Varanus brevicaudaShort-tailed Pygmy Monitor

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Geographic Range

The short-tailed monitor occupies arid regions of western Australia (Pianka 1969).

Habitat

The preferred habitat of the short-tailed monitor is arid regions dominated by spinifex. Spinifex are perennial grasses that form dense clumps, up to several feet in diameter, consisting of a central dense complex lattice-work and numerous outwardly directed needle-like spines (Pianka 1969).

Physical Description

The short-tailed monitor is the smallest of the Varanid lizards, attaining a maximum adult length of only 8 inches. Newly hatched short-tailed monitors look just like the adults, but weigh about 1 to 2 grams and are only 1 to 2 inches in total length (Pianka 1969). Like all monitors the short-tailed monitor has a long neck, well developed limbs with five toes on both fore and hind limbs, strong claws, and a powerful tail that cannot undergo autotomy (Cogger and Zweifel 1998). The body color is usually a drab olive to brown color with lighter ocelli on the trunk. It is nearly impossible to determine the sex of most monitors by their outward appearance and the short-tailed monitor is no exception, as it is monomorphic (Bartlett and Bartlett 1996).

  • Range mass
    8 to 10 g
    0.28 to 0.35 oz

Reproduction

The short-tailed monitor is oviparous. Short-tailed monitors exhibit internal fertilization. The male has paired intromittent organs known as hemipenes. Although the hemipenes are paired they are only used one at a time, depending on which is more convenient. The clutch size will vary from 1 to 4 eggs, with a clutch size of 2 being the most frequent (Pianka 1969). The female will dig a burrow in which to deposit her eggs. The eggs will hatch between 70 and 100 days depending upon incubation temperature. The hatchlings are precocial and grow quickly attaining maturity and adult length within the first year to year and a half (Bartlett and Bartlett 1996).

Behavior

Like most monitors, the short-tailed monitor is solitary in nature and avoids contact with other monitors, especially those that are bigger and could be a predator. However, due to the fact that short-tailed monitors are so small and mainly live in burrows it is not uncommon to find a very large population in a very small area (Pianka 1969). These monitors are very active diurnal foragers. The short-tailed monitor has a very keen sense of sight and hearing but attains the bulk of its sensory information through chemical signals picked up by the Jacobson's organ. These signals include mate recognition and competitor recognition. During the breeding season males will engage in combat for the right to mate with a female. The males will raise up on their hind limbs supported by their tails and wrestle until one is thrown to the ground (Cogger and Zweifel 1998).

Food Habits

The short-tailed monitor is strictly carnivorous. Unlike most lizards, monitors are active foragers with a very high metabolic rate. Monitors use their long forked tongue much like a snake to pick up chemical cues in the air. The tongue, when flicked out, draws in these chemical cues and transfers them to the Jacobson's organ where the cues are analyzed (Pough et al. 1998). The diet of the short-tailed monitor is composed primarily of insects, other arthropods, smaller lizards (especially of the family -Ctenotus-, Australian skinks), and like most other monitors, they will not turn down carrion (Bartlett and Bartlett 1996). However, any prey item that has a chance of fitting in the monitor's throat will be actively pursued and consumed. One short-tailed monitor was reportedly found with a -Ctenotus calurus- in its stomach that made up 16.5% of the monitor's total weight. The monitor is able to eat such large prey because it utilizes a technique for feeding called mesokinesis. Mesokinesis is the dislocation of the jaw to swallow prey larger than the head (Pianka 1969).

Economic Importance for Humans: Positive

While the short-tailed monitor does not have a positive economic value there is a slight scientific interest in how it deals with the arid conditions in which it lives (Pianka 1969).

Conservation Status

Although there is no special concern about the short-tailed monitor all monitors from Australia are protected under CITES. There has been no extensive pressure from the pet trade on the short-tailed monitor (Pough et al. 1998).

Contributors

John Herman (author), Michigan State University, James Harding (editor), Michigan State University.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

native range

the area in which the animal is naturally found, the region in which it is endemic.

References

Bartlett, R., P. Bartlett. 1996. Monitors, Tegus, and Related Lizards. NY: Barron's Educational Series, Inc..

Cogger, H., R. Zweifel. 1998. Encyclopedia of Reptiles and Amphibians, 2nd Edition. San Diego: Academic Press.

Pianka, E. May 1969. Habitat Specificity, Speciation, and Species Density in Australian Desert Lizards. Ecology, 50: 498-502.

Pough, F., R. Andrews, J. Cadle, M. Crump, A. Savitzky. 1998. Herpetology. New Jersey: Prentice-Hall, Inc..