Monitor lizards can be distinguished from all other lizards at a glance by their deeply forked tongues. Heloderms and tegus also have forked tongues but they are less well developed. Furthermore these lizards have large scales on their heads whereas monitors have much smaller ones. Monitor lizards eat food that fights back and tries to escape and so are fastmoving creatures equipped with sharp teeth and strong limbs. Most of their senses are extremely acute and they are considered to be the most perceptive and intelligent of the lizards. In many aspects of their morphology and ecology monitor lizards seem more closely aligned to snakes than to typical lizards. Like snakes they are able to swallow very large prey items whole and like many snakes they have a particularly well developed sense of smell. However there is no evidence of a close relationship between the monitor lizards and any snake (Joger 1991) and the remarkably similar attributes of these reptiles appear to have evolved completely independently of each other.
The large size of these splendid lizards, combined with the relatively easy availability of many species, mean that monitors are often the subject of laboratory experiments. Most of the experiments conducted on them are related to their ecology only in a very roundabout way and are not discussed here. Useful works on the anatomy and morphology of monitor lizards include descriptions of the skull and other bones (Mertens 1942, Brongersoma 1958), teeth (Rieppel 1979), jaws (Sinclair & Alexander 1987), shoulder joints (Haines 1952), toes (Landsmeer 1981), skin (Fuchs 1977, Smith & Hylander 1985), tongue and throat musculature (Smith 1986), the snout (Bellairs 1949), the heart (Webb et al 1971), kidneys (Green I972b), alimentary tract (Bishai 1959), pancreas (Sabnis & Ranjgekar 1967, Moscona 1990), jaw muscles (Smith 1982), chromosomes (King & King 1975; King et al 1982), haemoglobin structure (Abbasi & Braunitzer 1991). Auffenberg's important monographs (1981, 1988, 1994) contain many important insights into the morphology of varanids and the book by Green and King (1993) is particularly recommended as further reading.
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