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AGAMID PREDATION BY A STRIPED SKINK Ctenotus orientalis (LACERTILIA:SCINCIDAE).

By Rob Valentic and Chris Hay.

INTRODUCTION

In their review of the Ctenotus uber orientalis complex (Storr, 1971), Hutchinson and Donnellan (1999) recently elevated orientalis to full species status. In relation to the diet of Ctenotus; Greer (1989), citing other references, stated; “Ctenotus appear to be opportunistic predators, although some species are known to take more than 10% plant material by volume.” Ehmann (1992) wrote that the diet of Ctenotus uber consists of insects, spiders, centipedes and small lizards. Bennett (1997) recorded the diet of presumably ACT populations of Ctenotus orientalis as insects and occasionally smaller skinks. Bedford (1992) reported an interesting field observation of an adult C. orientalis ascending and consuming berries from fruiting salt bushes. Whilst commenting on the paucity of published information available on the biology of most Ctenotus species, Ehmann (1992) stated; “any field naturalist with a keen eye and interest could add materially to our knowledge of these skinks.”

The following account provides details of an adult C. orientalis expelling a recently ingested hatchling Mallee Dragon Ctenophorus fordi.

OBSERVATION 

Date: 27th January 2001.

Time: 13:55 hrs (Eastern Standard Time/ Daylight Savings Time).

Weather Conditions: Fine with no cloud cover and a gusting westerly breeze. Air temperature: 25.6C, Relative humidity: 32% (Measurements taken with a Smart Digital Thermo-hygrometer).

Location:  The Big Desert Wilderness area, central-western Victoria. 5.8 km north of Moonlight Tank (3545’S, 14123’E) on the verge of the Murrayville Track (distance reading taken using the vehicle odometer).

Habitat: Pale, clay-based interdune plain between a sand dune system running paralell and aligned east-west. Mallee/heath vegetation with an upper layer comprised predominately of Callitris verrucosa and Eucalyptus baxteri and with a dense ground cover of small shrubs including Hakea spp. and sparse large clumps of Triodia irritans.

Notes: Whilst stalking an adult male Painted Dragon Ctenophorus pictus that had retreated into a shaded area beneath a dense stand of Callitris verrucosa, an adult C. orientalis was inadvertently flushed from the cover of a small shrub nearby. After the skink was secured by hand for photographic purposes, one of us (Chris Hay) remarked that unfortunately the lizard had been squashed, as its tongue was protruding externally. Upon closer inspection the ‘tongue’ was in fact the hind limbs and tail of a hatchling C.fordi.

 

The hand pressure applied when pinning the skink was sufficient to almost expel the recently ingested agamid. The hatchling dragon had been swallowed head first and was gently removed for closer scrutiny. Length and weight measurements of both lizards were taken using a set of Mitutoyo Digimatic Callipers and Mettler Digital Scales.

 

Ctenotus orientalis

Snout to vent length: 55.4mm.  Tail length: 87.6mm (original tail).  Total length: 143.1mm.  Head length: 10.8mm (taken from the anterior edge of the rostral scale to the posterior edge of the parietals).  Head width: 8.0mm (at the widest point).  Weight: 3.95 grams (exclusive of the prey item).

 Striped Skink Ctenotus orientalis from Big Desert, western Victoria, Australia.
An adult Striped Skink Ctenotus orientalis from Big Billy Bore, Big Desert, western Victoria.

Ctenophorus fordi.

Snout to vent length: 26.3mm.  Tail length: 19.5mm.  Total length: 45.9mm (note that the posterior two thirds of the tail was missing).  Head width: 6.7mm (at the widest point).  Weight: 1.21 grams. The C. orientalis was subsequently released at the point of capture.

Mallee Dragon Ctenophorus fordi from Big Desert, western Victoria, Australia.
An adult Mallee Dragon Ctenophorus fordi from Big Billy Bore, Big Desert, western Victoria.

DISCUSSION 

That C. orientalis is capable of consuming prey more than a third of its own body weight is noteworthy. Greer (1989), referring to the genera stated: “Not surprising, larger individuals and species are able to eat larger prey than smaller forms and hence take a greater variety of prey.” Upon closer inspection of the hatchling C. fordi it was noted that there were several tiny lacerations on top of the head consistent with bite marks. The entrails were protruding out of the cloacal opening, possibly caused by the jaw pressure applied by the C. orientalis to the trunk of the body or by human hand pressure on capture. The proximal tail stub was also closely examined and the break appeared to be very recent and may have occurred during the ensuing struggle with the skink. 

Pianka’s (1969c) studies suggested that sympatric Ctenotus species minimise competition by differing their daily and seasonal activity periods, prey item sizes and foraging patterns. Hatchling C. fordi were found to be abundant during our visit to the Big Desert and the absence of flowering or fruiting shrubs was also noted. It is possible that smaller lizard species may be exploited as an important seasonal food source for adult C. orientalis during this period.

ACKNOWLEDGMENT

I would like to thank the anonymous referees who took the time to review this paper and for their helpful comments and improvements to the manuscript.

 

REFERENCES

Bedford, G. 1992.  A Different Feeding Strategy in the Striped Skink (Ctenotus uber orientalis) from South Australia. Herpetofauna, 22(2): 31-32.

Bennett, R. 1997.  Reptiles and Frogs of the Australian Capital Territory. National Parks Association of the ACT Inc.,Woden, 86pp.

Ehmann, H. 1992.  Encyclopedia of Australian Animals. Reptiles. Angus and Robertson, Sydney, 495pp.

Greer, A. E. 1989.  The Biology and Evolution of Australian Lizards. Surrey Beatty and Sons Pty Ltd, Chipping Norton, 264pp.

Hutchinson, M. N. & Donnellan, S. C. 1999. Genetic variation and taxonomy of the lizards assigned to Ctenotus uber orientalis Storr (Squamata: Scincidae) with description of a new species. Records of the South Australian Museum. 32 (2):173-189.

James, C. D. 1991.  Temporal variation in diet and trophic partitioning by coexisting lizards (Ctenotus: Scincidae) in central Australia. Oceologia 85:.553-561.

Pianka, E. R. 1969c.  Sympatry of desert lizards (Ctenotus) in Western Australia. Ecology 50: 1012-1030.

Storr, G.M. 1971. The genus Ctenotus (Lacertilia: Scincidae) in South Australia. Records of the South Australian Museum. 16:1-15.

Taylor, J. A. 1986.  Food and foraging behaviour of the lizard Ctenotus taeniolatus. Australian Journal of Ecology 10: 281-295.