The pattern of adult monitors generally consists of a single ground colour with varying amounts of ocellations, spots and bands over the body surfaces. Usually it seems that the purpose of this arrangement is to make the animals difficult to see amongst rocks and vegetation. It is impossible to appreciate the camouflage of monitor lizards without seeing them in their natural environment. The amount of pattern tends to increase in wetter climates in species such as Bengal and desert monitors, with specimens from very dry areas often having no pattern at all.
Newly hatched monitor lizards rarely resemble their parents. Youngsters of many species are almost unrecognisable because of their gaudy colouration. Dumeril's monitor provides the most extreme example. The purpose of their extraordinary appearance is unknown. A commonly held explanation is that bright colours serve as camouflage. Although the patterns of many adult monitors allow them to blend in with their surroundings and it is cenainly true that juvenile monitors are very rarely seen in the wild, but it is difficult to envisage a hatchling Dumeril's monitor being able to remain inconspicuous in any habitat. Another possibility is that the brighter colours and prominent markings of the young mimic the warning colouration of dangerous animals. Hatchling Dumeril's monitors show a strong resemblance to young spitting cobras (Bennett 1995) and the young of lace goannas, Nile monitors and water monitors are boldly banded with jet black and bright yellow. Juvenile water monitors are considered to be poisonous and / or venomous by people in some parts of their range. Whether this belief is shared by other potential predators would be best determined by experiment.
Dramatic colouration differences between young and adult such as in Dumeril's monitor are also found in many territorial coral reef fishes such as the emperor angelfish, Pomacantlms imperator'. It is thought that in these animals the juveniles' colour protects them from aggression by adults, who do not recognise them as belonging to the same species. Thus for the early stages in life they are able to move about without regard for their elders' territories and without having to waste energy and risk injury or death in fights. Such an explanation seems unlikely in the case of monitor lizards, who are present at much lower densities than coral reef fishes and seem to recognise each other by smell rather than by sight. But in at least one species (the Bengal monitor) young lizards are known to remain with their siblings for some time after hatching. Perhaps the prominent markings of some juveniles make it easier for the fraternal group to recognise each other and keep close together. Some of the rock goannas start life with very brightly coloured tails, they may act as a decoy to aerial predators, as has previously been suggested for the light tail tips of adults. Hatchlings with red or orange colouration such as Dumeril's monitor and Rosenberg's goanna loose their bright colours completely within a few weeks but in general the patterns of the juveniles fade slowly with age. As the monitors grow larger they frequent different habitats. In many species the shift from an arboreal to a terrestrial existence is often accompanied by a reduction in pattern; bands break down into spots which disintegrate further as the animals grow, so that very old specimens may have virtually no pattern at all.
Some aquatic species (Mertens' goanna and the Jobi monitor) have areas of bright colour on the throat which may serve as a warning signal when the animals inflate their throats in anger (Horn 1977). Other species (Gray's and the Yemen monitor) have prominent lighter markings on the snout that may serve as sexual stimuli (Auffenberg 1988). Races of black monitor lizards are known from a number of species (e.g. water monitors, mournful goanna and New Guinea tree monitors). Often, but not always, they seem to be associated with coastal areas. The colour is likely to have a significant effect on their thennoregulatory behaviour, presumably allowing them to absorb heat faster than would be possible with a lighter coloured skin. In the Western Australian deserts the black form of the mournful goanna apparently tolerates body temperatures of over 45°C, several degrees higher than the temperature considered lethal for most species! To date there have been few studies conducted to look for differences in the thermoregulatory behaviour of melanistic races (Green & King 1978 King 1980).
Attribution / Courtesy: Daniel Bennett. 1995. A Little Book of Monitor Lizards. Viper Press U.K.
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