Lizards 4: Green Tree Monitor

Juvenile V.prasinus

Turning from desert living vegetarians, the subject of this post is an insectivorous species from the rain forests of New Guinea. The Green Tree Monitor Varanus prasinus is widely distributed in rain forests across the island, but as it spends all its time in the canopy it is not seen frequently unless a tree is cut down.

The V.prasinus species group has numerous local forms on islands around New Guinea and also northern Australia, but many of these are threatened as a result of deforestation or collection for the international pet trade. Bristol has successfully bred V.prasinus on several occasions, and in fact there is clutch on show in the incubator room as the date of this post. The adults live in one of the larger display vivaria.

Green Tree Monitors feed basically on any insect or small vertebrate they come across as they forage through the canopy. Some of these can be quite large – there is at least one record of one being found to have eaten a 12cm Eurycantha stick insect. Having handled Eurycanthas myself this is no mean feat – males especially are equipped with massive spines on their hind legs which can easily draw blood, and I imagine one could easily kill a prasinus if they managed to get one in a vulnerable spot.

They are unusual among reptiles in using their front legs in investigating holes which might contain prey in the way a primate would, instead of just probing with their tongues as most reptiles do. As part of the enrichment routine for the adults, they have a special feeder which is given to them intermittently. This consists of a log suspended from a wire which is drilled with a series of holes. Pinkies (baby mice) are placed in these holes and the monitors have to jump from surrounding launch points onto the log, reach into the holes and pull out the food before they can eat it.

In the wild Green Tree Monitors apparently lay their eggs in termite mounds, although I suspect other site may also be used. Arboreal or terrestrial termite mounds make good nest sites for many animals – the nests are usually very resistant to assault by any creature not equipped with excavation abilities and the nests are maintained by the rightful inhabitants at constant temperature and humidity, which is good for incubation. The eggs take around 120 days to hatch and the youngsters grow fast, reaching maturity (in captivity at least) at about 1 year.

At present neither V.prasinus, nor any other of the prasinus species group, is listed as Endangered by the IUCN Red List, but as many of the forms are probably restricted to islands suffering deforestation the real situation is probably more serious for some forms than it appears. Needless to say, anyone who wishes to keep one should only think about it if they can provide very large accommodation appropriate to these magnificent lizards, and only buy captive bred individuals.