This arboreal pygmy monitor species is arboreal, preferring habitats with trees, which offer small hollows that provide the lizards with tight-fitting safe diurnal and nocturnal retreats. They have been found in mulga and myall trees (Acacia), desert she-oaks (Casuarina) and gum trees (Eucalyptus) These varanids do, however, descend to the ground to forage. They feed on grasshoppers and both terrestrial and arboreal geckos. Varanus gilleni actually "harvest" the exceedingly fragile tails of geckos that are too large to subdue intact.
Once I observed a small individual of the large monitor species Varanus gouldi attempting to subdue and eat a large individual Varanus gilleni. It was truly an epic battle, two varanids almost the same size, tails entwined, rolling over and over in the dust! The gouldi had the gilleni by the nape of its neck, and the gilleni was wrapped around the gouldi, struggling to free itself. My ex-wife Helen and I came upon this prehistoric scene while driving slowly down the track deep in the Great Victoria Desert. I had collated all the museum locality data for desert Varanus for papers I intended to write, and I was acutely aware that V. gilleni was known only from a handful of localities in central Western Australia. This was the first one that I had ever seen in the wild. At that point in time, no Varanus gilleni had ever been collected from the Great Victoria Desert; this would be about a 400-500 km extension of the known geographic range. Thus, it was quite important to collect the specimen as a voucher specimen, and a permanent record. However, the fighting monitors were also a sight of a lifetime that I very much wanted to preserve on film. As Helen kept her eyes on the two struggling lizards, I tried frantically to dig out the telephoto lens and get it onto the camera. (Monitors usually run from humans on foot but will sometimes hold their ground before a vehicle.) Before I could get the lens and camera ready, she shouted, "There they go", and jumped out to follow them. I ran after, wringing my hands at losing both the picture and the rare specimen! Luckily, however, the gouldi released the gilleni, and Helen managed to save the day when she came upon the stunned gilleni, quickly stepping on it, and then grabbing it. After photographing this beautiful maroon and gold lizard, it became the first validated and permanent record of that species from that part of Western Australia (a dozen years later, I managed to collect a two more V. gilleni about 200 km to the west of this locality).
To read about other Varanus in the Great Victoria desert of Western Australia, click Desert Varanus.