Varanus is a genus of largely carnivorous lizards. There are about 60 species in 10 subgenera. The species cover a vast area, occurring through Africa, the Indian Subcontinent, to China, down Southeast Asia to Brunei, 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.
Monitor lizards have long necks, powerful tails and claws, and well-developed limbs. The species include the largest living lizard, the Komodo dragon, and the crocodile monitor.
The various species cover a vast area, occurring through Africa, the Indian Subcontinent, to China, down Southeast Asia to Thailand, Brunei, 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 and the Perhentian Islands in the Malaysian state of Pahang. The West Nile monitor is now found in South Florida and in Singapore.
Habits and diet
Monitor lizards are, as a rule, 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 and yet show a range in size that is equivalent to a mouse and an elephant. 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 ensures 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. Monitor lizards are oviparous, laying from 7 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.
Varanids last shared a common ancestor with their closest living relatives, earless "monitors", during the Late Cretaceous.
During the Late Cretaceous era, 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, 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, a giant goanna formally known as Megalania prisca). This species is an iconic member of the Pleistocene megafauna of Australia, thought to have survived up until around 50,000 years ago.
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, 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. Among others, black-throated monitors, timor monitors, Asian water monitors, Nile monitors, mangrove monitors, emerald tree monitors, black tree monitors, roughneck monitors, dumeril's monitors, peach-throated monitors, crocodile monitors and Argus monitors have been kept in captivity.
"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 PNG trade language.
The meat of monitor lizards is eaten by some tribes in India, Thailand, Australia and in West Africa as a supplemental meat source. The meat of monitor lizards is used in Nepal for medicinal and food purpose.
The skin of monitor lizards is used in making a carnatic music percussion instrument called a kanjira.
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