years old.
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 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.
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.
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