The Gray's monitor (Varanus olivaceus ) is a large (180 cm, >9 kg) monitor lizard known only from lowland dipterocarp forest in southern Luzon, Catanduanes, and Polillo Island, all islands in the Philippines. It is also known as Gray's monitor lizard, butaan, and ornate monitor. It belongs to the subgenus Philippinosaurus. It is largely arboreal and extremely shy. The Northern Sierra Madre monitor lizard was thought to be of same species with Gray's monitor until a research concluded in 2010 that northern populations of Gray's monitor was a distinct species, now known as V. bitatawa.
It is well known for its diet, which consists primarily of ripe fruit, especially Pandanus. A number of prey items are, however, also consumed, including snails, crabs, spiders, beetles, birds and eggs. Monitors are generally carnivorous animals, which makes the Gray's monitor somewhat of an exception amongst the varanid family. Such an unusual diet may be as a result of competition over food with the water monitors, which share their range. One of the only fruits readily eaten by this species in captivity is grapes, with these and fruit powder supplementing a captive diet of insects and rodents.
Details of the breeding habits of this species, certainly in the wild, are very limited due to the secretive nature of this species. The optimum egg-laying time for this species is known to be between July and October, when a clutch of up to 11 eggs will be laid. Rather than digging a nest, the most likely place for this species to lay eggs is thought to be in tree hollows, where they also spend much of their days resting. Young are often observed at their smallest in May to July, and as such estimates of incubation time lay at around 300 days. In captivity, however, incubation has been recorded over 219 days.
It is classed as vulnerable by the IUCN because most of its habitat has been destroyed over the last 60 years, and it is now thought to live in an area as small as 20,000 km², of which only around 2,000 km² are occupied by this species. It is thought that habitat destruction is not the only cause for the decreasing population trend, but that they are also still hunted for food and collected for the pet trade. It may be more threatened than initially estimated, since the northern population were confirmed to be of different species (V. bitatawa ) in 2010. The range of the species has now been enclosed to the Polillo islands and the southern portions of Luzon, a very fragmented area with sparse rainforests left. A new assessment by the IUCN is highly recommended, as the species may be endangered or critically endangered already.
