What Is a Thorny Devil's False Head? A Clever Defense

What is the false head?

The thorny devil is a small ant-eating lizard of Australia’s deserts, covered almost entirely in cone-shaped spines. Just behind its real head, on the nape of the neck, sits a prominent hump crowned with spines. When the real head is hidden, this hump is large and rounded enough to pass for one, which is why it is called a false head.

The knob is made of soft tissue, not bone, and is covered with the same hard, pointed scales as the rest of the body. In profile, with the real head lowered, it looks remarkably like a small, horned head facing forward.

How the false-head trick works

When a thorny devil feels threatened, it lowers its real head and tucks it down between its forelegs. The spiny knob is left raised in the position the head would normally occupy, and the lizard often stays completely still.

The idea is that many predators aim for the head, the part of the body where a bite or peck would do the most damage. By presenting the knob instead, the lizard protects its eyes, brain and throat. If an attacker does grab the knob, it gets a mouthful of spines, and the injury to the lizard, if any, is to tissue it can better afford to lose.

The false head also makes the lizard much harder to swallow. Many predators swallow small prey head first. With its real head tucked down and the spiny knob and shoulders forming a wide, prickly front end, a thorny devil is a very awkward shape to get down a throat.

Does it really work?

This explanation is widely accepted, but direct tests are difficult. Thorny devils are rare to encounter, superbly camouflaged and slow, so watching real predator attacks in the wild is almost impossible.

Some evidence comes from the field. The ecologist Eric Pianka, who studied thorny devils in Western Australia for many years, once caught an individual whose neck knob was damaged as though a mammal had chewed on it, while the rest of the lizard survived. That is exactly what you would expect if the knob takes the hit instead of the real head. Most scientists see the false head as one part of a package of defenses rather than a guaranteed escape.

The thorny devil’s other defenses

The false head works together with several other tricks.

• Camouflage. Blotches of sandy yellow, tan, chestnut and reddish brown, plus a broken, spiky outline, make the lizard blend into sand, leaf litter and spinifex grass. Even experts searching for thorny devils often walk right past them.
• Color change. Thorny devils are darker and more olive when cold or alarmed and paler and brighter when warm, which helps them match their background and control their body temperature.
• Freezing and a jerky walk. They move slowly with a rocking, stop-start gait and often freeze in mid-step with a foot raised. A motionless lizard is much harder for a hunting bird to notice.
• Spines. The whole upper body is armed with sharp, conical spines, which make the lizard painful and difficult to grasp and swallow.
• Puffing up. When disturbed, a thorny devil can gulp air and inflate its body, making itself bigger and spikier still.
• Hiding. Thorny devils spend nights and the hottest part of the day under shrubs or inside loose spinifex tussocks, and retreat to these shelters when danger appears.

Together these defenses work well, but not perfectly. Pianka found a thorny devil whole in the stomach of a sand goanna, a monitor lizard, and another inside a racehorse goanna. Australian bustards and black-breasted buzzards are also known to eat them, while snakes, dingoes and introduced red foxes are likely predators.

False heads elsewhere in nature

The thorny devil is not the only animal that tries to fool predators about where its head is. Many hairstreak butterflies have eyespots and thin tails at the rear of their hind wings that look like a head with antennae; birds that peck at this false head often tear off only a piece of wing. Some fish, such as the foureye butterflyfish, have a dark eyespot near the tail and a stripe hiding the real eye.

Other animals use different kinds of bluff. The atlas moth’s wingtips resemble snake heads, as explained in Why do atlas moths look like snakes?, and pangolins rely on armor rather than deception, curling into a ball of overlapping scales, as described in How do pangolins protect themselves?. The thorny devil combines a little of both strategies: armor all over and a decoy where the head should be.