Varanus are morphologically conservative but vary widely in size, which makes this genus ideal for comparative studies of the evolution of body size. Small body size has evolved at least twice, once in Australia and another time in an Asian clade. Large body sizes appear to have evolved in several lineages of varanids. These lizards range from the diminutive Australian pygmy monitor Varanus brevicauda (only about 6.7-7.9 in or 17-20 cm in total length and 0.28-0.71 oz or 8-20 g in mass) to Indonesian Komodo dragons (Varanus komodoensis), which attain lengths of 9.9 ft (3 m) and weights of 331 lb (150 kg). The largest living lizard, Varanus komodoensis, was not officially described scientifically as a species until 1912, but Chinese and Dutch travelers must have known of its existence centuries before. Flicking their long forked yellow tongues, surely these great reptiles must have given rise to the very concept of dragons breathing fire. Ancient cartographers marked the Lesser Sunda Islands on their maps of southeastern Asia with an ominous warning: "Here be dragons."
The teeth of most varanids are serrated along the rear edge, which facilitates cutting and tearing skin and flesh of prey as these big lizards pull back on their bite. V. komodoensis routinely kills deer and pigs this way, and one Komodo dragon actually eviscerated a water buffalo. Komodo dragons and Megalania are/were ecological equivalents of large saber-toothed cats, using their slashing bite to disembowel large mammals.
Varanids have more aerobic capacity and a greater metabolic scope than other lizards and range over larger areas. Because of
their body size, large individual monitor lizards retain body heat in their nocturnal retreats and can emerge the next morning with body temperatures well above ambient air temperatures. Their mass confers a sort of "inertial homeothermy," which has implications for understanding the evolution of endothermy.
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