Monitor lizard (genus Varanus): overview, biology, distribution and conservation

Monitor lizards live in the tropics, subtropics and to a lesser extent in the temperate zones of the old world. The range of the genus extends in Africa over almost the entire continent and reaches from there over the Arabian Peninsula, Central Asia, continental Southeast Asia and the Southeast Asian islands to Australia. In addition, some species reach the westernmost island groups of the Pacific, such as the Solomon Islands, the Marshall Islands, the Caroline Islands and the Marianas. The northernmost occurrence of all monitor lizards is the desert monitor lizard, whose distribution extends from North Africa and Central Asia to the Aral Sea and the Caspian Sea at about 46° northern latitude. Monitor lizards are absent from Madagascar, Tasmania, and New Zealand, as well as Europe, America, and Antarctica.

Monitor lizards have their greatest diversity in Australia, where 30 species have been recorded. In places, up to eleven species occur sympatrically. Another "hotspot" for monitor lizards is New Guinea, which shares five species with Australia. On New Guinea, the Solomon Islands and the Bismarck Archipelago together 16 species of monitor lizards are known. Up to ten species may be found on the Moluccas, but the number is considered uncertain. The Philippines harbors eight species, including all fruit-eating species of the subgenus Philippinosaurus. Since 2010, three additional species have been described from the Philippines. The occurrence of the Raunacked Monitor (V. rudicollis) in the Philippines is not certain, which would make a total of 12 species. In the rest of the Malay Archipelago there are 12 species, in continental Asia 7 species. In Africa, finally, there are 5 species.

In their large range, monitor lizards inhabit a variety of habitats at different altitudes, such as rainforest, desert and mangroves. Some species also colonize anthropogenic and urban areas. The tropical habitats of monitor lizards are characterized by seasonal fluctuations. These can usually be divided into a food-rich rainy season and a food-poor dry season.

Appearance

Monitor lizards correspond in their appearance to the basic form of a lizard with four legs and a tail. The head is usually moderately high, comparatively long and tapers to a point towards the snout; however, there are a number of species with high skulls or even short heads and blunt snouts. Monitor lizards, like snakes, have a long forked tongue that may reach twice the length of the head. The eye has a round pupil. The nostrils are round or slit-shaped and may be located directly at the tip of the snout, directly in front of the eye, or in between. The position and shape of the nostril can be used to distinguish individual species. The head is well set off from the rather long neck. The limbs are comparatively long and always bear 5 fingers. The hind limbs are longer than the forelimbs. The tail is round in cross section in most species, but in some aquatic (semiaquatic) species the tail is compressed laterally as a rudder tail. Thus it produces more propulsion in the water when flapping sideways. The representatives of the V. prasinus group, on the other hand, have long thin prehensile tails, which they use as climbing aids. The tail length usually exceeds the head-torso length. Monitor lizards look similar to the not closely related rail lizards (Teiidae).

The coloration of monitor lizards is highly variable and ranges from the bright green emerald monitor (V. prasinus) to the completely black (melanistic) Panay monitor (V. mabitang). Dot patterns or transverse banding often occur, and the base color usually represents a camouflage color in the respective habitat. Longitudinal stripes are rare in monitor lizards. The ventral side is in most cases lighter than the upper side of the body. Juveniles are clearly brighter or more colorful than adults in some species.

Size and weight

Monitor lizards are known for their wide range in body size and weight: within no other vertebrate genus is the difference between small and large species greater. The smallest monitor is the short-tailed monitor (V. brevicauda), which can reach a maximum total length of 23 cm and a maximum weight of 17 g. The largest monitor living today is the Komodo dragon (V. komodoensis), which can reach a maximum length of around 3 m with a weight of over 70 kg. The Komodo dragon is the largest lizard living today; other medium-sized to large monitors are also among the largest lizards living today. Within the monitor lizards developed several times independently of each other particularly large or small species.

Males grow slightly larger than females in most species because the growth rate of females decreases more rapidly than males during individual development (ontogenesis). In addition, males sometimes have slightly different proportions of the head and limbs, but without more detailed examination it is not possible to determine the sex of most species with any certainty.

Scaling

Monitor lizards have mostly small, simply built scales. The head carries small, polygonal scales. The snout shield (rostrale) is only indistinctly developed. This is also true for the not very distinct nasal shields (Nasalia); instead the nostrils are surrounded by many small scales. In some species conspicuously enlarged scales are formed within the supraocular rows (Supraocularia). Also the nuchal scales are enlarged or pointed in some species; examples are V. nuchalis and the Raunackenwaran (V. rudicollis). The scales of the trunk are elongated oval and arranged in fairly regular transverse rows. On the tail, many dwarf monitors of the subgenus Odatria have spiny scales. With laterally compressed tails, the two uppermost scale rows of the tail usually form keels. Some monitor lizards (e.g., Gray's monitor lizard, V. olivaceus) also have specially keeled scales on the underside of the body for extra grip when climbing.