|Rock monitor (Varanus albigularis)|
Monitor lizards are generally large lizards, although some can be as small as 20 cm (7.9 in) in length. A total of 73 species are currently recognized; however, given that several species-groups are in need of taxonomic review, this number is certain to be increased with future research. They have long necks, powerful tails and claws, and well-developed limbs. Most species are terrestrial, but arboreal and semiaquatic monitors are also known. While most monitor lizards are carnivorous, three arboreal species, Varanus bitatawa, Varanus mabitang, and Varanus olivaceus, are primarily frugivores. They are oviparous, laying from seven to 37 eggs, which they often cover with soil or protect in a hollow tree stump. Most species of monitor lizard have a predominantly carnivorous diet, eating eggs, smaller reptiles, fish, birds and small mammals. Some species of monitor lizard also eat fruit and vegetation depending on where they live. 
The various species of Varanus cover a vast area, occurring through Africa, the Indian Subcontinent, to China, down Southeast Asia to Indonesia, the Philippines, New Guinea, Australia and islands of the Indian Ocean, and the South China Sea. A large concentration of monitor lizards occurs on Tioman Island in the Malaysian state of Pahang. Some are now found in South Florida, particularly in the Everglades, where they are becoming a dangerous threat to cattle and to other wildlife, including even alligators.
Monitor lizards differ greatly from other lizards in several ways, possessing a relatively high metabolic rate for reptiles, and several sensory adaptations that benefit the hunting of live prey. Recent research indicates the varanid lizards may have some venom. This discovery of venom in monitor lizards, as well as in agamid lizards, led to the Toxicofera hypothesis: that all venomous lizards and snakes share a common venomous ancestor.
Snakes were believed to be more closely related to monitor lizards than any other type of extant reptile; however, it has been more recently proposed that snakes are the sister group of the clade of iguanians and anguimorphs. Like snakes, monitor lizards have forked tongues which they use to sense odors.
During the Pleistocene epoch, giant monitor lizards lived in Southeast Asia and Australasia, the best known fossil being the megalania (Varanus priscus unless it falls in its own genus, in which case it is Megalania prisca). This species is an iconic member of the Pleistocene megafauna of Australia.
The generic name Varanus is derived from the Arabic word waral ورل, (alternative word waran). The name comes from a common Semitic root ouran, waran, or waral, meaning "lizard". The occasional habit of varanids to stand on their two hind legs and to appear to "monitor" their surroundings has been suggested to have led to this name, as it was Latinized into Varanus. Its common name is derived from the Latin word monere meaning "to warn".
In Austronesia where varanids are common, they are known under a large number of local names. They are usually known as biawak (Malay and Indonesian), bayawak (Filipino), binjawak or minjawak (Javanese), or variations thereof. Other names include hokai (Solomon Islands), bwo or puo (Maluku), halo (Cebu), galuf of kaluf (Micronesia and the Caroline Islands), batua or butaan (Luzon), alu (Bali), hora or ghora (Komodo group of islands), phut (Burmese) and guibang (Manobo).
In Tamil and Malayalam, monitor lizards are known as udumbu, ghorpad घोरपड in Marathi, uda in Kannada, in Sinhalese as kabaragoya, in Telugu as udumu, in Punjabi and Magahi (and other Bihari languages) as goh, in Assamese as gui xaap, and in Bengali as goshaap or guishaap and गोह (goh) in Hindi. Due to confusion with the large New World lizards of the family Iguanidae, the lizards became known as "goannas" in Australia. Similarly, in South Africa, they are referred to as leguaan, or likkewaan from the Dutch for iguana. The generic name inspired the name of the Japanese movie monster Varan.
Varanid lizards are very intelligent, and some species can even count. Careful studies feeding V. albigularis at the San Diego Zoo varying numbers of snails showed 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, D.C., recognize their keepers and seem to have distinct personalities.
Monitor lizards have become a staple in the reptile pet trade. The most commonly kept monitors are the savannah monitor and Ackies monitor, due to their relatively small size, low cost, and relatively calm dispositions with regular handling. Black throated monitors, white throated monitors, water monitors, Nile monitors, mangrove monitors, emerald tree monitors, black tree monitors, acanthurus monitors, quince monitors, crocodile monitors and Komodo dragons have been kept in captivity. Like all reptiles kept as pets, monitors need an appropriately sized enclosure, hiding places, and an appropriate substrate. Some water monitors also need a large water dish in which they can soak their entire bodies. In the wild, monitors will eat anything they can overpower, but crickets, superworms, feeder fish, and the occasional rodent (for calcium) make up most of the smaller captive monitors' diets. Boiled eggs, silkworms, and earthworms can also be fed to them. Larger species, such as Nile monitors, Asian water monitors, crocodile monitors, perenties, and Komodo dragons, will eventually require larger prey. Paleontologist and biology professor at Temple University, Michael Balsai has observed V. prasinus eating fruit in captivity, as has herpetologist and author, Robert G. Sprackland.
In parts of Pakistan, different parts of monitor lizards are used for a variety of medical purposes. The flesh is eaten for 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 aphrodesiac lubricant.
"Large scale exploitation" of monitor lizards is undertaken for their skins, which are described as being "of considerable utility" in the leather industry.
All but five species of monitor lizard 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 order 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.
In Tamil Nadu and all other parts of South India, catching or killing of monitor lizards is banned under the Protected Species Act.
- V. bengalensis, Bengal monitor
- V. b. bengalensis, Bengal monitor
- V. b. nebulosus or V. nebulosus, Clouded monitor
- V. dumerilii, Dumeril's monitor
- V. flavescens, Golden monitor
- V. rudicollis, Roughneck monitor
- V. beccarii, Black tree monitor
- V. boehmei, Golden-spotted tree monitor
- V. bogerti, Bogert's tree monitor
- V. caerulivirens, Turquoise monitor
- V. cerambonensis, Ceram monitor
- V. doreanus, Blue-tailed monitor
- V. finschi, Finsch's monitor
- V. indicus, Mangrove monitor
- V. jobiensis, Peach-throated monitor
- V. juxtindicus, Rennell Island monitor
- V. keithhornei, Canopy goanna, Nesbit River monitor
- V. kordensis, Biak tree monitor
- V. macraei, Blue-spotted tree monitor
- V. melinus, Quince monitor
- V. lirungensis, Talaud mangrove monitor
- V. obor, sago monitor
- V. prasinus, Emerald tree monitor
- V. rainerguentheri Rainer Günther’s monitor
- V. reisingeri, Reisinger's tree monitor
- V. telenesetes, Mysterious tree monitor, Rossell tree monitor
- V. yuwonoi, Tricolored monitor
- V. zugorum, Silver monitor, Zug's monitor
- V. acanthurus, Ridge-tailed monitor
- V. a. acanthurus, Ridge-tailed monitor
- V. a. brachyurus, Common ridge-tailed monitor
- V. a. insulanicus, Island ridge-tailed monitor
- V. auffenbergi, Auffenberg's monitor, Peacock monitor
- V. baritji, White's dwarf monitor, black-spotted ridge-tailed monitor, lemon-throated monitor
- V. brevicauda, Short-tailed monitor
- V. bushi, Pilbara stripe-tailed monitor, Bush's monitor
- V. caudolineatus, Stripe-tailed monitor
- V. eremius, Rusty desert monitor, pygmy desert monitor
- V. gilleni, Pygmy mulga monitor
- V. glauerti, Kimberley rock monitor
- V. glebopalma, Twilight monitor, black-palmed rock monitor
- V. hamersleyensis, Hamersley Range rock monitor
- V. kingorum, Kings' rock monitor
- V. mitchelli, Mitchell's water monitor
- V. pilbarensis, Pilbara rock monitor
- V. primordius, Blunt-spined monitor
- V. scalaris, Banded tree monitor
- V. semiremex, Rusty monitor
- V. similis, Spotted tree monitor
- V. storri, Storr's monitor
- V. s. storri, Eastern Storr's monitor
- V. s. ocreatus, Western Storr's monitor
- V. timorensis, Timor monitor
- V. tristis, Black-headed monitor
- V. t. orientalis, Freckled monitor
- V. salvadorii, Crocodile monitor
- V. bitatawa, Northern Sierra Madre forest monitor, bitatawa
- V. mabitang, Panay monitor
- V. olivaceus, Gray's monitor
- V. albigularis, White-throated monitor
- V. exanthematicus, Savanna monitor, Bosc's monitor
- V. niloticus, Nile monitor
- V. ornatus, ornate monitor
- V. yemenensis, Yemen monitor
- †V. darevskii (extinct)
- V. griseus, Desert monitor [endangered]
- V. g. griseus, Desert monitor, grey monitor
- V. g. caspius, Caspian monitor
- V. g. koniecznyi, Indian desert monitor, Thar desert monitor
- V. cumingi, Cuming's water monitor, yellow-headed water monitor
- V. c. cumingi, Cuming's water monitor
- V. c. samarensis, Samar water monitor
- V. marmoratus, Marbled water monitor, Philippine water monitor
- V. nuchalis, Large-scaled water monitor, white-headed water monitor, Negros water monitor
- V. palawanensis Palawan water monitor
- V. rasmusseni Rasmussen's water monitor
- V. salvator, Water monitor
- V. s. salvator, Sri Lankan water monitor
- V. s. andamanensis, Andaman water monitor
- V. s. bivittatus, Two-striped water monitor, Javan water monitor
- V. s. macromaculatus, Asian water monitor
- V. s. ziegleri, Ziegler's water monitor
- V. togianus, Togian water monitor
- V. giganteus, Perentie
- V. gouldii, Sand monitor, Gould's monitor
- V. komodoensis, Komodo dragon
- V. mertensi, Mertens' monitor
- V. panoptes
- V. p. panoptes, Argus monitor
- V. p. horni, Horn's monitor
- V. p. rubidus, Yellow-spotted monitor
- †V. priscus, Megalania (extinct)
- V. rosenbergi, Rosenberg's monitor, heath monitor
- V. spenceri, Spencer's monitor
- V. varius, Lace monitor
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- Varanus keithhornei, The Reptile Database
- Varanus prasinus, The Reptile Database
- Varanus baritji, The Reptile Database
- Varanus spinulosus, The Reptile Database
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