Nile crocodile
From Wikipedia, the free encyclopedia
| iNile crocodile | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
 |
||||||||||||||||
|
|
||||||||||||||||
| Scientific classification | ||||||||||||||||
|
||||||||||||||||
|
|
||||||||||||||||
| Crocodylus niloticus (Laurenti, 1768) |
The Nile crocodile is one of the 3 species of crocodiles found in Africa, and the second largest species of crocodile. Its range covers most of Africa south of the Sahara, and the island of Madagascar. Known as a man-eater, it has been hated, revered (especially in Ancient Egypt where crocodiles were mummified), and worshipped. The Nile crocodile can, and sometimes will, easily snatch and devour a human. While it is no longer threatened with extinction as a species, the population in many countries is in danger of vanishing.
Contents |
[edit] Biology and appearance
The Nile crocodile's size, repute, widespread range, and overlap with humanity has continued to make them particularly well known among the crocodylians.
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 mighty jaws. They have nictitating membranes to protect their eyes, and despite the myths they do 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 belly, but they can also "high walk". 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 equally fast by moving their body and tail in a sinouous fashion, but they can sustain this form of movement much longer.
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. 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, reaching lengths of up to 5 m (16 ft), or rarely up to 6 m (20 ft). Good sized males weigh 500 kg (1100 lb), and truly exceptional specimens may exceed 1200 kg (2,465 lb). Like all crocodiles they are sexually dimorphic, with the males up to 30% larger than the females, though the difference is less than 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. It is not known why some crocodiles grow larger than others, but it seems to be based on a favorable environment. Therefore it is unlikely that a modern crocodile in the wild will break any records: before the heavy hunting of the 1940s and 1950s, the larger population base and more extensive wetland habitats meant more giants. The largest Nile Crocodile on record was an old male shot in Tanzania in 1905. It measured 6.5 m (21.3 feet) long and weighed approximately 1400 kg (3,087 lb).
There is also the infamous Gustav who patrols the rivers near Lake Tanganyika, in Burundi. Reputed to be over 6.5m and weigh over 1000kg, he is held responsible for hundreds of human deaths. Over 60 years old, he has evaded capture for years. He was last seen in 2005 so may have died.
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] Reproduction and maturity
For males, the onset of sexual maturity occurs when they are about 3 m (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. 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 2 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.
Once the eggs are laid, the expectant mother covers the eggs with sand, and 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. 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] Diet and eating behavior
Hatchlings eat insects and small aquatic invertebrates, and quickly graduate to amphibians, reptiles, and birds. But even as an adult, 70% of a Nile crocodile's diet is fish and other small vertebrates, though adult crocodiles can potentially eat nearly any vertebrate that comes to take a drink at the edge of the water, the exception being mature elephants and hippopotamuses. Adult Nile Crocodiles are known to eat zebras, young hippopotamuses, buffalo, rhinoceroses , warthogs, hyenas, baboons, antelope like the Blue Wildebeest, giraffe, big cats[1] like leopards and lions[2][3] and other crocodiles.
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 semi-circle 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 is probably not a true symbiotic relationship.
[edit] Habitat and range
The preferred habitat of Nile crocodiles is along rivers, in freshwater marshes, or along lakes; in some cases they thrive in more brackish water, along estuaries or in mangrove swamps.
They are found in most of Africa south of the Sahara Desert, extending as far south as Kruger National Park and the Waterberg Massif in South Africa; they also occur in northern Madagascar, and along the Nile River basin. Historically, they were present on the islands of Comoros, which lie between Madagascar and Mozambique, but no more. In more recent times, Nile crocodiles were present in Israel, Jordan, and Algeria. Their absence is blamed on an increasingly arid climate, and the corresponding reduction of their wetland habitat; diminutive (both in individual and in population size) remnant populations are known from Mauretania (Tagant Plateau, possibly gone by 1996), Algeria and Chad (Guelta d'Archei). They are also no longer found in the Nile Delta, or along the nearby coast of the Mediterranean Sea. Their range nowadays corresponds roughly with the Afrotropic ecozone.
[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.
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 south and east Africa, like 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.
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] Gods, mummies, and magic
The people of Ancient Egypt worshiped Sobek, a crocodile-god associated with fertility, protection, and the power of the Pharaoh. 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.
The crocodile is also sometimes associated with Set, the god of evil.
[edit] Nile crocodiles in fiction
The crocodile has seeped into modern consciousness, and appears regularly in horror stories and films. For instance, the villain in the B-movie Crocodile (2000) is a 9 m (30 ft), 100 year-old Nile crocodile called "Flat Dog", who eats teenagers.
- In Terry Pratchett's Discworld series, one of the Discworld's many gods is Offler the crocodile god. He is mainly worshipped in Klatch and other hot countries near large rivers.
- In Matthew Reillys Seven Deadly Wonders book, in the opening chapter, The Nine come into contact with several of these crocodiles in a tomb.
[edit] Alternate names
The Nile crocodile is called Mamba in Swahili, Garwe in Shona, Ngwenya in Ndebele, and Olom in a Nubian dialect.
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".
