What Do Thorny Devils Eat? Only Ants, by the Hundreds

Which ants do thorny devils eat?

The thorny devil is what biologists call an obligate myrmecophage, which simply means an animal that must eat ants. It does not switch to beetles, termites or plants when ants are scarce, as many other desert lizards do.

Its favorite prey are small, abundant ants that travel in long foraging trails. Studies in Western Australia found that most of the diet came from:

• Small Iridomyrmex ants, especially Iridomyrmex rufoniger, which run along trails on the ground.
• Ochetellus flavipes, listed in older studies as Iridomyrmex flavipes, which the ecologist Eric Pianka singled out as a favorite.
• Crematogaster ants, often taken from trails running up the stems of low currant bushes.
• Smaller numbers of other ants, including Camponotus, Pheidole, Monomorium and Polyrhachis species.

Researchers occasionally find small stones, bits of stick, seeds or tiny flowers in thorny devil stomachs. These were almost certainly being carried by ants and were swallowed along with them.

How many ants does a thorny devil eat?

Exact numbers are hard to pin down, and published figures vary. In early field studies, Eric and Helen Pianka estimated that a single meal can contain anywhere from about 675 to 2,500 ants. Another widely cited estimate puts daily intake at around 750 ants.

Feeding rates also vary with conditions. Pianka reported rates of about 24 to 45 ants per minute, while other observations recorded an average of about 3 ants per minute, with bursts of up to one ant every second. The difference reflects how many ants are passing, how warm the lizard is and how hungry it is.

How a thorny devil catches ants

Thorny devils are sit-and-wait predators. A lizard picks a spot beside an ant trail, usually close to the cover of a shrub or a spinifex tussock, and stays put. As the workers stream past, it flicks out its tongue, picks one up and swallows it, then picks up the next.

High-speed filming has shown how unusual this is. A 2005 study by Jay Meyers and Anthony Herrel compared the thorny devil with a desert horned lizard and two lizards with mixed diets. The thorny devil did not lunge forward with its body, protruded its tongue faster and spent much less time processing each ant. As a result, a single feeding event was faster than in any other lizard of its group studied so far. A bearded dragon eating ants in the same study took about four times as long per ant, because it chewed each one.

The thorny devil’s teeth are also adapted to its diet. It has lost the small front teeth that many lizards use to grab prey, since the tongue does that job, and the lower and upper cheek teeth fit together to form a shearing edge suited to crunching hard-bodied ants.

When do thorny devils feed?

Thorny devils feed only when they are warm. In some study areas they fed mainly in the morning, before about 11 a.m., and again in the late afternoon, from about 3 to 6 p.m., and avoided the fierce midday heat. A typical morning begins with basking in the sun, then a walk to the lizard’s regular toilet site and back, with a stop to feed at an ant trail on the way.

Through the year, feeding follows the lizard’s two active seasons: autumn (March to May) and late winter to early summer (August to December). In the hottest and coldest months they spend most of their time in burrows and eat little. Females build up fat reserves before the breeding season and use them to produce their eggs.

Why does a thorny devil need so many ants?

Ants are small, and much of each one is a tough outer skeleton made of chitin, which is hard to digest. Many ants also contain formic acid and other defensive chemicals. As a result, a thorny devil absorbs only about 59% of the energy in its food, noticeably less than a lizard eating a mix of insects. To make up for this, it eats enormous numbers of ants and has a large stomach to hold them.

The evidence is easy to spot in the field. Thorny devil droppings are distinctive oval, black, glossy pellets that crumble to reveal masses of ant exoskeletons. Each lizard tends to use the same toilet site day after day, away from where it feeds and basks, so neat piles of pellets build up.

Other ant-eaters and the horned lizard connection

Thorny devils share their habitat with other ant-eaters. The short-beaked echidna eats ants and termites across Australia, as described in What do echidnas eat?. Several small dragon lizards that live alongside the thorny devil eat a much broader mix of insects, so they compete with it less directly.

The thorny devil’s closest ecological match is on another continent. North American horned lizards, which also look spiky and flattened and eat mostly ants, belong to a different family. In the 1890s, the naturalist William Saville-Kent noticed the resemblance and predicted that horned lizards would turn out to be ant-eaters as well, a prediction that proved correct. You can meet one of them, the Texas horned lizard, in our guide to lizards in Texas.