The spiny-tailed monitor (Varanus acanthurus) or ridge-tailed monitor is an Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

n species of lizard belonging to the genus of monitor lizard

Monitor lizard

Monitor lizards are usually large reptiles, although some can be as small as in length. They have long necks, powerful tails and claws, and well-developed limbs. Most species are terrestrial, but arboreal and semiaquatic monitors are also known...

s (Varanus).

Diagnosis

The spiny-tailed monitor, a medium-sized monitor lizard, can attain a total length of up to 70 cm. The tail is about 1,3-2,3 times longer than head and body. The upper side is a rich, dark brown and painted with bright yellowish to cream spots, which often enclose a few dark scales. Its tail is round in section and features very spinose scales. There are 70-115 scales around the middle of the body. The spiny-tailed monitor is distinguished from the similar-looking species V. baritji and V. primordius by the presence of pale longitudinal stripes on the neck.

Distribution and habitat

This arid-adapted lizard is found in northern Western Australia

Western Australia

Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...

, in the Northern Territory

Northern Territory

The Northern Territory is a federal territory of Australia, occupying much of the centre of the mainland continent, as well as the central northern regions...

 and in the eastern and northeastern parts of Queensland

Queensland

Queensland is a state of Australia, occupying the north-eastern section of the mainland continent. It is bordered by the Northern Territory, South Australia and New South Wales to the west, south-west and south respectively. To the east, Queensland is bordered by the Coral Sea and Pacific Ocean...

. The spiny-tailed monitor inhabits arid areas, and is associated with rocky ranges and outcrops.

Subspecies and taxonomy

The spiny-tailed monitor is part of an exclusively Australian radiation

Radiation

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 of small monitor lizards, the subgenus Odatria.

In the past, three subspecies of V. acanthurus were recognised: The continental subspecies V. a. acanthurus and V. a. brachyurus, and V. insulanicus, that only inhabits Groote Eylandt

Groote Eylandt

Groote Eylandt is the largest island in the Gulf of Carpentaria in northeastern Australia. It is the homeland of, and is owned by, the Anindilyakwa people who speak the isolated Anindilyakwa language)....

 and the islands of the Wessel group

Wessel Islands

The Wessel Islands are a group of islands belonging to the Northern Territory of Australia. They extend in a more or less straight line from Buckingham Bay and the Napier Peninsula of Arnhem Land, and Elcho Island, to the northeast. Marchinbar Island is the largest of the group...

. In 2006, the results of a study on the mtDNA of Australian monitors were published, according to which the two continental subspecies do not form natural (monophyletic), taxonomic entities. V. a. insulanicus was proven to be monophyletic, but it is more closely related to V. baritji than to other V. acanthurus. Therefore V. a. insulanicus might represent a distinct species. Wilson and Swan (2010) still accept V. a. insulanicus as a valid subspecies of V. acanthurus, which is easily distinguished from other spiny-tailed monitors by its dark colouration and more-banded pattern.

Ecology

Spiny-tailed monitors, as all monitor lizards, are diurnal, solitary ground-dwellers. This species is most often found in its shelter, mainly under rock slabs, boulders or in rock crevices. Only rarely they hide in spinifex. As the spiny-tailed monitor has a lower level of activity compared to other monitor lizards, as well as a lower metabolic rate, it is often found in its hideout at day, and most likely it is a sit-and-wait-predator. They prey mainly on insects, especially orthopterans, beetles and cockroaches, and on small lizards such as skinks, geckoes or small dragon lizards

Agamidae

Agamids, lizards of the family Agamidae, include more than 300 species in Africa, Asia, Australia, and a few in Southern Europe. Many species are commonly called dragons or dragon lizards. Phylogenetically they may be sister to the Iguanidae, and have a similar appearance. Agamids usually have...

. Less often, this species feeds on other insects, spiders, snails or young marsupials. Approximately 70% of its water requirement comes from food.

Reproduction

In captivity, a clutch consists of up to 18 eggs. The young hatch after 3-5 months of incubation, and measure 15 cm. The knowledge on reproduction in the wild is sparse. Males most likely mature at 12 cm snout-vent length, females mature at 10-14 cm snout-vent length. Ovulation occurs in August and November. The eggs are deposited in self-dug tunnels.