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Varanus olivaceus Print E-mail

Gray's monitor spends much of its time in trees, but finds almost all of its food on or below the surface. It shelters in thickets of vegetation on branches, in rock crevices and in tree hollows. That this species likes to wedge itself into tight crevices is demonstrated by the large numbers of scratches seen on their backs and bellies. Unlike other well known monitor lizards, Gray's monitor relies largely on its cryptic colouration for defence and when threatened tends to remain immobile amongst tangles of vegetation.

The most extraordinary feature of the ecology of this lizard is its diet. Juveniles feed largely on snails and crabs but between the ages of 12-18 months the diet changes to include a large number of fruits. No other monitor lizard is known to feed on plant matter in the wild, but in Gray's monitor it forms a major part of the diet. Other important foods are snails, crabs, spiders, beetles, birds and their eggs. Fruits are only eaten when they are perfectly ripe and are collected from the forest floor. Both sugary and oily fruits are eaten, but the latter are consumed in larger numbers. Oily fruits are generally avoided by vertebrates because they contain chemicals that render proteins indigestible, so the ability to feed on them probably reduces competition between the monitor lizard and the other fruit eating birds and mammals with which it shares its range.

Gray's monitor is sympatric with the water monitor V.salvator, and it has been suggested that it is only able to survive competition from this non specialised, gregarious animal by adapting to a largely frugivorous diet. Although a wide range of fruits are available in tropical forests, Gray's monitor selects only a few of them. They appear to know the location of the trees that bear edible fruit within their home range. However although some trees bear large amounts of fruit over several months the lizards are very picky and rarely spend long feeding below an particular tree but move about to collect a variety of fruit. Nor do they gorge themselves with food in the manner of many large monitor species. Food in the stomach accounts for only about 2% of their body weight, although many other species are known to consume individual prey items weighing 20% or more of their own weight. The need for a wide variety of different fruits is due to complex nutritional requirements and the need to avoid ingesting too much of any of the toxins present in the fruit. Least fruit is eaten between December and February, most sugary fruits are eaten between May and July and oily fruits most abundant from August to November and from February to March.

The seasonal differences in amount of food available is reflected in the amount of body fat accumulated by the lizards. Fat levels are lowest from June to August and highest in October (the level varies somewhat between the sexes). Body fat may account for 5-12% of body weight depending on time of year. As a consequence of its unusual diet the alimentary tract of Gray's monitor is unlike that of any other species. They possess a caecum in which microbial degradation of plant matter may occur and the large intestine is much longer than in carnivorous monitor lizards, suggesting a need for more complete digestion of food items. The ability to feed on sedentary food items present in large numbers is also probably responsible for the very small ranges in which these lizards are active; from just over 2000m2 to 27,100m2. However if the area of the trees used by the lizards is also accounted for, the mean activity range (14,800m2) increases ten-fold.



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Monitoring Individuals 2
butaan4.jpgWe tape spool and line devices to butaan that have been caught and release them at the exact point of capture as soon as possible. Spool and line data gives us a detailed account of the animals' movementes for a few hours, days or weeks after release.  We have also used spool and line very effectively on other animals, including the endemic Polillo forest snail Helicostyla portei


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