Varanus acanthurus Boulenger, 1885

Size

The spiny-tailed monitor (also called ackie monitor), a moderately small species of monitor lizard, reaches a maximum total length of 70 cm (27 in), though unconfirmed reports exist of wild individuals growing as long as 34 inches. Its tail is between 1.3 and 2.3 times as long as its head and body combined.

Upper Body Coloration

The upper body is a rich dark brown, marked with bright yellowish to cream spots that often contain a few dark scales.

Tail Structure

The tail is round in cross-section and covered in very spiny scales.

Scale Count

There are between 70 and 115 scales around the midsection of the body.

Distinguishing Features

This species can be told apart from similar-looking species V. baritji and V. primordius by the presence of pale longitudinal stripes on its neck.

General Distribution

This arid-adapted lizard occurs in northern Western Australia, the Northern Territory, and the western and north-western regions of Queensland. It lives in association with arid rocky ranges and outcrops.

V. a. acanthurus Range

The subspecies Varanus acanthurus acanthurus is native to northern Australia, ranging from Broome on the west coast, through the Kimberley and the Top End, to the Gulf of Carpentaria.

V. a. brachyurus Range

Subspecies V. a. brachyurus is found in the center, western, and eastern parts of the species' full range, reaching as far west as Carnarvon and as far east as Mt. Isa.

V. a. insulanicus Range

Subspecies V. a. insulanicus has a range restricted to Groote Eylandt and the Wessel Islands.

Activity Pattern

Spiny-tailed monitors are diurnal, typically solitary ground-dwelling lizards.

Shelter Locations

They are most often found in shelters, mainly under rock slabs, wedged between boulders, in rock crevices, or in burrows. They only rarely shelter in spinifex.

Shelter Function

Sheltering underground gives them access to humid microclimates, which helps them maintain enough hydration to survive the arid conditions of their habitat.

Foraging Behavior

Their diet matches that of intensive foragers, meaning they likely spend a considerable amount of time hunting.

Captive Sexing Methods

There are multiple methods to sex captive ackie monitors, and the hemipenal transillumination technique is generally considered the easiest and most accurate. Sex can also be estimated using visible physical traits, but this method is usually far less accurate.

Male Physical Traits

Male ackie monitors are typically larger, have blockier heads, and have grippy scales on the underside of the tail.

Female Physical Traits

Females are usually smaller, with narrower, pointier heads, and have smooth scales under the tail.

Captive Reproduction

In captivity, females lay clutches of up to 18 eggs. Hatchlings emerge after three to five months of incubation and measure 15 cm (6 in) in total length.

Wild Reproduction Notes

Little information is available about reproduction in wild spiny-tailed monitors.

Sexual Maturity Sizes

Males most likely reach sexual maturity at a snout-vent length of 30 cm (12 in), while females mature at a snout-vent length of 25 to 36 cm (10 to 14 in).

Reproductive Timing

Ovulation occurs in August and November.

Egg Laying Behavior

Females lay their eggs in tunnels they dig themselves.

Communal Nesting

In the wild, female spiny-tailed monitors have been observed sharing large burrows and nesting communally.