Varanus tristis consume other lizards as well as baby birds (and probably bird eggs as well, Pianka 1971, 1982). These lizards drag the base of their tails and leave a very distinctive track with a wide tail mark (Pianka 1971). Tracks typically run more or less directly from tree to tree (these monitors climb most of the trees they visit looking for food). V. tristis activity is highly seasonal, and the animals rely on building up fat reserves during times of plenty to get them through lean periods (Pianka 1971). Once, while camped at a study site near Lorna Glen Homestead where I had never seen a tristis track in many weeks of work over several months in the midst of a prolonged drought, I noticed a beady black eye peering out of a small black hole in a burned out Eucalyptus tree; chopping the hollow tree open revealed an extremely emaciated tristis, literally skin and bones, waiting for the drought to break!
Varanus tristis also eat other lizards including agamids, geckos and various species of Ctenotus. One specimen had actually eaten a small thorny devil Moloch horridus (estimated intact volume about 5 ml)! A tristis weighing 220 gm had eaten a large adult Pogona minor (estimated volume 50 ml), constituting 22.7% of that Varanus's body mass. Another tristis weighing 330 gm had eaten a 57 ml Pogona minor, which constituted 17.3% of its mass.
Based on enlarged testes and yolked ovarian eggs, both sexes appear to achieve sexual maturity at about 200 mm SVL. Large adult males are found with females in the same trees or dead marble gum hollow logs in October-November (probably the mating season). Fecundity is considerably higher in V. tristis (mean clutch size = 10.1 eggs) than it is in other pygmy montitors (Pianka 1994). One hatchling captured in early March measured 72 mm SVL; this individual was not black like adults but was boldly banded with black and beige.
At a desert site near Agnew, W.A., I heard a Galah Cockatoo screeching loudly nearby as if in distress. When I first sighted the cockatoo, it was on the ground with its crest held high, and its wings partially outstretched. The Galah flew up onto a fallen log under a marble gum tree, and then into the tree, which proved to be its nesting tree. A large male (288 mm SVL, 307 gms) tristis clambered over the same log toward the tree. The Galah continued to screech, and then began to harass the lizard. When the lizard climbed about three meters up the tree, and went around out of sight on the other side, the cockatoo attacked and actually drove the monitor back down the tree. The Galah's mate was present and also screeching. These large climbing predatory lizards must constitute a potent threat to hole-nesting parrots.
Pianka, E. R. 1971. Notes on the biology of Varanus tristis. Western Australian Naturalist 11: 180-183. Download pdf
Pianka, E. R. 1982. Observations on the ecology of Varanus in the Great Victoria desert. Western Australian Naturalist 15: 37-44. Download pdf
Pianka, E. R. 1986. Ecology and Natural History of Desert Lizards. Analyses of the Ecological Niche and Community Structure. Princeton University Press, Princeton, New Jersey.
Pianka, E. R. 1994. Comparative ecology of Varanus in the Great Victoria desert. Australian Journal of Ecology 19: 395-408. Download pdf.
Thompson, G. G., E. R. Pianka, and M. de Boer. 1999. Body temperatures of an arboreal monitor lizard, Varanus tristis (Squamata: Varanidae) during the breeding season. Amphibia- Reptilia 20: 82-88. Download pdf
Thompson, G. G. and E. R. Pianka. 1999. Reproductive ecology of the black-headed goanna Varanus tristis (Squamata: Varanidae). Journal of the Royal Society of Western Australia 82: 27-31. Download pdf
Thompson, G. G., M. de Boer, and E. R. Pianka. 1999. Activity areas and daily movements of an arboreal monitor lizard, Varanus tristis (Squamata: Varanidae) during the breeding season. Australian Journal of Ecology 24: 117-122. Download pdf
Farlow, J. O. and E. R . Pianka. 2000. Body form and trackway pattern in Australian desert monitors (Squamata: Varanidae): Comparing zoological and ichnological diversity. Palaios 15: 235-247. Abstract, Download pdf
To read about other Varanus in the Great Victoria desert of Western Australia, click Desert Varanus.