Nile monitor
| Nile monitor | ||||||||||||
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Nile monitor in Mapungubwe National Park ( South Africa ) |
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| Scientific name | ||||||||||||
| Varanus niloticus | ||||||||||||
| ( Linnaeus , 1766) |
The Nile monitor ( Varanus niloticus ) is a kind of Squamata (Squamata). With lengths of over 2 m, it belongs to the large species of monitor lizards ( Varanus ). In the past, the rainforest Nile monitor ( Varanus ornatus ) was considered a subspecies of the Nile monitor, but today both forms are considered as separate species.
Nile monitors inhabit large parts of Africa. They prefer to live near bodies of water and are good swimmers. The diet consists mainly of arthropods , small vertebrates , crustaceans , mollusks , eggs and carrion. The species is of great importance for the leather and pet trade as well as food.
features
Nile monitors in western Africa normally reach a total length of about 1.7 m, in the rest of the distribution area they become larger: In Chad an animal with a total length of 2.14 m was measured; the record in length is held by a 2.4 m long specimen from South Africa . Males become slightly larger than females ( sexual dimorphism ); in Gambia an average head-trunk length of 64.4 cm for males and 52.3 cm for females was determined. The tail is about 1.5 times as long as the head and trunk, flattened laterally as a row tail and has a low dorsal crest.
Adults are gray-brown to olive-brown with yellow eye spots and ribbons on the head, back, legs and tail. The back and throat are light with black lines. The Nile monitor has six to nine transverse rows of eye spots on the back between the front and rear legs. This characteristic enables a very reliable differentiation from the visually very similar rainforest Nile monitor, which has only five, rarely fewer, of these bands.
distribution and habitat
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Natural occurrence
The Nile monitor lives practically all of sub-Saharan Africa, but is absent from the deserts of Somalia , Namibia , Botswana and South Africa . In the rainforests of West and Central Africa ( Congo Basin , Gabon , Cameroon , Guinea , Equatorial Guinea , Togo , Zaire ) it occurs together with the rainforest Nile monitor. To the north, its distribution area extends along the upper reaches of the Nile to Egypt . The very adaptable species lives in a variety of habitats, including savannas, scrubland, undergrowth, forests, swamps, mangroves and the banks of lakes and rivers, sometimes also in urban areas, where they search garbage dumps for food. The only apparently indispensable requirements for its habitat are free areas for sunbathing and the at least temporary availability of water.
Nile monitors as neozoa
A stable, reproductive population of Nile monitor lizards has formed in Florida since 1990 . The wetlands are a suitable habitat and apparently the monitor lizards can cope with the climatic conditions. These stocks go back to illegally released pets . Young Nile monitors are available in Florida for as little as $ 10, but they quickly get too big for private keeping. Should the species penetrate protected areas, it could endanger rare native species such as the owl owl ( Athene cunicularia ) or eat the eggs of sea turtles . At the moment it is planned to fight the species together with the also naturalized dark tiger python ( Python bivittatus ) with baits provided with high doses of paracetamol . Death is apparently painless, but it is not yet clear how poisoning of native species can be prevented.
Way of life
behavior
Nile monitors are diurnal, the peak of activity is in the early afternoon. They sunbathe on embankments, termite mounds , open spaces and on trees, in more urban living spaces also on streets, roofs and stone walls. As a shelter for the night or to protect against heat, Nile monitors either use burrows they have dug themselves in sandy soils or expand burrows of other animals; occasionally trees are also used as sleeping places. The entrances are mostly located in sloping terrain (e.g. embankments), and an oval entrance leads into a tunnel up to 6 m long, which ends in an enlarged chamber. Already built or anthropogenic shelters are preferred to self-built tunnels.
Like all monitor lizards, Nile monitors are primarily solitary animals. In an investigation in the Abuko Nature Reserve (Gambia), male Nile monitors used up to 50,000 m² large roaming areas , while the females limited themselves to areas of a maximum of 15,000 m². Hatchlings moved in areas of about 30 m². The grazing areas strongly overlapped, territorial behavior could only rarely be observed. All home areas contained at least one larger watering hole.
The Nile monitor is a good climber and a very good swimmer, they sometimes stay under water for up to an hour. The heart rate can be reduced by up to 85% while diving. Trees are climbed either in search of food or to sunbathe.
nutrition
Nile monitors are carnivorous and will eat almost any animal that they can overwhelm. The food spectrum depends on the habitat, the season and the age of the animal. During the dry season about 50% of the food requirement is met by carrion , as invertebrates are difficult to find at this time. Carrion plays a much smaller role during the rainy season, and arthropods , amphibians , reptiles , birds , mollusks, crustaceans and small mammals are mainly hunted. Cannibalism on younger specimens occurs. Poisonous snakes are also part of the food spectrum, since the Nile monitor is apparently largely insensitive to poisons. In the course of ontogeny , the structure of the teeth in the Nile monitor changes, which is also reflected in the food spectrum. The young animals with sharp and pointed teeth, typical of war, feed 80% on arthropods, in subadult specimens arthropods still make up 75% of the diet. Elderly animals have duller, thicker teeth, have a more generalist diet, and crustaceans and mollusks complement the food spectrum.
During the rainy season, Nile monitors actively chase their prey on the ground, on trees and in the water. During the dry season, Nile monitors often lie in wait for their prey, for example near water holes. It was also reported that two monitor lizards worked together to catch prey: one of the monitor lizards distracted a female crocodile guarding its nest, while the other monitor tore open the nesting mound and ate eggs and young animals. Similar behavior has also been observed in bird nests.
Reproduction and development
The breeding season usually begins at the end of the rainy season, in western Africa, for example, from September to November, or from March to May in southern Africa. Males fight for the right to mate in a shortened commentary battle; The clinch phase , which is typical for monitor lizards , does not occur , in which both opponents stand up and try to wrestle each other down. Instead, Nile monitors only show the behavior prior to the clinch phase , which includes nodding the head as a threatening gesture and exerting lateral pressure when the animals sit next to each other in preparation for the clinch phase . The rest of the behavior was apparently lost secondarily. The females lay 5–60 eggs in termite mounds or self-dug caves , depending on their size . The young hatch after six to nine months at the beginning of the next rainy season, when the richest food supply is available. They are 16.5–30 cm long when hatched.
Systematics
The Nile monitor was the first scientifically described monitor , it was described by Carl von Linné in 1766 as a Lacerta monitor . Based on its hemipenis morphology , it is now placed in the subgenus Polydaedalus along with some other African monitor lizards . The rainforest Nile monitor ( Varanus ornatus ) was formerly a subspecies of the Nile monitor, but was raised to the status of a species because the two species occur sympatric without a gradual transition being recognizable. This species is thus the sister taxon of the Nile monitor, according to DNA analyzes the steppe monitor ( Varanus exanthematicus ) is in turn the sister taxon of these two species.
Nile monitors and humans
In earlier times the Nile monitor reached the mouth of the Nile, and the ancient Egyptians depicted the Nile monitor on monuments. Today, the Nile monitor is one of the monitors most frequently offered in pet shops. Between 1970 and 2005, according to records from the CITES authorities, 309,759 live Nile monitors were exported . This figure is only exceeded by the steppe monitor with around twice as many imports. The problem is that these monitor lizards are often sold as small young animals for low prices, and the often insufficiently informed buyers are confronted with very large and powerful lizards after a while.
The Nile monitor is also of great economic importance as food and for the leather trade. Resident hunters usually hunt down the Nile monitor with 60–100 cm long fishing lines whose hooks they bait with fish, frogs or pieces of meat. These lines are tied to vegetation in the morning and the monitor lizards are collected in the afternoon. Since around 50–80% of all sub- adult to adult specimens in an area are caught after just 7–9 days of catching , such actions can only be carried out every 3–4 years; the young animals cannot bite into the hooks that are too large and are fully grown by the next catch. The CITES authorities determined an annual export of 70,000 hides from Cameroon, 80,000 from Chad, 180,000 from Mali and 100,000 from Sudan between 1990 and 1993 . Overall, the number of monitor lizards killed per year is likely to be several million.
The monitor populations seem to be largely able to withstand the pressure from hunting; Because of the stable populations and the wide distribution, the Nile monitor is not considered endangered.
swell
- S. Lenz (2004): Varanus niloticus . In: ER Pianka & DR King (Eds.): Varanoid Lizards of the World , pp. 133-138. Indiana University Press, Bloomington & Indianapolis. ISBN 0-253-34366-6
- ↑ a b K. M. Enge et al. (2004): Status of the Nile Monitor (Varanus niloticus) in Southwestern Florida . Southeastern Naturalist 3 (4), pp. 571–582 ( full text ; PDF; 320 kB)
- ↑ RE Mauldin & PJ Savariey (2010): acetaminophen as to oral toxicant for Nile monitor lizards (Varanus niloticus) and Burmese pythons (Python molurus bivittatus) . Wildlife Research 37, pp. 215–222 ( full text )
- ↑ S. Lenz (1995): On the biology and ecology of the Nilwarans, Varanus niloticus (Linnaeus 1766) in Gambia, West Africa . Mertensiella 5
- ↑ SC Wood & K. Johansen (1974): Respiratory adaptations to diving in the nile monitor lizard, Varanus niloticus . Journal of Comparative Physiology A 89 (2), pp. 145-158
- ↑ JC Ast (2001): Mitochondrial DNA Evidence and Evolution in Varanoidea (Squamata) . Cladistics 17, pp. 211–226 ( full text ; PDF; 276 kB)
- ^ W. Neugebauer (1979): Family Warane In: Bernhard Grzimek (Ed.): Grzimeks Tierleben Kriechtiere , S. 324. Bechtermünz Verlag, Augsburg 2000 (unchanged reprint of the dtv edition from 1979). ISBN 3-8289-1603-1
- ↑ AP Pernetta (2009): Monitoring the Trade: Using the CITES Database to Examine the Global Trade in Live Monitor Lizards (Varanus spp.) . Biawak 3 (2), pp. 37–45 ( full text ; PDF; 2.0 MB)
- ↑ a b V. de Buffrénil & G. Hémery: Harvest of the Nile monitor, Varanus niloticus, in Sahelian Africa. Part I: Impact of Intensive Harvest on Local Stocks . Mertensiella 16 ( Advances in Monitor Research III ), pp. 181-194
