Nile crocodile
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| Crocodylus niloticus (Laurenti, 1768) |
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 Nile Crocodile range
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The Nile Crocodile Crocodylus niloticus is a African reptile of the family Crocodylidae.
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[edit] Distribution and habitat
In antiquity, Nile crocodiles occurred in the Nile delta and the Jabbok. 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. It is known from fossil remains that they once inhabited Lake Edward.[1] The Nile crocodile's current range of distribution extends from the Senegal River, Lake Chad, Wadai and 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.[1]
In West Africa, Nile crocodiles occur most frequently in coastal lagoons, estuaries and in the rivers bordering the equatorial forest belt. 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 km off Santa Lucia Bay in 1917.[1]
[edit] Biology and appearance
Nile crocodiles have a dark bronze colouration above, with black spots on the back and a dirty yellow 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. Their eyes are green.[1]
Like all crocodiles, they are quadrupeds with four short, splayed legs; long, powerful tails; a scaly hide with rows of ossified scutes running down their back and tail; and powerful jaws. They have nictitating membranes to protect their eyes and have lachrymal glands, and can cleanse their eyes with tears.
Nostrils, eyes, and ears are situated on the tops of their head, so the rest of the body can remain concealed underwater. Their coloration also helps them hide: Juveniles are grey, dark olive, or brown; with darker cross-bands on their tail and body. As they mature they become darker and the cross-bands fade, especially those on the body. The underbelly is yellowish, and makes high-quality leather.
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 speeds, briefly reaching up to 12 to 14 km/h (7.5 to 8.5 mi/h). They can swim much faster by moving their body and tail in a sinouous fashion, and they can sustain this form of movement much longer at about 30 to 35 km/h.
They have a four-chambered heart, like a bird, which is especially efficient at oxygenating their blood. They normally dive for only a couple of 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 2 hours. They have an ectothermic metabolism, so they can survive a long time between meals — though when they do eat, they can eat up to half their body weight at a time.
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.
Their jaws are capable of exerting impressive force as they hold on to their prey. The bite force exerted by an adult Nile crocodile has been shown by Doctor Brady Barr to measure 5,000 lbf (22 kN) or more. 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.[2] Their mouths are filled with a total of 64 to 68 cone-shaped teeth. On each side of the mouth, there are 5 teeth in the front of the upper jaw (the premaxilla), 13 or 14 in the rest of the upper jaw (the maxilla), and 14 or 15 on either side of the lower jaw (the mandible). Hatchlings quickly lose a hardened piece of skin on the top of their mouth called the egg tooth, which they use to break through their egg's shell at birth.
[edit] Size
The Nile crocodile is the largest African crocodilian and is sometimes regarded as the second largest crocodilian after the Saltwater crocodile, usually measuring between 3.3 and 5 m (11 and 16 feet), exceptionally to 6 m (20 ft).[3] Good sized males weigh 500 kg (1100 lb)[4], but can also reach up to 2000 lbs (909 kg).[5] The largest accurately measured male was shot near Mwanza, Tanzania and measured 6.45 m (21.3 ft) and weighed aproximately 1,090 kg (2,400 lb).[6] Like all crocodiles they are sexually dimorphic, with the males up to 30% larger than the females, though the difference is even less in some species, like the Saltwater crocodile.
Seven meters (23 ft) and larger specimens 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 approximately 20-30 feet 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.
There is some evidence that Nile crocodiles from cooler climates like the southern tip of Africa are smaller, and may reach lengths of only 4 m (13 ft). Dwarf Nile crocodiles also exist in Mali and in the Sahara desert, which reach only 2 to 3 m (6.5 to 10 ft) in length. Their reduced size is probably the result of the less than ideal environmental conditions, not genetics.
[edit] Diet and eating behavior
Hatchlings eat insects and small aquatic invertebrates, and quickly graduate to amphibians, reptiles, and birds. But even as an adult, a significant portion of a Nile crocodile's diet is fish and other small vertebrates. However adult crocodiles prefer to consume larger prey to conserve energy. In the absence of large prey, they gradually have to move onto smaller prey. Nile crocodiles have a very broad diet and can potentially eat nearly any animal that comes to take a drink at the edge of the water.[7] The most frequently recorded mammalian prey taken by Nile crocodiles are waterbuck, sitatunga, lechwe, wildebeest, zebra, warthog, goats, sheep and cattle. Other crocodiles, including their own species have been recorded to be eaten. Cape buffalo and giraffes have also been taken. Lion claws have occasionally been found in crocodile stomachs, and there is at least one record of a group of crocodiles killing a female rhinoceros in the Tana river.[1]
Adult Nile crocodiles use their bodies and tail 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 body 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 kills, although they avoid rotting meat. Groups of Nile crocodiles may travel hundreds of meters (yards) from a waterway to feast on a carcass.
Once their prey is dead, they rip off and swallow chunks of flesh. When groups of Nile crocodiles are sharing a kill, they use each other for leverage, biting down hard and then twisting their body to tear off large pieces of meat. This is called the death roll. They may also get the necessary leverage by lodging their prey under branches or stones, before rolling and ripping.
Nile crocodiles are reputed to have a symbiotic relationship with certain birds like the spur-winged plover. According to reports, the crocodile opens its mouth wide, and then the bird picks pieces of meat from between the crocodile's teeth. This has proven hard to verify, and may not be a true symbiotic relationship.
[edit] Mating and breeding
For males, the onset of sexual maturity occurs when they are about 3 metres (10 ft) long while for females, it occurs when they reach 2 to 2.5 m (6.5 to 8 ft) 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, though they are much less vocal than American alligators.[1] The larger males of a population tend to be more successful. Once a female has been attracted, the pair warble and rub the underside 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 couple of meters (yards) from the bank and up to 500 mm (20 in) deep, and lays between 25 and 80 eggs. The number of eggs varies between different populations, but averages around 50. Multiple females may nest close together.
The eggs resemble hen eggs, but have a much thinner shell.
Unlike most other crocodilians, female Nile crocodiles will bury their eggs in sand rather than incubate them in rotting vegetation.[1] After burying the eggs, the female then guards them for the 3 month incubation period. The father-to-be will often stay nearby, and both parents will fiercely attack anything that approaches their eggs. The impending mother 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, monitor lizards, and other animals while the mother is temporarily absent.
The hatchlings start to make a high-pitched chirping noise before hatching, which is the signal for the mother to rip open the nest. Both the mother and father may pick up the eggs in their mouths, and roll them between their tongue and the upper palate of their mouth to help crack the shell, and release their offspring. Once they are hatched, the female may lead the hatchlings to water, or even carry them there, in her mouth.
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 5-degree 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 to protect them, either in their mouth or in her gular or throat pouch, to keep the babies safe. The mother will sometimes carry her young on her back to avoid them getting 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.
Crocodile longevity is not well established, but larger species like the Nile crocodile live longer, and may have an average life span of 70–100 years.
[edit] Environmental status
From the 1940s to the 1960s, the Nile crocodile was hunted, primarily for high-quality leather, though also for meat and 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.
There are an estimated 250,000 to 500,000 individuals in the wild. The Nile crocodile is also widely distributed, with strong, documented populations in many countries in east Africa, including Somalia, Ethiopia, Kenya, and Zambia. 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 of Nile crocodiles 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 like the barbel catfish, a predator that can overeat fish populations that other species, like birds, depend on. The Nile crocodile also consumes dead animals that would otherwise pollute the waters. The primary threat to Nile crocodiles, in turn, are humans. While illegal poaching is no longer a problem, they are threatened by pollution, hunting, and 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, like the Salties, the Nile crocodile lives in close proximity to human populations, so contact is more frequent. While there are no solid numbers, the Nile crocodile probably kills a couple of hundred people a year, which is more than all the other crocodiles combined.[8] Some estimates put the number of annual victims in the thousands.[9]
The Conservation Status of the Nile crocodile under the 1996 World Conservation Union (IUCN) Red List is "Lower Risk" (Lrlc). The Convention on International Trade in Endangered Species of Wild Fauna and Flora (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.
[edit] In Myth
The people of Ancient Egypt worshiped Sobek, a crocodile-god associated with fertility, protection, and the power of the Pharaoh.[10] They had an ambivalent relationship with Sobek, as they did (and do) with the Nile crocodile; sometimes they hunted crocodiles and reviled Sobek, and sometimes they saw him as a protector and source of pharonic power.
Sobek was depicted as a crocodile, as a mummified crocodile, or as a man with the head of a crocodile. The center of his worship was in the Middle Kingdom city of Arsinoe in the Faiyum Oasis (now Al Fayyum), known as "Crocodopolis" by the Greeks. Another major temple to Sobek is in Kom-Ombo, and other temples were scattered across the country.
According to Herodotus in the 5th century BC, some Egyptians kept crocodiles as pampered pets. In Sobek's temple in Arsinoe, a crocodile was kept in the pool of the temple, where it was fed, covered with jewelry, and worshipped. When the crocodiles died, they were embalmed, mummified, placed in sarcophagi, and then buried in a sacred tomb. Many mummified crocodiles and even crocodile eggs have been found in Egyptian tombs.
Spells were used to appease crocodiles in Ancient Egypt, and even in modern times Nubian fishermen stuff and mount crocodiles over their doorsteps to ward against evil.
[edit] Alternate names
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 Mamba in Swahili, Garwe in Shona, Ngwenya in Ndebele, and Olom in a Nubian dialect.
[edit] See also
[edit] References
- ^ a b c d e f g Guggisberg, C.A.W. (1972). Crocodiles: Their Natural History, Folklore, and Conservation, pp.195. ISBN 0715352725.
- ^ National Geographic documentary; "Bite Force", Brady Barr.
- ^ Somma, Louis A. (June 19, 2002). Crocodylus niloticus Laurenti, 1768. USGS Nonindigenous Aquatic Species Database. Retrieved July 14, 2006 from the USGS.
- ^ Nile Crocodile. (n.d.). Retrieved December 16, 2004 from SeaWorld/Busch Gardens, Animal Bytes
- ^ <http://www.reptileworldzoo.com/gators.htm>
- ^ Wood, The Guinness Book of Animal Facts and Feats. Sterling Pub Co Inc (1983), ISBN 978-0851122359
- ^ Nile Crocodile: Photos, Video, E-card, Map - National Geographic Kids
- ^ Crocodile Attack in Australia: An Analysis of Its Incidence and Review of the Pathology and Management of Crocodilian Attacks in General
- ^ Wood, The Guinness Book of Animal Facts and Feats. Sterling Pub Co Inc (1983), ISBN 978-0851122359
- ^ Sobek, God of Crocodiles, Power, Protection and Fertility.... Retrieved on 2007-03-17.
- Crocodile Specialist Group (1996). Crocodylus niloticus. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 12 May 2006. Database entry includes a brief justification of why this species is of least concern
- Britton, Adam. (n.d.). Crocodylus niloticus (Laurenti, 1768). Retrieved December 16, 2004 from Crocodilian Species List.
- 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. December 16, 2004 from Status Survey and Conservation Action Plan, Second Edition: Crocodiles.
