Monitor lizards can count to six and smell in stereo
Rock monitors fed different numbers of snails can tell quantities apart up to six. Nile monitors team up to raid crocodile nests, one acting as decoy. Their forked tongues sample scent in stereo, and a four-chambered-style heart gives them the highest resting metabolism of any living reptile.
About 94 species make up the genus Varanus, found across Africa, Asia and Oceania, with the Nile monitor now established as an invader in South Florida. They share one basic body plan, with long necks, strong tails and claws, yet span an extraordinary size range: from about 20 cm in Varanus sparnus to more than 3 m in the Komodo dragon. The extinct megalania may have topped 7 m. Europe once had them too, until the Middle Pleistocene.
The name is a Linnaean mistake. In 1758 he labelled the Nile monitor Lacerta monitor, probably because he wrongly linked the German Waran, borrowed from an Arabic word meaning lizard beast, to warnen, to warn. Later stories about the lizards rearing up to keep watch, or warning people of venomous animals, came afterwards. Australians call them goannas, and South Africans leguaans, both from confusion with American iguanas.
Their physiology is unusual for reptiles. A well-developed ventricular septum separates the lung and body circuits during each heartbeat, letting them keep mammal-like pressure differences, which suits their habit of actively chasing prey across large territories. They have no taste buds at all; instead the forked tongue delivers molecules to the Jacobson's organ, and Merten's water monitor can even smell underwater. Most species appear to be venomous, with glands in the lower jaw. Some venoms stop blood clotting, so bites bleed heavily, while the desert monitor's venom instantly paralyses rodents but not birds.
Most eat animals, and deer make up about half of an adult Komodo dragon's diet, yet three Philippine tree species live mainly on fruit. Females lay from seven to 38 eggs, and Komodo dragons can reproduce without mating. Humans exploit them widely: their skins stretch over Papua New Guinea's kundu drums and the South Indian kanjira, and dried Bengal monitor parts are illegally sold in India under the false name Hatha Jodi, passed off as a rare Himalayan root.
Source: Monitor lizard
