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Goanna/ Monitor

Spotted tree Monitor
Varanus timorensis

Lace Monitor
Varanus varius

Monitors/ Goannas, Family Varanidae

Monitors are found in Africa, Asia and Australia.  There are at least 50 species in the family. Today’s species all are seen to be closely related and are all currently placed in the one genus Varanus. The biggest of the group is the Komodo Dragon of Indonesia which can grow to 3 metres in length and has been known to eat humans. Monitors are most diverse in Australia. When Europeans arrived, they used the word ‘iguana’ (a different family of lizards found mainly in the American tropics) which corrupted into the present day word ‘goanna’, which is not an aboriginal derived name. ‘Goanna’ and ‘monitor’ refer to the same animals in Australia, and can be used interchangeably. In Australia there are over 25 species, with most occurring nowhere else.

Physical description

Although much of the length of a goanna is tail, they are generally fairly large lizards. Monitors move walk their body held up but still low to the ground and with alternate movements of the opposite limbs; that is the front and rear limb move on one side, then the limbs on the other side move forward. This results in a sinewy snake-like motion. Goannas have large feet and sharp claws that in many species are used for climbing and/or digging. Monitors have a large tail, which is used for balance, for storing fat (Vincent and Wilson 1999) and in larger species as a whip-like weapon. 

Feeding ecology

Monitors have a good sense of smell, with many species using this to find their food, even if it is buried. They have distinct forked tongues which they flick in and out like snake. Like a snakes, they are catching the air molecules and ‘tasting’ them with their Jacobson’s gland which is located in the roof of their mouth. They may be trying to sense predators, other goannas and of course food.

Their diet ranges widely, but most species are essentially predators. Their prey varies from invertebrates to mammals, depending on the size of the goanna species. Many of the larger, more commonly seen species are also scavengers, sniffing out for dead animals, and thus also hanging around areas where humans have left food, such as picnic areas. Monitors have large stomachs and are capable of gorging a lot of food at once; goannas have been recorded with food inside representing over 40% of their body weight (Greer 1989).

The predatory nature of goannas is reflected by the teeth. The teeth are long, sharp and, in larger species, are also slightly serrated. The teeth of monitors are also recurved; such teeth are more efficient at gripping prey (Steel 1997). However, for recurved teeth to work they need to grip with their tip, and thus such animals need a wide gape. Goannas, like many other lizards and snakes, have fibrous tissue between bones that make up the skull, thus allowing a certain amount of flexibility, and creating what is often referred to as ‘cranial kinesis’ (Steel 1997). They kill and break apart their prey by lacerating them with their sharp teeth and by bashing them against the ground. Monitors are not capable of masticating their food, but swallow in chunks. The prey is ‘thrown’ back into the throat and the head moved forward in a lunging feeding action.

Sexual reproduction 

Goannas are usually seen alone. They are sometimes seen in groups if brought together by a food source. Males are sometimes observed in ritualized combat. This often involves the males pushing against each other; in larger species they may both stand upright. There are minimal differences between the sexes of goannas, and is it is difficult to tell male and females apart. The males, like many other lizards and snakes, have a double penis called a ‘hemipene’. The act of sex involves the male and female wrapping together closely. In tropical northern Australia this takes place in the wet season for terrestrial species (Greer 1989). Goannas usually lay their eggs in a hole dug in the ground if they are terrestrial species, or in hollow stumps if they are an arboreal species. Occasionally the latter types of goannas may also dig open a termite nest and lay their eggs inside.

Script: Courtesy of  Damon Ramsey BSc.(Zool) Biologist Guide

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Tropical North Queensland, Australia.
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