gondwana reptile productions by rob valentic about us

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The feeding strategy of a Yellow-spotted monitor Varanus panoptes panoptes (Storr, 1980).

A field observation. 

Text & Photos: Rob Valentic

INTRODUCTION:

The Yellow-spotted Monitor Varanus panoptes panoptes is a large, predominantly terrestrial monitor, widespread and favouring heavy soils in humid areas of northern Australia (Wilson & Knowles, 1992).  Indeed, anyone who has witnessed an adult erect on its rear legs in the hot expanses of an alluvial floodplain will testify that it is a formidable sight.  The following report is based on a single adult specimen that was followed for several hours as it foraged in and around our makeshift camp in the ‘Top End’ of the Northern Territory.

LOCATION:

Shady Camp is located on the banks of the lower Mary River (1230’E, 13143’S).  A concrete barrage has been constructed across the river which marks the confluence of fresh and tidal (estuarine) waters.

Habitat:   Riparian alluvial floodplain with deep cracking in the heavy clay soil. Open grassland with scattered Banyan Trees Ficus virens in the campground.
Seasonal climate:  The following observation took place on the 17th November 1990.  The indigenous people of the Kakadu area refer to this period as ‘Gunumeleng’ on their seasonal calendar.  It is the pre-monsoonal season of very hot weather with increasing humidity. Clouds build up each afternoon causing  thunderstorms and irregular rainfall.  During this period, the herpetofauna becomes increasingly active as air pressure fluctuates, and typically dormant dry season species such as Frilled Lizards Chlamydosaurus kingii leave the tree tops and are seen with more frequency on the ground.

OBSERVATION:   

Time:  08:15 to 10:05hrs (Central Standard Time).

Weather conditions:  Air temperature: 33C, with a gentle breeze and full cloud cover. Relative humidity: 90%+.
 
An adult male Varanus p. panoptes (total length estimated at > 1.5 metres) was observed active around the camp.  Having camped in the area for several days I was familiar with the beast, as its retreat site was beneath the concrete foundation of the toilet only a few metres from camp.

The monitor appeared to focus intently on the black-soil substrate immediately in front of it as it ambled slowly around the gas bottle, its tongue flickering constantly and its head tilted downwards.  As I watched and pondered how great it was to have such a guest in camp, the lizard suddenly stopped, fixing its gaze immediately in front of the snout. It then began to vigorously scratch the stratum using the digits of both fore feet as a rake.  A small frog subsequently leapt from the disturbed soil and was seized in the jaws of the monitor, whilst in mid-flight, and was consumed with the head raised at an acute angle.  The monitor progressed again and continued this foraging pattern within an area of some sixty square metres before retiring beneath the ‘bush dunny’ at 10:10hrs. During these observations, a distance of  some two metres was maintained between myself and the lizard, which did not seem at all perturbed by my close proximity.  A total of eighteen frogs were consumed in the above manner.  The monitor discovered a further specimen which I stole from him for identification purposes.  It was identified as an Ornate Burrowing Frog Opisthodon ornatus, a species also widespread across the northern half of the continent.  From close scrutiny it appeared that all other frogs taken by the monitor were of the same species.

Yellow spotted Monitor Lizard Varanus panoptes panoptes from Longreach, Queensland, Australia.                  Ornate Burrowing Frog Limnodynastes ornatus from Charlieville, Queensland, Australia.
A bipedally erect adult Yellow-spotted Monitor Varanus p. panoptes from north of Longreach, Queensland                    An Ornate Burrowing Frog Opisthodon ornatus from Charleville, Queensland.

DISCUSSION:

A close inspection of the immediate stratum was made whenever the monitor stopped, but no discernible evidence of any imperfection in soil level or consistency could be detected.  Perhaps the vision of V. p. panoptes was considerably more adept at noting such indications (if any) than mine.  Perhaps scent particles from the concealed frogs were transferred via the constantly flickering  tongue-tips.  In any case, the success rate was exemplary, as on any occasion the monitor would stop and commence scratching, he did not fail to locate and devour another O. ornatus.

REFERENCE:

Wilson, S.K. & Knowles, D.G. 1992. Australia's Reptiles - A Photographic Reference to the Terrestrial Reptiles of Australia. Cornstalk Publishing, Pymble NSW. 447pp.