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Varanus niloticus Print E-mail

Nile monitor, Water leguaan

Varanus niloticus niloticus     Linnaeus 1758
Varanus niloticus ornatus       Daudin 1803

ImageThe Nile monitor is the largest lizard in Africa and also one of the most widespread. It is known from all parts of Africa except desert regions (Mertens 1942, Luxmoore et al 1988). More than a hundred years ago Nile monitors were reported to live in Palestine (Tristram 1888). V.griseus is the only monitor found there now but the two species are so different that it would be very difficult to confuse them. Perhaps the Nile monitor once lived along the banks of the Jordan River, but today it seems almost certain that they are restricted to the African continent.

The subspecies V.niloticus ornatus (the ornate Nile monitor or African forest monitor) is currently recognised on the basis of having a light coloured tongue and 3-5 rows of light markings over the back, compared to 6-9 rows in the nominate race (Mertens 1942, Loveridge 1953). V.n.ornatus is restricted to the rainforests and coastal grasslands of central and western Africa. There is growing evidence that this poorly-known subspecies is a very different animal to the Nile monitor. However, observations on monitor lizards in African forests are surprisingly scarce. I have never seen this lizard in the wild, but examined a burrow in 1994 that locals claimed was its home. The burrow was immense and dug on a rock strewn hillside close to a large bat colony. In Ghana this lizard is known as "mampam sika" (money monitor) on account of the large golden spots on its back. Few people claim to have seen it, and those that do say that it attains ahuge size, lives in the same burrow for all of its adult life if unmolested and is so strong and fierce that it is impossible to capture without killing it. If one is caught, something very very good will happen to the hunter shortly afterwards, or something very very bad. Later this year we hope to make some investigations into the status and biology of this extraordinarily beautiful monitor lizard.
Nile monitors are found almost wherever there are permanent bodies of water. They are absent from deserts but present in most other habitats, from grasslands and desert fringes to rainforests, where they are found along rivers, swamps, pools, lakes and seashores. They will readily inhabit human settlements and cultivations where they are not persecuted. Barbour & Loveridge (1928) record them as high as 2000m above sea level.

In 1929 Ingleton published a photograph of a Nile monitor from South Africa which he claimed measured 213cm TL and weighed almost 19kg. He also claimed to have shot another 250cm long. No specimens of this size exist there today but Buffrenil (1992) considered that animals of this size could occur around Lake Chad. Charlton (1973), Patterson (1987) and Fitzsimmons (1934) suggest maximum lengths of about 150cm. Auerbach (1985) gives a figure of over 200cm. Specimens of 188cm (74cm SVL) have been recorded from Orange Free State in South Africa (de Waal 1978) and Boycott & Morgan (1988) refer to a female of 170cm, so it seems likely that in southern Africa occasional specimens may attain lengths of 200cm or more, but they must be rare. A male from Sudan with a SVL of 57.5cm weighed 5.9kg (Hirth & Latif 1979). A healthy male from the coast of Ghana (SVL 49cm) weighed 3,011g and what appeared to be a gravid female from the same location (SVL 44cm) weighed 1,478g (pers.obs.). Around Lake Chad males reach a maximum size of 207cm, females 155cm TL. Typically, males measure 150-170cm TL and weigh 5-10kg, females 134cm TL, 3kg. In this area, where the animals are predated upon by man, they attain ages of around eight years (Buffrenil 1992, Buffrenil et al 1994).  McGraw (1992) stated that Nile monitors reach 200cm in length around Lake Malawi National Park, and that those from Mumbo and Boadzulu Islands were particularly large. Hatchlings weigh about 30g and measure around 30cm TL.

ImageChanges in skull and tooth morphology with age are well documented in this species (Dumeril & Bibron 1856, Lonnberg 1903, Schmidt 1919, Peyer 1929, Rieppel & Labhardt 1979). Young specimens have sharp pointed teeth that are gradually replaced with teeth that are broader and develop ridges on the crown. These changes are generally attributed to a shift in diet from fast moving prey such as insect and lizards to a diet of more sessile, but better armoured, prey such as molluscs and crustaceans that must be crushed in the jaws before being swallowed. Where crabs and snails abound they may form a large part of the diet, but in general Nile monitors eat anything they can fit into their mouths. In the Shai Hills of Ghana they appear to be associated with large bat colonies (Bennett & Akonnor, ms). Elsewhere beetles, spiders, orthopterans, snakes, lizards, young crocodiles, fish, small mammals (including domestic cats), birds and their eggs, frogs, toads, crabs, snails, slugs, turtles, termites, caterpillars and reptile eggs (including those of crocodiles, agamids and varanids) are all included in the diet (Hesse 1889; Lonnberg 1903; Roosevelt 1910; Schmidt 1919; Barbour & Loveridge 1928; Cowles 1930; Pitman 1931; Loveridge 1933; Darby 1955; Cott 1960; Cisse 1972; Borquin & Channing 1980; Dial & Vaughan 1987; Patterson 1987; Losos & Greene 1988; Lang & Branch 1990; Van Rhyn 1991; McGraw 1992; Yeboah 1993). They also feed readily on carrion, including the remains of lion kills (Edroma & Ssali 1983), often forage on human waste tips and will even consume human faeces (Bennett & Akonnor ms).   Important prey items vary with habitat, but the great variety taken reflects that fact that Nile monitors forage on or below the ground, in trees or in water. Cisse (1972) suggested that they regularly revisit previous excavations to search for food, and may ambush nesting birds. They may use similar strategies to catch migrating shoals of fish (Pienaar 1978). Cott (1960), Modha (1965) and others consider them to be amongst the most important predators of crocodile eggs and young. Ground nesting birds such as water Dikkop often nest in the same areas as crocodiles. It has been suggested that this strategy reduces the danger of other animals, particularly monitor lizards, preying on their eggs (Pitman 1957, Cott 1961). Pitman (1931) suggested that the monitors work in pairs to raid the nests, one diverting the female crocodile's attention and drawing her away from the nest whilst the other rushes in and digs up the eggs. Crocodiles will eat Nile monitors when they can catch them. Other known predators include mongooses and cobras (Branch & Branch 1992; Hinkel 1987).

Nile monitors are most often seen basking on rocks and branches or in the water. Adults can easily outrun people over short distances, even over open ground and will almost invariably make for water when pursued. They retreat to burrows and abandoned termite mounds at night, but in warm weather they may remain outside, sleeping on branches or half submerged in water. Rose's (1962) report that they line their burrows with cow dung is unsubstantiated. Cisse (1972) found that in Senegal the animals left their burrows in the morning and did not return until late afternoon. This is confirmed by Edroma & Ssali (1983) who also note that whereas in warm weather rock or sand are the usual basking substrates, in cooler weather grass and branches are preferred. Of 35 burrows examined by them, none had been dug by the lizards themselves.



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