What Is a Cassowary's Casque For? Helmet Theories Explained

What is a cassowary’s casque?

The casque is the tall, helmet-like crest on top of a cassowary’s head. All three living cassowary species have one. The southern cassowary has the largest, usually about 13–18 cm (5–7 in) high, while the dwarf cassowary of New Guinea has the smallest.

From the outside, the casque looks like solid horn. It is covered by a hard sheath of keratin, the same protein that makes up fingernails and bird bills. Inside, the casque is far lighter than it looks. Its core is a spongy, fibrous material that sits on top of the bones of the skull. Older accounts described the casque as hollow, but later studies found it is filled with a network of fine fibers rather than empty.

Chicks hatch without a casque. It begins as a small bump and grows for years, reaching full size by the time birds are about 3–5 years old, when they also gain full adult colors. Females, the larger and dominant sex in the southern cassowary, usually have taller casques than males.

Theory 1: A display and status signal

One of the oldest and most widely accepted ideas is that the casque is a signal to other cassowaries. Because the casque grows with age and is largest in big, dominant females, its size may show age, health and social rank.

This fits how cassowaries look. The casque sits right above the bright blue skin and red wattles of the head and neck, which are themselves display features. Many animals have showy head ornaments, from deer antlers to hornbill casques, that play a role in attracting mates or warning off rivals.

Recent research has found that cassowary casques can glow in ultraviolet light, a part of the spectrum many birds can see, which may support a role in visual signaling. These results are new, and how cassowaries use any such signal is not yet known.

Theory 2: Making or receiving deep sounds

Cassowaries make some of the deepest calls of any bird. Their low booms sit close to the lower limit of human hearing. Very low sounds travel well through dense rainforest, where it is hard to see other birds.

Biologist Andrew Mack, who studied cassowaries in New Guinea, suggested that the casque might help the bird produce, amplify or even sense these low sounds. The idea is attractive because the casque is large, light and has an internal structure that could resonate.

So far, though, there is no direct evidence that the casque actually changes the sound of a cassowary’s call. It remains a reasonable hypothesis that needs testing.

Theory 3: A radiator for shedding heat

The best-tested idea is that the casque helps control body temperature. Cassowaries are large, dark-feathered birds living in hot, humid tropical forests, and they need ways to lose heat.

In a study published in Scientific Reports in 2019, researchers at La Trobe University in Australia used thermal imaging cameras on cassowaries in zoos at different air temperatures. They found that the casque released heat when the weather was hot and released very little when it was cool, so it behaved like a “thermal window”. The bare skin of the neck and wattles acted in a similar way.

This suggests the casque helps cassowaries stay cool in the heat. It does not rule out other uses. Many animal structures do more than one job, and a casque could work as both a radiator and a display signal.

Theory 4: A helmet or battering ram

Popular books and documentaries often say cassowaries use their casques as a helmet to crash through dense undergrowth head first, or to push aside leaf litter while searching for fruit. Some writers have also suggested it is a weapon for fights between rivals.

There is little evidence for these ideas. Field biologists report that cassowaries usually move through the forest with the head held low and the neck tucked, but do not normally ram vegetation with the casque. The casque’s light, spongy core and its tendency to crack and chip make it poorly suited to heavy impacts. Fights between cassowaries involve kicking, not head-butting.

So what is the casque for?

The most likely answer is that the casque has more than one function. The 2019 heat study gives good evidence that it helps shed heat. Its growth with age, its link with dominance and its position among the bird’s brightest colors make a role in display very likely. The sound idea is possible but untested, and the battering-ram idea is now considered unlikely.

Do other birds have casques?

Casques are not unique to cassowaries. Hornbills of Africa and Asia carry casques on top of their bills, and some, like the helmeted hornbill, use them in head-butting contests. The helmeted guinea fowl of Africa has a bony knob on its head.

Some dinosaurs had head crests too. Certain oviraptorosaurs, feathered relatives of birds, had tall crests that look strikingly like a cassowary’s casque, which is one reason cassowaries are often called “living dinosaurs”. In fact, all birds are dinosaurs, as explained in are birds dinosaurs?. For more on the cassowary’s fearsome image, see is the cassowary the most dangerous bird?.