Thorny Devil (Moloch horridus): Australia's Spiny Desert Lizard

What does a thorny devil look like?

The thorny devil is short, round-bodied and flattened, with a small head, short legs and a thick, tapering tail. Almost its whole upper surface is studded with conical spines, and even its belly is covered in small, pointed scales. Two especially large spines sit above the eyes like horns, which is where the “devil” in its name comes from. The genus name Moloch refers to an ancient Near Eastern deity, and horridus means rough or bristly.

On the back of the neck is a rounded, spiny hump of soft tissue. When threatened, the lizard tucks its real head down and leaves this knob in its place, which is why it is often called a false head. You can read more about this trick in What is a thorny devil’s false head?.

Coloration is a patchwork of desert shades: sandy yellow, tan, chestnut and reddish brown. The colors shift with temperature and mood. A cold or alarmed thorny devil is darker and duller, often olive, while a warm, active one turns paler and brighter.

Despite how fierce the spines look, most are not supported by bone. They are hardened skin and scale, and only the two horns over the eyes rest on small bony bumps of the skull.

How big is a thorny devil?

Thorny devils are small. Adults reach up to about 21 cm (8.3 in) in total length, including the tail, and most are smaller. Females are clearly larger and heavier than males, the reverse of many other dragon lizards, in which males are usually the bigger sex.

Weight changes a lot through the year. A female may lose around 40% of her body mass when she lays her eggs and then regain it within weeks once ants are plentiful.

Where do thorny devils live?

Thorny devils are found only in Australia. Their range covers much of the arid interior and west of the continent: large parts of Western Australia, much of the Northern Territory and South Australia, and the far west of Queensland.

What matters most to them is soil. They live on sandy and sandy loam ground and are rarely found on hard, stony country. Typical habitats are spinifex sandplains and red sand-ridge deserts, mulga acacia scrub, and the mallee belt of low eucalypt woodland in southern South Australia and southwestern Western Australia. They shelter beneath shrubs and inside spinifex tussocks and dig their own shallow burrows to escape extreme heat and cold.

The thorny devil is one of several remarkable lizards of Australia’s dry country; others are covered in our list of desert animals. Another familiar inland lizard, the central bearded dragon, belongs to the same family but is far less specialized.

What does a thorny devil eat?

The thorny devil is an ant specialist. It eats ants and almost nothing else, mostly small species of the genera Iridomyrmex and Crematogaster. It finds a busy ant trail, settles beside it and picks up the passing workers one at a time with its tongue.

Because ants are small and not very nourishing, a thorny devil must eat a great many of them. Field estimates of a single meal range from several hundred to around 2,500 ants. Its stomach is large, its teeth form shearing blades suited to hard ant bodies, and a single feeding strike is faster than in any other lizard of its group studied so far. Its droppings are distinctive black, glossy pellets packed with ant remains, usually left at a regular toilet site. For the full story, see What do thorny devils eat?.

How do thorny devils drink?

One of the thorny devil’s most famous abilities is collecting water with its skin. Between its overlapping scales runs a network of fine, interconnected grooves. When any part of the body touches water, the grooves soak it up by capillary action, the same force that draws water up a paper towel, and carry it across the body to the corners of the mouth. The lizard then swallows it with gulping jaw movements.

Laboratory studies by Philipp Comanns and colleagues in 2016 found that thorny devils can drink this way when standing in a puddle or wet by rain. Standing on damp sand did not fill the channels enough for drinking, but thorny devils in the wild shovel moist sand onto their backs, which may solve the problem. The idea that they drink mainly from dew turned out to be less likely than once thought. Read more in How do thorny devils drink?.

Behavior: a slow, rocking walk

Thorny devils are active by day and live alone. They walk slowly, with a stiff, jerky, rocking motion, and often freeze in mid-stride with one foot raised. This odd gait probably makes them harder for predators to pick out against swaying shrubs and grass.

Their year has two active seasons. They are busiest in autumn (March to May) and from late winter to early summer (August to December), and they are largely inactive in the hottest months (January and February) and the coldest (June and July), when they stay in their burrows.

In summer and autumn a thorny devil keeps to a small patch of ground with a few shrubs, an ant trail or two and a toilet site. A typical morning starts with basking in the sun, followed by a short walk to the toilet site and back, with a stop to feed at an ant trail on the way. In late winter and early spring, when mating takes place, they travel much farther, and some individuals have been tracked moving more than 500 m (1,600 ft) in a day.

Defenses: spines, puffing up and camouflage

The thorny devil has few ways to run from danger, so it relies on not being seen and on being hard to swallow. Its colors and outline blend so well with sand, stones and dry leaves that even experienced field biologists struggle to find one standing in the open.

If discovered, it freezes, tucks its real head down between its front legs to present the spiny false head, and can gulp air to puff itself up. A larger, spikier lizard is much more awkward for a bird or monitor lizard to grab and swallow.

Reproduction and life cycle

Mating takes place mainly in late winter and spring. In captivity, males have been seen approaching a female and bobbing their heads; a female that is not receptive rolls sideways to throw the male off. Some mating also happens in autumn, which suggests females may be able to store sperm.

Between about September and December, the female digs a nesting burrow in sand, often on a south-facing dune slope, and lays one clutch a year of 3 to 10 eggs, most often about 8. The eggs lie roughly 20–30 cm (8–12 in) underground. She fills in the burrow and smooths the surface to hide it. The eggs hatch after about 90 to 132 days, and the tiny hatchlings, weighing around 2 g, dig their way out. Young males and females grow at the same pace for their first year, after which females grow faster. Growth continues until at least five years of age.

Lifespan, predators and threats

Lifespan in the wild is poorly known. An early researcher who kept thorny devils for up to eight years thought a lifespan of up to about 20 years was likely in nature, and that figure is widely quoted, but it is an estimate rather than a measured record.

The spines do not make thorny devils invulnerable. Known predators include the Australian bustard, the black-breasted buzzard and monitor lizards such as the sand goanna and the racehorse goanna; one thorny devil was found whole in a sand goanna’s stomach. Snakes, dingoes and introduced red foxes may also take them. Their slow walk also puts them at risk when they cross roads.

Conservation status

The IUCN Red List classifies the thorny devil as Least Concern (assessed in 2017). Its range is enormous, much of it remote and lightly used, and no major threat to the species as a whole has been identified. Like other native Australian reptiles, it is protected under state and territory wildlife laws.

Because thorny devils are so well camouflaged and so specialized, they are difficult to survey. Scientists still know relatively little about how their numbers change over time, how long they live, or how changes in fire patterns and ant communities affect them.

Myths and facts

• Myth: Thorny devils soak up water through their skin. Fact: the skin does not absorb water into the body. It moves water along the surface, through channels between the scales, to the mouth, where the lizard drinks it.
• Myth: The spines are venomous or dangerous. Fact: the thorny devil is harmless to people. It has no venom, and its spines are only a defense.
• Myth: It is a kind of horned lizard. Fact: North American horned lizards look and live in a strikingly similar way and also eat ants, but they belong to a different family. The resemblance is a classic case of convergent evolution, where unrelated animals evolve similar features in response to similar lifestyles.