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Monitor lizards have long necks, powerful tails and claws, and well-developed limbs. The adult length of extant species ranges from 20 cm (7.9 in) in some species, to over 3 m (10 ft) in the case of the Komodo dragon, though the extinct varanid known as Megalania (Varanus priscus) may have been capable of reaching lengths more than 7 m (23 ft). Most monitor species are terrestrial, but arboreal and semiaquatic monitors are also known. While most monitor lizards are carnivorous, eating eggs, smaller reptiles, fish, birds, insects, and small mammals, some also eat fruit and vegetation, depending on where they live.
Most monitor lizards are almost entirely carnivorous, consuming prey as varied as insects, crustaceans, arachnids, myriapods, mollusks, fish, amphibians, reptiles, birds, and mammals. Most species feed on invertebrates as juveniles and shift to feeding on vertebrates as adults. Deer make up about 50% of the diet of adults of the largest species, Varanus komodoensis. In contrast, three arboreal species from the Philippines, Varanus bitatawa, Varanus mabitang, and Varanus olivaceus, are primarily fruit eaters. Although normally solitary, groups as large as 25 individual monitor lizards are common in ecosystems that have limited water resources.
The genus Varanus is considered unique among animals in that its members are relatively morphologically conservative, yet show a very large size range. Finer morphological features such as the shape of the skull and limbs do vary, though, and are strongly related to the ecology of each species.
Monitor lizards maintain large territories and employ active-pursuit hunting techniques that are reminiscent of similar-sized mammals. The active nature of monitor lizards has led to numerous studies on the metabolic capacities of these lizards. The general consensus is that monitor lizards have the highest standard metabolic rates of all extant reptiles.
Monitor lizards have a high aerobic scope that is afforded, in part, by their heart anatomy. Whereas most reptiles are considered to have three-chambered hearts, the hearts of monitor lizards - as with those of boas and pythons - have a well developed ventricular septum that completely separates the pulmonary and systemic sides of the circulatory system during systole. This allows monitor lizards to create mammalian-equivalent pressure differentials between the pulmonary and systemic circuits, which in turn ensure that oxygenated blood is quickly distributed to the body without also flooding the lungs with high-pressure blood.
Anatomical and molecular studies indicate that all varanids (and possibly all lizards) are partially venomous. The venom of monitor lizards is diverse and complex, as a result of the diverse ecological niches monitor lizards occupy.
Monitor lizards are oviparous, laying from seven to 37 eggs, which they often cover with soil or protect in a hollow tree stump. Some monitor lizards, including the Komodo dragon, are capable of parthenogenesis.
The family Varanidae probably originated in Asia at least 65 million years ago, although some estimates are as early as the late Mesozoic (112 million years ago). Monitor lizards probably expanded their geographic range into Africa between 49 and 33 million years ago, possibly via Iran, and to Australia and the Indonesian archipelago between 39 and 26 million years ago.
During the Late Cretaceous Period, monitor lizards or their close relatives are believed to have evolved into amphibious and then fully marine forms, the mosasaurs, some of which reached lengths of 12 m (39 ft) or more.
Snakes were believed to be more closely related to monitor lizards than any other type of extant reptile; however, snakes have been more recently proposed to be the sister group of the clade of iguanians and anguimorphs. Like snakes, monitor lizards have forked tongues, which they use to sense odors.
The genus Varanus first emerged in Laurasia. During the late Oligocene to early Miocene, the group had dispersed to Australia, New Guinea, and the Solomon Islands three separate times. By the late Miocene, the genus was also present in Africa, Arabia, Asia, and Eastern Europe.
By the Pleistocene Epoch, giant monitor lizards lived in Southeast Asia and Australasia, the best known fossil species being the megalania (Varanus priscus, a giant goanna formerly known as Megalania prisca). This species is an iconic member of the Pleistocene megafauna of Australia, thought to have survived until around 50,000 years ago.
V. indicus species complex (V. indicus, V. cerambonensis, V. caerulivirens, V. colei, V. obor, V. lirugensis, V. rainerguentheri, V. zugorum) 
V. doreanus species complex (V. doreanus, V. finschi, V. semotus, V. yuwonoi)
V. gouldii species complex (V. gouldii, V. rosenbergi, V. panoptes)
V. bengalensis species complex (V. bengalensis, V. nebulosus)
V. acanthurus species complex (V. acanthurus, V. baritji, V. primordius, V. storri)
V. exanthematicus species complex (V. exanthematicus, V. albigularis, V. yemenensis)
V. timorensis species complex (V. timorensis, V. auffenbergi, V. scalaris, V. similis, V. tristis)
V. niloticus species complex (V. niloticus, V. stellatus)
V. salvator species complex (V. salvator, V. cumingi, V. nuchalis, V. togianus, V. marmoratus)
The tree monitors of the V. prasinus species complex (V. prasinus, V. beccarii, V. boehmei, V. bogerti, V. keithhornei, V. kordensis, V. macraei, V. reisingeri, V. telenesetes) were once in the subgenus Euprepriosaurus, but as of 2016, form their own subgenus Hapturosaurus.
V. jobiensis was once considered to be a member of the V. indicus species complex, but is now considered to represent its own species complex.
The generic name Varanus is derived from the Arabic word waral/waran /, from a common Semitic root ouran, waran, or waral, meaning "dragon" or "lizard beast".
In English, they are known as "monitors" or "monitor lizards". The earlier term "monitory lizard" became rare by about 1920. The name may have been suggested by the occasional habit of varanids to stand on their two hind legs and to appear to "monitor", or perhaps from their supposed habit of "warning persons of the approach of venomous animals".
Due to confusion with the large New World lizards of the family Iguanidae, the lizards became known as "goannas" in Australia. Similarly, in South African English, they are referred to as leguaans, or likkewaans, from the Dutch term for the Iguanidae, leguanen.
Some species of varanid lizards can count; studies feeding V. albigularis varying numbers of snails showed that they can distinguish numbers up to six.V. niloticus lizards have been observed to cooperate when foraging; one varanid lures the female crocodile away from her nest, while the other opens the nest to feed on the eggs. The decoy then returns to also feed on the eggs.Komodo dragons, V. komodoensis, at the National Zoo in Washington, DC, recognize their keepers and seem to have distinct personalities.
Monitor lizard meat, particularly the tongue and liver, is eaten in parts of India and Malaysia, and is traditionally considered to also be an aphrodisiac.
In parts of Pakistan and southern India, as well in Northeastern India particularly Assam the different parts of monitor lizards are used for a variety of medical purposes. The flesh is eaten for the relief of rheumatic pain, abdominal fat is used as a salve for skin infections, oil and fat are used to treat hemorrhoids or chronic pain, and the oil is used as an aphrodisiac lubricant (sande ka tel).
"Large-scale exploitation" of monitor lizards is undertaken for their skins, which are described as being "of considerable utility" in the leather industry. In Papua New Guinea, monitor lizard leather is used for membranes in traditional drums (called kundu) , and these lizards are referred to as kundu palai or "drum lizard" in Tok Pisin, the main Papuan trade language.
The meat of monitor lizards is eaten by some tribes in India, the Philippines, Australia, South Africa and West Africa as a supplemental meat source. Both meat and eggs are also eaten in Southeast Asia such as Vietnam and Thailand as a delicacy.
The meat of monitor lizards is used in Nepal for medicinal and food purpose.
According to IUCN Red List of threatened species, most of the monitor lizards species fall in the categories of least concern, but the population is decreasing globally. All but five species of monitor lizards are classified by the Convention on International Trade in Endangered Species of Wild Fauna and Flora under Appendix II, which is loosely defined as species that are not necessarily threatened with extinction but may become so unless trade in such species is subject to strict regulation to avoid use incompatible with the survival of the species in the wild. The remaining five species - V. bengalensis, V. flavescens, V. griseus, V. komodoensis, and V. nebulosus - are classified under CITES Appendix I, which outlaws international commercial trade in the species.
The yellow monitor (V. flavescens) is protected in all countries in its range except Bhutan, Nepal, India, Pakistan, and Bangladesh.
In Tamil Nadu and all other parts of South India, catching or killing of monitor lizards is banned under the Protected Species Act.
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^ abcdWeijola, Valter (2019-03-14). "A molecular phylogeny for the Pacific monitor lizards (Varanus subgenus Euprepiosaurus) reveals a recent and rapid radiation with high levels of cryptic diversity". Zoological Journal of the Linnean Society. 186 (4): 1053-1066. doi:10.1093/zoolinnean/zlz002.
^Koch, Andre (2010). "Updated checklist of the living monitor lizards of the world (Squamata: Varanidae)". Bonn Zoological Bulletin. 57: 127-136.
^Koch, Andre (2010-05-06). "Unravelling The Underestimated Diversity Of Philippine Water Monitor Lizards (Squamata: Varanus Salvator Complex), With The Description Of Two New Species And A New Subspecies". Zootaxa. 2446: 1-54. doi:10.11646/zootaxa.2446.1.1.
^Mendyk, Robert (2018). "An Annotated Bibliography of Captive Reproduction in Monitor Lizards (Varanidae: Varanus). Part III. Soterosaurus". Biawak. 12.
^Ghimire HR, Phuyal S, Shah KB (2014). "Protected species outside the protected areas: People's attitude, threats and conservation of the Yellow Monitor (Varanus flavescens) in the Far-western Lowlands of Nepal". Journal for Nature Conservation. 22 (6): 497-503. doi:10.1016/j.jnc.2014.08.003.
^Ziegler, Thomas; Schmitz, Andreas; Koch, Andre; Böhme, Wolfgang (2007). "A review of the subgenus Euprepiosaurus of Varanus (Squamata: Varanidae): morphological and molecular phylogeny, distribution and zoogeography, with an identification key for the members of the V. indicus and the V. prasimus species groups". Zootaxa1472: 1-28.
^Eidenmüller, Bernd; Wicker, Rudolf (2005). "Eine weitere neue Waranart aus dem Varanus prasinus-Komplex von den Insel Misol, Indonesian ". Sauria27 (1): 3-8. (Varanus reisingeri, new species). (in German).
^Bucklitsch, Yannick (2016-08-17). "Scale Morphology and Micro-Structure of Monitor Lizards (Squamata: Varanidae: Varanus spp.) and their Allies: Implications for Systematics, Ecology, and Conservation". Zootaxa. 4153 (1): 1-192. doi:10.11646/zootaxa.4153.1.1. PMID27615821.
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