Goannas, or monitors as they are known elsewhere in the world, are a very distinctive group of lizards. They range in size from about 12 cm to 1.3 m, not including the tail. They include the largest lizard living today, the Komodo Dragon of Indonesia.
There are 58 living species of goanna and most of them are carnivores. Goannas find their food by searching widely across the landscape, catching animals by stalking or digging them out of shelters and nests. They are aided in their search for food by their long forked tongues which they flick in and out, picking up chemicals in the air and on the ground. Goannas then "read" these chemicals with a special organ in the roof of the mouth.
Goannas evolved in the northern hemisphere in the Upper Cretaceous Period about 90 million years ago. However, in the Miocene Epoch, about 15 million years ago, goannas moved south into Africa and Australia. In Africa, only a few species evolved, but in Australia, goannas evolved into more than 20 species of which at least one is extinct. Goannas may have evolved in such profusion in Australia because when they arrived, there were relatively few medium-sized carnivores competing for the same resources. Africa, however, had a large number of medium-sized carnivores, especially mammals in the dog and weasel families.
One of the most distinctive features of the evolution of goannas in Australia is that their size range is enormous but their body shape has changed very little over time. The scarcity of competitors provided goannas with an evolutionary opportunity: to stick with one way of making a living and apply it over a range of sizes rather than changing the way of making a living within the same size range.
Allen E Greer
Herpetology
Australian Museum
Copyright © Australian Museum, 2003
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