Moloch horridus Gray, 1841, Western Australia.
OTHER COMMON NAMES English: Mountain devil.
The thorny devil is a very spiny, moderately sized, reddish and yellowish lizard with a round body and a short tail, about 4-6 in (10-15 cm) long. Adult females are larger and stouter than adult males; they range from 3.1 to 4.3 in (80-110 mm) in snout-vent length and weigh 1.2-3.1 oz (33-88.7 g). Adult males are all less than 3.8 in (96 mm) in snout-vent length and never weigh more than 1.7 oz (49 g).
The species occurs in the southern section of the Northern Territory, the northern section of South Australia, and in Western Australia.
Thorny devils are found through most of arid inland Australia, particularly on sandy soils, but they seldom occur on stony soils. They prefer two quite different habitats: spinifex sandy plain and the sand ridge deserts of the interior and the mallee belt of southern South Australia and southwestern Western Australia. The geographic distribution of the species corresponds more closely to the distribution of sandy and sandy loam soils than to any climatological field.
Thorny devils display a bimodal seasonal pattern of activity. These lizards move little during the coldest winter months (June and July) or the hottest summer months (January and February). They are active for a three-month Austral autumnal period (March, April, and May) and a five-month period that spans late winter, spring, and early summer (August through December), during which mating and egg deposition take place. During hot summer days, thorny devils are inactive, retreating into shallow underground burrows that they dig for themselves.
These lizards posses a curious knoblike spiny appendage on the backs of their necks, which has been likened to a false head. When threatened, they tuck their real heads down between their forelegs, leaving this false head in the position of the real head. This makes them difficult for most predators to swallow. When disturbed, thorny devils also inflate themselves with air, puffing up like little puffer fish. They can also change color rapidly; when warm and active, they are usually a pale yellow and red. When they are alarmed or when they are cold, however, they turn dark olive drab.
Thorny devils are obligate ant specialists, eating virtually nothing else. They consume several species of ants but are especially partial to very small Iridomyrmex ants, especially Iridomyrmex flavipes. Large numbers of these tiny ants are eaten per meal by an individual thorny devil (estimates range from 675 to 2,500).
Mating has been observed in the autumn, which suggests that thorny devils may have a mechanism of sperm storage. In contrast to the relatively sedentary summer to autumn existence, thorny devils move over much greater distances during August and September, when most mating takes place. Female thorny devils excavate nest chambers and lay clutches of eggs in September, October, and November. Eggs are laid from mid-
September through late December. Only a single clutch is laid per year, and clutch size varies from three to 10, with a mode of eight eggs per clutch. Seven clutches had reported incubation times of 90-132 days.
Hatchlings emerge in January and February, weighing an average of 0.06 oz (1.8 g) and measuring 2.5-2.6 in (63-65 mm) in total length (snout to tail tip). Hatchlings may eat their own egg cases to obtain calcium and other materials to support early growth.
These lizards are interesting to biologists because they are independently evolved ecological equivalents of North American horned lizards and one of the best examples of convergent evolution. ♦
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