Monitor lizards are large lizards in the genus Varanus. They are native to Africa, Asia, and Oceania, and one species is also found in the Americas as an invasive species. About 80 species are recognized.
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.[1]
The various species cover a vast area, occurring through Africa, the Indian subcontinent, to China, the Ryukyu Islands in southern Japan, south to Southeast Asia to Thailand, Malaysia, Brunei, Indonesia, the Philippines, New Guinea, Australia, and islands of the Indian Ocean and the South China Sea. The West African Nile monitor (Varanus stellatus) is now found in South Florida[2] and in Singapore. Monitor lizards also occurred in Europe in the Neogene, with the last known remains dating to the Middle Pleistocene.[3]
Most monitor lizards are almost entirely carnivorous,[4] 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.[5] In contrast, three arboreal species from the Philippines, Varanus bitatawa, Varanus mabitang, and Varanus olivaceus, are primarily fruit eaters.[6][7][8] Although normally solitary, groups as large as 25 individual monitor lizards are common in ecosystems that have limited water resources.[citation needed]
The genus Varanus is considered unique among animals in that its members are relatively morphologically conservative, yet show a very large size range.[9] 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.[10][11]
Monitor lizards maintain large territories and employ active-pursuit hunting techniques that are reminiscent of similar-sized mammals.[12] 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.[13]