Nile crocodile
The Nile crocodile is a large crocodilian native to freshwater habitats in Africa, where it is present in 26 countries. It is widely distributed in sub-Saharan Africa, occurring mostly in the eastern, southern, and central regions of the continent, and lives in different types of aquatic environments such as lakes, rivers, swamps and marshlands. It occasionally inhabits deltas, brackish lakes and rarely also saltwater. Its range once stretched from the Nile Delta throughout the Nile River. Lake Turkana in Kenya has one of the largest undisturbed Nile crocodile populations.
Generally, the adult male Nile crocodile is between in length and weighs. However, specimens exceeding in length and in weight have been recorded. It is the largest predator in Africa, and may be considered the second-largest extant reptile in the world, after the saltwater crocodile. Size is sexually dimorphic, with females usually about 30% smaller than males. The crocodile has thick, scaly, heavily armoured skin.
Nile crocodiles are opportunistic apex predators; a very aggressive crocodile, they are capable of taking almost any animal within their range. They are generalists, taking a variety of prey, with a diet consisting mostly of different species of fish, reptiles, birds, and mammals. As ambush predators, they 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 a powerful bite that is unique among all animals, and sharp, conical teeth that sink into flesh, allowing a grip that is almost impossible to loosen. They can apply high force for extended periods of time, a great advantage for holding down large prey underwater to drown.
Nile crocodiles are relatively social amongst themselves. 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 first 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 but also males, making the Nile crocodiles one of few reptile species whose males contribute to parental care. 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 common and is not endangered, despite some regional declines or extirpations in the Maghreb.
Etymology and naming
The binomial name Crocodylus niloticus is derived from the Greek κρόκη, kroke, δρῖλος, drilos, referring to its rough skin; and niloticus, meaning "from the Nile River". The Nile crocodile is called timsah al-nil in Arabic, mamba in Swahili, yaxaas in Somali, garwe in Shona, ngwenya in Ndebele, ngwena in Venda, kwena in Sotho and Tswana, and tanin ha-yeor in Hebrew. It also sometimes referred to as the African crocodile, Ethiopian crocodile, and common crocodile.Taxonomy
Although no subspecies are currently formally recognized, as many as seven have been proposed, mostly due to variations in appearance and size noted in various populations throughout Africa. These have consisted of C. n. africanus, C. n. chamses, C. n. cowiei, C. n. madagascariensis, C. n. niloticus, C. n. pauciscutatus and C. suchus.In a study of the morphology of the various populations, including C. suchus, the appearance of the Nile crocodile sensu lato was found to be more variable than that of any other currently recognized crocodile species, and at least some of these variations were related to locality. For example, a study on Lake Turkana in Kenya found that the local crocodiles have more osteoderms in their ventral surface than other known populations, and thus are of lesser value in leather trading, accounting for an exceptionally large local population there in the late 20th century. The segregation of the West African crocodile from the Nile crocodile has been supported by morphological characteristics, studies of genetic materials and habitat preferences. The separation of the two is not recognized by the IUCN as their last evaluations of the group was in 2008 and 2009, years before the primary publications supporting the distinctiveness of the West African crocodiles.
Evolution
Although originally thought to be the same species as the West African crocodile, genetic studies using DNA sequencing have revealed that the Nile crocodile is actually more closely related to the crocodiles of the Americas, namely the American, Cuban, Morelet's, and Orinoco crocodiles. The extant Nile crocodiles first appeared during the Late Miocene-Early Pliocene in Kenya based on fossil remains dated to approximately, and the related fossil species C. checchiai from the same strata was similar in size and shared similar physical characteristics to this specific species.At one time, the fossil species Rimasuchus lloydi was thought to be the closest relative of the Nile crocodile, but more recent research has indicated that Rimasuchus, despite its very large size, is more closely related to the dwarf crocodile among living species. Two other fossil species from Africa retained in the genus Crocodylus appear to be closely related to the Nile crocodile: C. anthropophagus from Plio-Pleistocene Tanzania and C. thorbjarnarsoni from Plio-Pleistocene Kenya. C. anthropophagus and C. thorbjarnarsoni were both somewhat larger, with projected total lengths up to. As well as being larger, C. anthropophagus and C. thorbjarnarsoni, as well as Rimasuchus spp., were all relatively broad-snouted, indicating a specialization at hunting sizeable prey, such as large mammals and freshwater turtles, the latter much larger than any in present-day Africa. Studies have since shown these other African crocodiles to be only more distantly related to the Nile crocodile.
Below is a cladogram based on a 2018 tip dating study by Lee & Yates simultaneously using morphological, molecular, and stratigraphic data, as revised by the 2021 Hekkala et al. paleogenomics study using DNA extracted from the extinct Voay. Hall's New Guinea crocodile placement suggested in 2023 study by Sales-Oliveira ''et al.''
Characteristics and physiology
Adult 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 them; juveniles are grey, multicoloured, or brown, with dark cross-bands on the tail and body. The underbelly of young crocodiles is yellowish green. As they mature, Nile crocodiles become darker and the cross-bands fade, especially those on the upper-body. A similar tendency in 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. 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. 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.
Skull and head morphology
The mouths of Nile crocodiles are filled with 64 to 68 sharply pointed, cone-shaped teeth. 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, 13 or 14 are in the rest of the upper jaw, and 14 or 15 are on either side of the lower jaw. 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, with the largest one being in length with a mandibular length of. Nile crocodiles with skulls this size are likely to measure in the range of, 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 was found to have weighed, including the large tendons used to shut the jaw.