Shorttailed monitor

Varanus brevicauda

SUBFAMILY

Varanines

TAXONOMY

Varanus brevicauda Boulenger, 1898, Sherlock River, Nickol Bay, Western Australia.

OTHER COMMON NAMES English: Short-tailed pygmy monitor.

PHYSICAL CHARACTERISTICS

The short-tailed monitor is the smallest varanid. Adult size is reached at a snout-vent length (SVL) of 3.5-4.3 in (90-110 mm) and a weight of 0.35-0.6 oz (10-17 g). Hatchlings are about 1.8 in (45 mm) SVL and weigh only 0.07-0.1 oz (2-3 g).

DISTRIBUTION

Central and Western Australia.

HABITAT

Red sandy desert dominated by spinifex (Triodia) grasses. BEHAVIOR

Short-tailed monitors are terrestrial, spending most of their time within tussocks of spinifex grass. These small monitors probably climb around within these grass tussocks. Their tails are very muscular and prehensile and they "hang on for dear life" when inside a spinifex grass tussock using their legs as well as their tail. The typical monitor lizard threat posture and behavior has been conserved in the evolution of these diminutive monitors, which hiss and lunge with their throat inflated as if they are a serious threat. This tiny varanid is seldom encountered active above ground; the vast majority of specimens are collected in pit traps. Two were dug up in shallow burrows

during August (one must have been active immediately prior to being exhumed, as crisp, fresh tail lash marks were at the burrow's entrance and the lizard had a body temperature of 95.7°F [35.4°C], 10 degrees above ambient air temperature). Mark-recapture studies show that short-tailed monitors do not move very far. Dozens have been pit trapped on a flat sand-plain covered with large, long unburned, clumps of spinifex, possibly the preferred habitat of this monitor. One female weighing 0.32 oz (9.1 g) contained an adult 0.05-oz (1.5-ml) Ctenotus calurus skink; this prey item constituted 16.5% of the short-tailed monitor's body weight.

FEEDING ECOLOGY AND DIET

Since these sedentary lizards seldom leave the protective cover of spinifex tussocks, they must forage within tussocks. They eat large insects and, occasionally, small lizards.

REPRODUCTIVE BIOLOGY

In the Great Victoria Desert, the smallest male V. brevicauda with enlarged testes was 3.2 in (82 mm) SVL and the smallest gravid female was 3.7 in (94 mm) SVL. In central Australia, sexual maturity is reached in males at about 2.75 in (70 mm) SVL and in females at about 3.3 in (83 mm) SVL. Males may become reproductive at an age of about 10 months, but females probably do not mature until their second spring at an age of about 22 months. A male fell into the same pit only hours after a female was removed from that pit trap, suggesting that males may follow scent trails to find females. Clutch size is usually two to three eggs, although larger clutches have been reported. Relative clutch masses of two females with oviductal eggs was 16.7% of a female's body weight. Mating occurs in the spring (September-October) and eggs are laid in November. Hatch-lings emerge in late January to February and are about 1.6-1.8 in (42-45 mm) SVL and weigh only about 0.07 oz (2 g). Incubation takes about 70-84 days at 64.4-77°F (18-25°C).

CONSERVATION STATUS

Not threatened.

SIGNIFICANCE TO HUMANS

As the smallest of all monitors, this diminutive species offers insight into the evolution of small body size. German herpeto-culturists have successfully bred this species in captivity. ♦

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