The Nile crocodile (Crocodylus niloticus) is an African crocodile, the largest freshwater predator in Africa, and may be considered the second-largest extant reptile in the world, after the saltwater crocodile (Crocodylus porosus). The Nile crocodile is quite widespread throughout sub-Saharan Africa, occurring mostly in the central, eastern, and southern regions of the continent, and lives in different types of aquatic environments such as lakes, rivers, and marshlands.[3] 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 adult male Nile crocodile is between 3.5 and 5 m (11.5 and 16.4 ft) in length and weighs 225 to 750 kg (500 to 1,650 lb). However, specimens exceeding 6.1 m (20 ft) in length and weighing up to 1,090 kg (2,400 lb) have been recorded. Sexual dimorphism is prevalent, and females are usually about 30% smaller than males. They have thick, scaly, heavily armored skin.
Nile crocodiles are opportunistic apex predators; a very aggressive species of crocodile, they are capable of taking almost any animal within their range. They are generalists, taking a variety of prey. Their diet consists mostly of different species of fish, reptiles, birds, and mammals. They are ambush predators that can wait for hours, days, and even weeks for the suitable moment to attack. They are agile predators and wait for the opportunity for a prey item to come well within attack range. 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 relatively social crocodiles. They share basking spots and large food sources, such as schools of fish and big carcasses. Their strict hierarchy is determined by size. Large, old males are at the top of this hierarchy and have primary access to food and the best basking spots. Crocodiles tend to respect this order; when it is infringed, the results are often violent and sometimes fatal. Like most other reptiles, Nile crocodiles lay eggs; these are guarded by the females. 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 human deaths every year. It is a rather common species of crocodile and is not endangered despite some regional declines or extinctions.
Characteristics and physiology
Side viewAdult Nile crocodiles have a dark bronze colouration above, with faded blackish spots and stripes variably appearing across the back and a dingy off-yellow on the belly, although mud can often obscure the crocodile’s actual colour. The flanks, which are yellowish-green in colour, have dark patches arranged in oblique stripes in highly variable patterns. Some variation occurs relative to environment; specimens from swift-flowing waters tend to be lighter in colour than those dwelling in murkier lakes or swamps, which provides camouflage that suits their environment, an example of clinal variation. Nile crocodiles have green eyes. The colouration also helps to camouflage it; juveniles are grey, multicoloured, or brown, with dark cross-bands on the tail and body. The underbelly of young crocodiles is yellowish green. As it matures, Nile crocodiles become darker and the cross-bands fade, especially those on the upper-body. A similar tendency is coloration change during maturation has been noted in most crocodile species.
Most morphological attributes of Nile crocodiles are typical of crocodilians as a whole. Like all crocodilians, for example, 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, elongated jaws. Their skin has a number of poorly understood integumentary sense organs that may react to changes in water pressure, presumably allowing them to track prey movements in the water.[32] The Nile crocodile has fewer osteoderms on the belly, which are much more conspicuous on some of the more modestly sized crocodilians. The species, however, also has small, oval osteoderms on the sides of the body, as well as the throat.[30][33] The Nile crocodile shares with all crocodilians a nictitating membrane 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 under water. 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. As in all crocodilians, Nile crocodiles have exceptionally high levels of lactic acid in their blood, which allows them to sit motionless in water for up to 2 hours. Levels of lactic acid as high as they are in a crocodile would kill most vertebrates. However, exertion by crocodilians can lead to death due to increasing lactic acid to lethal levels, which in turn leads to failure of the animal’s internal organs. This is rarely recorded in wild crocodiles, normally having been observed in cases where humans have mishandled crocodiles and put them through overly extended periods of physical struggling and stress.
The mouths of Nile crocodiles are filled with 64 to 68 sharply pointed, cone-shaped teeth (about a dozen less than alligators have). For most of a crocodile's life, broken teeth can be replaced. On each side of the mouth, five teeth are in the front of the upper jaw (premaxilla), 13 or 14 are in the rest of the upper jaw (maxilla), and 14 or 15 are on either side of the lower jaw (mandible). The enlarged fourth lower tooth fits into the notch on the upper jaw and is visible when the jaws are closed, as is the case with all true crocodiles. 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 hatching. Among crocodilians, the Nile crocodile possesses a relatively long snout, which is about 1.6 to 2.0 times as long as broad at the level of the front corners of the eyes. As is the saltwater crocodile, the Nile crocodile is considered a species with medium-width snout relative to other extant crocodilian species.
In a search for the largest crocodilian skulls in museums, the largest verifiable Nile crocodile skulls found were several housed in Arba Minch, Ethiopia, sourced from nearby Lake Chamo, which apparently included several specimens with a skull length more than 65 cm (26 in), with the largest one being 68.6 cm (27.0 in) in length with a mandibular length of 87 cm (34 in). Nile crocodiles with skulls this size are likely to measure in the range of 5.4 to 5.6 m (17 ft 9 in to 18 ft 4 in), which is also the length of the animals according to the museum where they were found. However, larger skulls may exist, as this study largely focused on crocodilians from Asia. The detached head of an exceptionally large Nile crocodile (killed in 1968 and measuring 5.87 m (19 ft 3 in) in length) was found to have weighed 166 kg (366 lb), including the large tendons used to shut the jaw.
Details of the skull from a juvenile Nile crocodileThe bite force exerted by an adult Nile crocodile has been shown by Dr. Brady Barr to measure 22 kN (5,000 lbf). However, the muscles responsible for opening the mouth are exceptionally weak, allowing a person to easily hold them shut, and even larger crocodiles can be brought under control by the use of duct tape to bind the jaws together. The broadest snouted modern crocodilians are alligators and larger caimans. For example, a 3.9 m (12 ft 10 in) black caiman (Melanosuchus niger) was found to have a notably broader and heavier skull than that of a Nile crocodile measuring 4.8 m (15 ft 9 in). However, despite their robust skulls, alligators and caimans appear to be proportionately equal in biting force to true crocodiles, as the muscular tendons used to shut the jaws are similar in proportional size. Only the gharial (Gavialis gangeticus) (and perhaps some of the few very thin-snouted crocodilians) is likely to have noticeably diminished bite force compared to other living species due to its exceptionally narrow, fragile snout. More or less, the size of the tendons used to impart bite force increases with body size and the larger the crocodilian gets, the stronger its bite is likely to be. Therefore, a very large male saltwater crocodile, which had attained a length around 4.59 m (15 ft 1 in), was found to have the most powerful biting force ever tested in a lab setting for any type of animal.
The Nile crocodile is the largest crocodilian in Africa, and is generally considered the second-largest crocodilian after the saltwater crocodile. Typical size has been reported to be as much as 4.5 to 5.5 m (14 ft 9 in to 18 ft 1 in), but this is excessive for actual average size per most studies and represents the upper limit of sizes attained by the largest animals in a majority of populations. According to the Guinness Book of World Records, the typical mature size is between 3.4 and 3.7 m (11 ft 2 in and 12 ft 2 in), while Alexander and Marais (2007) state it is 2.8 to 3.5 m (9 ft 2 in to 11 ft 6 in) and Garrick and Lang (1977) claim from 3.0 to 4.5 m (9 ft 10 in to 14 ft 9 in) . According to Cott (1961), the average length and weight of Nile crocodiles from Uganda and Zambia in breeding maturity was 3.16 m (10 ft 4 in) and 137.5 kg (303 lb).[6] Per Graham (1968), the average length and weight of a large sample of adult crocodiles from Lake Turkana (formerly known as Lake Rudolf), Kenya was 3.66 m (12 ft 0 in) and body mass of 201.6 kg (444 lb). Similarly, adult crocodiles from Kruger National Park reportedly average 3.65 m (12 ft 0 in) in length. In comparison, the saltwater crocodile and gharial reportedly both average around 4 m (13 ft 1 in), so are about 30 cm (12 in) longer on average and the false gharial (Tomistoma schlegelii) may average about 3.75 m (12 ft 4 in), so may be slightly longer, as well. 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). However, compared to the narrow-snouted, streamlined gharial and false gharial, the Nile crocodile is rather more robust and ranks second to the saltwater crocodile in total average body mass amongst living crocodilians and third amongst all living reptiles - the massive leatherback turtle (Dermochelys coriacea) has a mean body mass slightly less than that of an average mature male saltwater crocodiles.
Behaviour
Adult Nile crocodile baskingTypically, Nile crocodiles are relatively inert creatures, as are most crocodilians and other large, cold-blooded creatures. More than half of the crocodiles observed by Cott (1961), if not disturbed, spent the hours from 9:00 am to 4:00 pm continuously basking with their jaws open if conditions were sunny. If their jaws are bound together in the extreme midday heat, Nile crocodiles may easily die from overheating. Although they can remain practically motionless for hours on end, whether basking or sitting in shallows, Nile crocodiles are said to be constantly aware of their surroundings and aware of the presence of other animals. However, mouth-gaping (while essential to thermoregulation) may also serve as a threat display to other crocodiles, for example when specimens have been observed mouth-gaping at night when overheating is not a risk. In Lake Turkana, crocodiles rarely bask at all through the day, unlike crocodiles from most other areas, for unknown reasons, usually sitting motionless partially exposed at the surface in shallows with no apparent ill effect from the lack of basking on land.
Crocodiles are deft and agile swimmers when motivated.In South Africa, Nile crocodiles are more easily observed in winter because of the extensive amount of time they spend basking at this time of year. More time is spent in water in overcast, rainy, or misty days. In the southern reaches of their range, as a response to dry, cool conditions that they cannot survive externally, crocodiles may dig and take refuge in tunnels and engage in aestivation. Pooley found in Royal Natal National Park that during aestivation, young crocodiles of 60 to 90 cm (24 to 35 in) total length would dig tunnels around 1.2 to 1.8 m (3 ft 11 in to 5 ft 11 in) in depth for most, some tunnels measuring more than 2.7 m (8 ft 10 in), the longest there being 3.65 m (12 ft 0 in). Crocodiles in aestivation are totally lethargic, entering a state similar to animals that hibernate. Only the largest individuals engaging in aestivation leave the burrow to sun on warmest days, otherwise these crocodiles rarely left their burrows. Aestivation has been recorded from May to August.
Nile crocodiles normally dive for only a few minutes at a time, but can swim under water up to 30 minutes if threatened, and if they remain fully inactive, they can hold their breath for up to 2 hours (which, as aforementioned, is due to the high levels of lactic acid in their blood). They have a rich vocal range, and good hearing. Nile crocodiles 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 individuals are capable on occasion of surprising bursts of speed, briefly reaching up to 14 km/h (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 than on land, with a maximum known swimming speed 30 to 35 km/h (19 to 22 mph), more than three times faster than any human.
Drawing depicting the mythical relationship between plovers and crocodiles – no reliable observations exist of this purported symbiosisNile crocodiles have been widely known to have gastroliths in their stomachs, which are stones swallowed by animals for various purposes. Although this clearly is a deliberate behaviour for the species, the purpose is not definitively known. Gastroliths are not present in hatchlings, but increase quickly in presence within most crocodiles examined at 2–3.1 m (6 ft 7 in–10 ft 2 in) and yet normally become extremely rare again in very large specimens, meaning that some animals may eventually expel them. However, large specimens can have a large number of gastroliths. One crocodile measuring 3.84 m (12 ft 7 in) and weighing 239 kg (527 lb) had 5.1 kg (11 lb) of stones inside it, perhaps a record gastrolith weight for a crocodile. Specimens shot near Mpondwe on the Semliki River had gastroliths in their stomach despite being shot miles away from any sources for stones, the same applies to the Kafue Flats, Upper Zambesi and Bangweulu Swamp, all of which often had stones inside them despite being nowhere near stony regions. Cott (1961) felt that gastroliths were most likely serving as ballast to provide stability and additional weight to sink in water, this bearing great probability over the theories that they assist in digestion and staving off hunger. However, Alderton (1998) stated that a study using radiology found that gastroliths were seen to internally aid the grinding of food during digestion for a small Nile crocodile.
Herodotus claimed that Nile crocodiles have a symbiotic relationship with certain birds, such as the Egyptian plover (Pluvianus aegyptius), which enter the crocodile's mouth and pick leeches feeding on the crocodile's blood, but no evidence of this interaction actually occurring in any crocodile species has been found, and it is most likely mythical or allegorical fiction. However, Guggisberg (1972) had seen examples of birds picking scraps of meat from the teeth of basking crocodiles (without entering the mouth) and prey from soil very near basking crocodiles, so felt it was not impossible that a bold, hungry bird may occasionally nearly enter a crocodile’s mouth, but not likely as a habitual behaviour.
Hunting and diet
Attacking a Burchell's zebraNile crocodiles are apex predators throughout their range. In the water, this species 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. No matter where they attack prey, this and other crocodilians take practically all of their food by ambush, needing to grab their prey in a matter of seconds to succeed.[11] 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. However, for such large animals, their stomachs are relatively small, not much larger than a basketball in an average-sized adult, so as a rule, they are anything but voracious eaters.[13] Young crocodiles feed more actively than their elders according to studies in Uganda and Zambia. In general, at the smallest sizes (0.3–1 m (1 ft 0 in–3 ft 3 in)), Nile crocodiles were most likely to have full stomachs (17.4% full per Cott); adults at 3–4 m (9 ft 10 in–13 ft 1 in) in length were most likely to have empty stomachs (20.2%). Interestingly, in the largest size range studied by Cott, 4–5 m (13 ft 1 in–16 ft 5 in), they were the second most likely to either have full stomachs (10%) or empty stomachs (20%). Other studies have also shown a large number of adult Nile crocodiles with empty stomachs. For example, in Lake Turkana, Kenya, 48.4% of crocodiles had empty stomachs. The stomachs of brooding females are always empty, meaning that they can survive several months without food.
The Nile crocodile mostly hunts within the confines of waterways, either attacking aquatic prey or terrestrial animals when they come to the water to drink or to cross. The crocodile mainly hunts land animals by almost fully submerging its body under water. Occasionally, a crocodile quietly surfaces so that only its eyes (to check positioning) and nostrils are visible, and swims quietly and stealthily towards its mark. The attack is sudden and unpredictable. The crocodile lunges its body out of water in practically the blink of an eye and grasps its prey. On other occasions, more of its head and upper body is visible, especially when the terrestrial prey animal is on higher ground, to get a sense of the direction of the prey item as the top of an embankment or on a tree branch. Crocodile teeth are not used for tearing up flesh, but to sink deep into it and hold 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 that the prey item cannot escape through the grip. Much prey taken is much smaller than the crocodile itself and such prey can be overpowered and swallowed with ease. When it comes to larger prey, success depends on the crocodile's body power and weight to pull the prey item back into the water, where it is either drowned or killed by sudden thrashes of the head or by tearing it into pieces with the help of other crocodiles.
Feeding on a dead wildebeest in the Masai MaraSubadult 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. Some crocodiles of the species may habitually use their tails to sweep terrestrial prey off balance, sometimes forcing the prey specimen into the water, where it can be more easily drowned. 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 under water, 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 also scavenge or steal kills from other predators, such as lions and leopards (Panthera pardus). Groups of Nile crocodiles may travel hundreds of meters from a waterway to feast on a carcass. They also feed on dead hippopotamuses (Hippopotamus amphibius) as a group (sometimes including three or four dozen crocodiles), tolerating each other. In fact, probably much of the food from crocodile stomachs may come from scavenging carrion, and the crocodiles could be viewed as performing a similar function at times as do vultures or hyenas on land. 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 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 two times of its size. Most hunting on land is done at night by lying in ambush near forest trails or roadsides, up to 50 m (170 ft) from the water's edge. Since their speed and agility on land is rather outmatched by most terrestrial animals, they must use obscuring vegetation or terrain to have a chance of succeeding during land-based hunts. In one case, an adult crocodile charged from the water up a bank to kill a bushbuck (Tragelaphus scriptus) and instead of dragging it into the water, was observed to pull the kill further on land into the cover of the bush. Two subadult crocodiles were once seen carrying the carcass of a nyala (Tragelaphus angasii) across land in unison. In South Africa, a game warden far from water sources in a savannah-scrub area reported that he saw a crocodile jump up and grab a donkey by the neck and then drag the prey off.
Attacks on humans
Much of the hunting of and general animosity towards Nile crocodiles stems from their reputation as a man-eater, which is not entirely unjustified.[185] Despite most attacks going unreported, the Nile crocodile along with the saltwater crocodile is estimated to kill hundreds (possibly thousands) of people each year, which may be more than all other crocodilian species combined. While these species are much more aggressive towards people than other living crocodilians (as is statistically supported by estimated numbers of crocodile attacks), Nile crocodiles are not particularly more likely to behave aggressively to humans or regard humans as potential prey than saltwater crocodiles. However, unlike other "man-eating" crocodile species, including the saltwater crocodile, the Nile crocodile lives within close proximity to human populations through most of its range, so contact is more frequent. This combined with the species’ large size renders a higher risk of attack. Crocodiles as small as 2.1 m (6 ft 11 in) are capable of overpowering and successfully preying on small apes and hominids, presumably including children and smaller adult humans, but a majority of fatal attacks on humans are by crocodiles reportedly exceeding 3 m (9 ft 10 in) in length.
In studies preceding the slaughter of crocodiles for the leather trade, when there were believed to be many more Nile crocodiles, a roughly estimated 1,000 human fatalities per annum by Nile crocodiles were posited with a roughly equal number of aborted attacks. A more contemporary study claimed 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. With the Nile crocodile and the saltwater crocodile, the mean size of crocodiles involved in non-fatal attacks was about 3 m (9 ft 10 in) as opposed to a reported range of 2.5–5 m (8 ft 2 in–16 ft 5 in) or larger for crocodiles responsible for fatal attacks. The average estimated size of Nile crocodiles involved in fatal attacks is 3.5 m (11 ft 6 in). 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.[188] In comparison, lions, in the years from 1990 to 2006, were responsible for an estimated one-eighth as many fatal attacks on humans in Africa as were Nile crocodiles. Although Nile crocodiles are more than a dozen times more numerous than lions in the wild, probably fewer than a quarter of living Nile crocodiles are old and large enough to pose a danger to humans.[188][189][190] Other wild animals responsible for more annual human mortalities either attack humans in self-defense, as do venomous snakes, or are deadly only as vectors of disease or infection, such as snails, rats and mosquitos.
Regional reportage from numerous areas with large crocodile populations nearby indicate, per district or large village, that crocodiles often annually claim about a dozen or more lives per year. Miscellaneous examples of areas in the last few decades with a dozen or more fatal crocodile attacks annually include Korogwe District, Tanzania, Niassa Reserve, Mozambique and the area around Lower Zambezi National Park, Zambia.[195][196] Despite historic claims that the victims of Nile crocodile attacks are usually "women and children",[2] there is no detectable trends in this regard and any human, regardless of age, gender, or size is potentially vulnerable. Incautious human behavior unfortunately is the primary drive behind crocodile attacks.[16] Most fatal attacks occur when a person is standing a few feet away from water on a non-steep bank, are wading in shallow waters, actively swimming or have limbs dangling over a boat or pier. Many victims are caught while crouching and people in jobs that might require heavy usage of water including laundry workers, fisherman, game wardens and regional guides are more likely to be attacked. Many fisherman and other workers who are not poverty-stricken will go out of their way to avoid waterways known to harbor large crocodile populations.
Most biologists who have engaged in months or even years of field work with Nile crocodiles, including Cott (1961), Graham and Beard (1968) and Guggisberg (1972), have found that with sufficient precautions, their own lives and the lives of their local guides were rarely, if ever, at risk in areas with many crocodiles. However, Guggisberg accumulated several earlier writings that noted the lack of fear of crocodiles among Africans, driven in part perhaps by poverty and superstition, that caused many observed cases of an "appalling" lack of caution within view of large crocodiles, as opposed to the presence of bold lions which engendered an appropriate panic. Per Guggisberg, this disregard (essentially regarding the crocodile as a lowly creature and thus non-threatening to humans) may account for the seemingly higher frequency of deadly attacks by crocodiles than by large mammalian carnivores. Most locals are well aware of how to behave in crocodile-occupied areas and some of the writings quoted by Guggisberg from the 19th and 20th century may require being taken with a "grain of salt".
SOURCE WIKIPEDIA
Nile crocodiles are opportunistic apex predators; a very aggressive species of crocodile, they are capable of taking almost any animal within their range. They are generalists, taking a variety of prey. Their diet consists mostly of different species of fish, reptiles, birds, and mammals. They are ambush predators that can wait for hours, days, and even weeks for the suitable moment to attack. They are agile predators and wait for the opportunity for a prey item to come well within attack range. 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 relatively social crocodiles. They share basking spots and large food sources, such as schools of fish and big carcasses. Their strict hierarchy is determined by size. Large, old males are at the top of this hierarchy and have primary access to food and the best basking spots. Crocodiles tend to respect this order; when it is infringed, the results are often violent and sometimes fatal. Like most other reptiles, Nile crocodiles lay eggs; these are guarded by the females. 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 human deaths every year. It is a rather common species of crocodile and is not endangered despite some regional declines or extinctions.
Characteristics and physiology
Side viewAdult Nile crocodiles have a dark bronze colouration above, with faded blackish spots and stripes variably appearing across the back and a dingy off-yellow on the belly, although mud can often obscure the crocodile’s actual colour. The flanks, which are yellowish-green in colour, have dark patches arranged in oblique stripes in highly variable patterns. Some variation occurs relative to environment; specimens from swift-flowing waters tend to be lighter in colour than those dwelling in murkier lakes or swamps, which provides camouflage that suits their environment, an example of clinal variation. Nile crocodiles have green eyes. The colouration also helps to camouflage it; juveniles are grey, multicoloured, or brown, with dark cross-bands on the tail and body. The underbelly of young crocodiles is yellowish green. As it matures, Nile crocodiles become darker and the cross-bands fade, especially those on the upper-body. A similar tendency is coloration change during maturation has been noted in most crocodile species.
Most morphological attributes of Nile crocodiles are typical of crocodilians as a whole. Like all crocodilians, for example, 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, elongated jaws. Their skin has a number of poorly understood integumentary sense organs that may react to changes in water pressure, presumably allowing them to track prey movements in the water.[32] The Nile crocodile has fewer osteoderms on the belly, which are much more conspicuous on some of the more modestly sized crocodilians. The species, however, also has small, oval osteoderms on the sides of the body, as well as the throat.[30][33] The Nile crocodile shares with all crocodilians a nictitating membrane 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 under water. 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. As in all crocodilians, Nile crocodiles have exceptionally high levels of lactic acid in their blood, which allows them to sit motionless in water for up to 2 hours. Levels of lactic acid as high as they are in a crocodile would kill most vertebrates. However, exertion by crocodilians can lead to death due to increasing lactic acid to lethal levels, which in turn leads to failure of the animal’s internal organs. This is rarely recorded in wild crocodiles, normally having been observed in cases where humans have mishandled crocodiles and put them through overly extended periods of physical struggling and stress.
The mouths of Nile crocodiles are filled with 64 to 68 sharply pointed, cone-shaped teeth (about a dozen less than alligators have). For most of a crocodile's life, broken teeth can be replaced. On each side of the mouth, five teeth are in the front of the upper jaw (premaxilla), 13 or 14 are in the rest of the upper jaw (maxilla), and 14 or 15 are on either side of the lower jaw (mandible). The enlarged fourth lower tooth fits into the notch on the upper jaw and is visible when the jaws are closed, as is the case with all true crocodiles. 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 hatching. Among crocodilians, the Nile crocodile possesses a relatively long snout, which is about 1.6 to 2.0 times as long as broad at the level of the front corners of the eyes. As is the saltwater crocodile, the Nile crocodile is considered a species with medium-width snout relative to other extant crocodilian species.
In a search for the largest crocodilian skulls in museums, the largest verifiable Nile crocodile skulls found were several housed in Arba Minch, Ethiopia, sourced from nearby Lake Chamo, which apparently included several specimens with a skull length more than 65 cm (26 in), with the largest one being 68.6 cm (27.0 in) in length with a mandibular length of 87 cm (34 in). Nile crocodiles with skulls this size are likely to measure in the range of 5.4 to 5.6 m (17 ft 9 in to 18 ft 4 in), which is also the length of the animals according to the museum where they were found. However, larger skulls may exist, as this study largely focused on crocodilians from Asia. The detached head of an exceptionally large Nile crocodile (killed in 1968 and measuring 5.87 m (19 ft 3 in) in length) was found to have weighed 166 kg (366 lb), including the large tendons used to shut the jaw.
Details of the skull from a juvenile Nile crocodileThe bite force exerted by an adult Nile crocodile has been shown by Dr. Brady Barr to measure 22 kN (5,000 lbf). However, the muscles responsible for opening the mouth are exceptionally weak, allowing a person to easily hold them shut, and even larger crocodiles can be brought under control by the use of duct tape to bind the jaws together. The broadest snouted modern crocodilians are alligators and larger caimans. For example, a 3.9 m (12 ft 10 in) black caiman (Melanosuchus niger) was found to have a notably broader and heavier skull than that of a Nile crocodile measuring 4.8 m (15 ft 9 in). However, despite their robust skulls, alligators and caimans appear to be proportionately equal in biting force to true crocodiles, as the muscular tendons used to shut the jaws are similar in proportional size. Only the gharial (Gavialis gangeticus) (and perhaps some of the few very thin-snouted crocodilians) is likely to have noticeably diminished bite force compared to other living species due to its exceptionally narrow, fragile snout. More or less, the size of the tendons used to impart bite force increases with body size and the larger the crocodilian gets, the stronger its bite is likely to be. Therefore, a very large male saltwater crocodile, which had attained a length around 4.59 m (15 ft 1 in), was found to have the most powerful biting force ever tested in a lab setting for any type of animal.
The Nile crocodile is the largest crocodilian in Africa, and is generally considered the second-largest crocodilian after the saltwater crocodile. Typical size has been reported to be as much as 4.5 to 5.5 m (14 ft 9 in to 18 ft 1 in), but this is excessive for actual average size per most studies and represents the upper limit of sizes attained by the largest animals in a majority of populations. According to the Guinness Book of World Records, the typical mature size is between 3.4 and 3.7 m (11 ft 2 in and 12 ft 2 in), while Alexander and Marais (2007) state it is 2.8 to 3.5 m (9 ft 2 in to 11 ft 6 in) and Garrick and Lang (1977) claim from 3.0 to 4.5 m (9 ft 10 in to 14 ft 9 in) . According to Cott (1961), the average length and weight of Nile crocodiles from Uganda and Zambia in breeding maturity was 3.16 m (10 ft 4 in) and 137.5 kg (303 lb).[6] Per Graham (1968), the average length and weight of a large sample of adult crocodiles from Lake Turkana (formerly known as Lake Rudolf), Kenya was 3.66 m (12 ft 0 in) and body mass of 201.6 kg (444 lb). Similarly, adult crocodiles from Kruger National Park reportedly average 3.65 m (12 ft 0 in) in length. In comparison, the saltwater crocodile and gharial reportedly both average around 4 m (13 ft 1 in), so are about 30 cm (12 in) longer on average and the false gharial (Tomistoma schlegelii) may average about 3.75 m (12 ft 4 in), so may be slightly longer, as well. 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). However, compared to the narrow-snouted, streamlined gharial and false gharial, the Nile crocodile is rather more robust and ranks second to the saltwater crocodile in total average body mass amongst living crocodilians and third amongst all living reptiles - the massive leatherback turtle (Dermochelys coriacea) has a mean body mass slightly less than that of an average mature male saltwater crocodiles.
Behaviour
Adult Nile crocodile baskingTypically, Nile crocodiles are relatively inert creatures, as are most crocodilians and other large, cold-blooded creatures. More than half of the crocodiles observed by Cott (1961), if not disturbed, spent the hours from 9:00 am to 4:00 pm continuously basking with their jaws open if conditions were sunny. If their jaws are bound together in the extreme midday heat, Nile crocodiles may easily die from overheating. Although they can remain practically motionless for hours on end, whether basking or sitting in shallows, Nile crocodiles are said to be constantly aware of their surroundings and aware of the presence of other animals. However, mouth-gaping (while essential to thermoregulation) may also serve as a threat display to other crocodiles, for example when specimens have been observed mouth-gaping at night when overheating is not a risk. In Lake Turkana, crocodiles rarely bask at all through the day, unlike crocodiles from most other areas, for unknown reasons, usually sitting motionless partially exposed at the surface in shallows with no apparent ill effect from the lack of basking on land.
Crocodiles are deft and agile swimmers when motivated.In South Africa, Nile crocodiles are more easily observed in winter because of the extensive amount of time they spend basking at this time of year. More time is spent in water in overcast, rainy, or misty days. In the southern reaches of their range, as a response to dry, cool conditions that they cannot survive externally, crocodiles may dig and take refuge in tunnels and engage in aestivation. Pooley found in Royal Natal National Park that during aestivation, young crocodiles of 60 to 90 cm (24 to 35 in) total length would dig tunnels around 1.2 to 1.8 m (3 ft 11 in to 5 ft 11 in) in depth for most, some tunnels measuring more than 2.7 m (8 ft 10 in), the longest there being 3.65 m (12 ft 0 in). Crocodiles in aestivation are totally lethargic, entering a state similar to animals that hibernate. Only the largest individuals engaging in aestivation leave the burrow to sun on warmest days, otherwise these crocodiles rarely left their burrows. Aestivation has been recorded from May to August.
Nile crocodiles normally dive for only a few minutes at a time, but can swim under water up to 30 minutes if threatened, and if they remain fully inactive, they can hold their breath for up to 2 hours (which, as aforementioned, is due to the high levels of lactic acid in their blood). They have a rich vocal range, and good hearing. Nile crocodiles 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 individuals are capable on occasion of surprising bursts of speed, briefly reaching up to 14 km/h (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 than on land, with a maximum known swimming speed 30 to 35 km/h (19 to 22 mph), more than three times faster than any human.
Drawing depicting the mythical relationship between plovers and crocodiles – no reliable observations exist of this purported symbiosisNile crocodiles have been widely known to have gastroliths in their stomachs, which are stones swallowed by animals for various purposes. Although this clearly is a deliberate behaviour for the species, the purpose is not definitively known. Gastroliths are not present in hatchlings, but increase quickly in presence within most crocodiles examined at 2–3.1 m (6 ft 7 in–10 ft 2 in) and yet normally become extremely rare again in very large specimens, meaning that some animals may eventually expel them. However, large specimens can have a large number of gastroliths. One crocodile measuring 3.84 m (12 ft 7 in) and weighing 239 kg (527 lb) had 5.1 kg (11 lb) of stones inside it, perhaps a record gastrolith weight for a crocodile. Specimens shot near Mpondwe on the Semliki River had gastroliths in their stomach despite being shot miles away from any sources for stones, the same applies to the Kafue Flats, Upper Zambesi and Bangweulu Swamp, all of which often had stones inside them despite being nowhere near stony regions. Cott (1961) felt that gastroliths were most likely serving as ballast to provide stability and additional weight to sink in water, this bearing great probability over the theories that they assist in digestion and staving off hunger. However, Alderton (1998) stated that a study using radiology found that gastroliths were seen to internally aid the grinding of food during digestion for a small Nile crocodile.
Herodotus claimed that Nile crocodiles have a symbiotic relationship with certain birds, such as the Egyptian plover (Pluvianus aegyptius), which enter the crocodile's mouth and pick leeches feeding on the crocodile's blood, but no evidence of this interaction actually occurring in any crocodile species has been found, and it is most likely mythical or allegorical fiction. However, Guggisberg (1972) had seen examples of birds picking scraps of meat from the teeth of basking crocodiles (without entering the mouth) and prey from soil very near basking crocodiles, so felt it was not impossible that a bold, hungry bird may occasionally nearly enter a crocodile’s mouth, but not likely as a habitual behaviour.
Hunting and diet
Attacking a Burchell's zebraNile crocodiles are apex predators throughout their range. In the water, this species 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. No matter where they attack prey, this and other crocodilians take practically all of their food by ambush, needing to grab their prey in a matter of seconds to succeed.[11] 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. However, for such large animals, their stomachs are relatively small, not much larger than a basketball in an average-sized adult, so as a rule, they are anything but voracious eaters.[13] Young crocodiles feed more actively than their elders according to studies in Uganda and Zambia. In general, at the smallest sizes (0.3–1 m (1 ft 0 in–3 ft 3 in)), Nile crocodiles were most likely to have full stomachs (17.4% full per Cott); adults at 3–4 m (9 ft 10 in–13 ft 1 in) in length were most likely to have empty stomachs (20.2%). Interestingly, in the largest size range studied by Cott, 4–5 m (13 ft 1 in–16 ft 5 in), they were the second most likely to either have full stomachs (10%) or empty stomachs (20%). Other studies have also shown a large number of adult Nile crocodiles with empty stomachs. For example, in Lake Turkana, Kenya, 48.4% of crocodiles had empty stomachs. The stomachs of brooding females are always empty, meaning that they can survive several months without food.
The Nile crocodile mostly hunts within the confines of waterways, either attacking aquatic prey or terrestrial animals when they come to the water to drink or to cross. The crocodile mainly hunts land animals by almost fully submerging its body under water. Occasionally, a crocodile quietly surfaces so that only its eyes (to check positioning) and nostrils are visible, and swims quietly and stealthily towards its mark. The attack is sudden and unpredictable. The crocodile lunges its body out of water in practically the blink of an eye and grasps its prey. On other occasions, more of its head and upper body is visible, especially when the terrestrial prey animal is on higher ground, to get a sense of the direction of the prey item as the top of an embankment or on a tree branch. Crocodile teeth are not used for tearing up flesh, but to sink deep into it and hold 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 that the prey item cannot escape through the grip. Much prey taken is much smaller than the crocodile itself and such prey can be overpowered and swallowed with ease. When it comes to larger prey, success depends on the crocodile's body power and weight to pull the prey item back into the water, where it is either drowned or killed by sudden thrashes of the head or by tearing it into pieces with the help of other crocodiles.
Feeding on a dead wildebeest in the Masai MaraSubadult 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. Some crocodiles of the species may habitually use their tails to sweep terrestrial prey off balance, sometimes forcing the prey specimen into the water, where it can be more easily drowned. 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 under water, 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 also scavenge or steal kills from other predators, such as lions and leopards (Panthera pardus). Groups of Nile crocodiles may travel hundreds of meters from a waterway to feast on a carcass. They also feed on dead hippopotamuses (Hippopotamus amphibius) as a group (sometimes including three or four dozen crocodiles), tolerating each other. In fact, probably much of the food from crocodile stomachs may come from scavenging carrion, and the crocodiles could be viewed as performing a similar function at times as do vultures or hyenas on land. 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 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 two times of its size. Most hunting on land is done at night by lying in ambush near forest trails or roadsides, up to 50 m (170 ft) from the water's edge. Since their speed and agility on land is rather outmatched by most terrestrial animals, they must use obscuring vegetation or terrain to have a chance of succeeding during land-based hunts. In one case, an adult crocodile charged from the water up a bank to kill a bushbuck (Tragelaphus scriptus) and instead of dragging it into the water, was observed to pull the kill further on land into the cover of the bush. Two subadult crocodiles were once seen carrying the carcass of a nyala (Tragelaphus angasii) across land in unison. In South Africa, a game warden far from water sources in a savannah-scrub area reported that he saw a crocodile jump up and grab a donkey by the neck and then drag the prey off.
Attacks on humans
Much of the hunting of and general animosity towards Nile crocodiles stems from their reputation as a man-eater, which is not entirely unjustified.[185] Despite most attacks going unreported, the Nile crocodile along with the saltwater crocodile is estimated to kill hundreds (possibly thousands) of people each year, which may be more than all other crocodilian species combined. While these species are much more aggressive towards people than other living crocodilians (as is statistically supported by estimated numbers of crocodile attacks), Nile crocodiles are not particularly more likely to behave aggressively to humans or regard humans as potential prey than saltwater crocodiles. However, unlike other "man-eating" crocodile species, including the saltwater crocodile, the Nile crocodile lives within close proximity to human populations through most of its range, so contact is more frequent. This combined with the species’ large size renders a higher risk of attack. Crocodiles as small as 2.1 m (6 ft 11 in) are capable of overpowering and successfully preying on small apes and hominids, presumably including children and smaller adult humans, but a majority of fatal attacks on humans are by crocodiles reportedly exceeding 3 m (9 ft 10 in) in length.
In studies preceding the slaughter of crocodiles for the leather trade, when there were believed to be many more Nile crocodiles, a roughly estimated 1,000 human fatalities per annum by Nile crocodiles were posited with a roughly equal number of aborted attacks. A more contemporary study claimed 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. With the Nile crocodile and the saltwater crocodile, the mean size of crocodiles involved in non-fatal attacks was about 3 m (9 ft 10 in) as opposed to a reported range of 2.5–5 m (8 ft 2 in–16 ft 5 in) or larger for crocodiles responsible for fatal attacks. The average estimated size of Nile crocodiles involved in fatal attacks is 3.5 m (11 ft 6 in). 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.[188] In comparison, lions, in the years from 1990 to 2006, were responsible for an estimated one-eighth as many fatal attacks on humans in Africa as were Nile crocodiles. Although Nile crocodiles are more than a dozen times more numerous than lions in the wild, probably fewer than a quarter of living Nile crocodiles are old and large enough to pose a danger to humans.[188][189][190] Other wild animals responsible for more annual human mortalities either attack humans in self-defense, as do venomous snakes, or are deadly only as vectors of disease or infection, such as snails, rats and mosquitos.
Regional reportage from numerous areas with large crocodile populations nearby indicate, per district or large village, that crocodiles often annually claim about a dozen or more lives per year. Miscellaneous examples of areas in the last few decades with a dozen or more fatal crocodile attacks annually include Korogwe District, Tanzania, Niassa Reserve, Mozambique and the area around Lower Zambezi National Park, Zambia.[195][196] Despite historic claims that the victims of Nile crocodile attacks are usually "women and children",[2] there is no detectable trends in this regard and any human, regardless of age, gender, or size is potentially vulnerable. Incautious human behavior unfortunately is the primary drive behind crocodile attacks.[16] Most fatal attacks occur when a person is standing a few feet away from water on a non-steep bank, are wading in shallow waters, actively swimming or have limbs dangling over a boat or pier. Many victims are caught while crouching and people in jobs that might require heavy usage of water including laundry workers, fisherman, game wardens and regional guides are more likely to be attacked. Many fisherman and other workers who are not poverty-stricken will go out of their way to avoid waterways known to harbor large crocodile populations.
Most biologists who have engaged in months or even years of field work with Nile crocodiles, including Cott (1961), Graham and Beard (1968) and Guggisberg (1972), have found that with sufficient precautions, their own lives and the lives of their local guides were rarely, if ever, at risk in areas with many crocodiles. However, Guggisberg accumulated several earlier writings that noted the lack of fear of crocodiles among Africans, driven in part perhaps by poverty and superstition, that caused many observed cases of an "appalling" lack of caution within view of large crocodiles, as opposed to the presence of bold lions which engendered an appropriate panic. Per Guggisberg, this disregard (essentially regarding the crocodile as a lowly creature and thus non-threatening to humans) may account for the seemingly higher frequency of deadly attacks by crocodiles than by large mammalian carnivores. Most locals are well aware of how to behave in crocodile-occupied areas and some of the writings quoted by Guggisberg from the 19th and 20th century may require being taken with a "grain of salt".
SOURCE WIKIPEDIA
