The function of these odd-looking features is poorly understood. It was once thought they acted as a kind of crash helmet for bashing through undergrowth or as a weapon to settle dominance disputes. But behavioral observations don't support this. Another suggestion is that the casque is used as a visual signal of identity or sexual maturity.
However, Mack and his colleagues suggest the casque may act as a kind of sound amplifier or could help in picking up incoming calls. They plan to investigate this further as they develop an acoustic monitoring system to track these elusive but vulnerable birds.
A better understanding of cassowary calls could also shed light on the bird's Jurassic ancestors.
"These investigations are exciting because many dinosaur fossils exhibit casques at least superficially similar to those of living cassowaries," said Mack. "No one knows for sure what purpose these served in these dinosaurs, so further study of living cassowaries might provide clues to how dinosaurs communicated."
Cassowaries belong to a primitive group of mainly flightless birds called Palaeognathae. They are thought to have more in common with dinosaurs than most other birds.
Certainly, the cassowary's clawed wings, scaly legs, featherless heads, wrinkled necks, and large size give them a dinosaur-like appearance. They can also be highly aggressive if approached. Over 220 attacks on humans have been recorded.
Then there're those casques. Types of "duck-billed" dinosaur, such as the Corythosaurus and Parasaurolophus, which lived more than 65 million years ago, had similar crests. Many scientists think they used these for sound production.
Darren Naish, a paleobiologist at the University of Portsmouth, England, said: "The notion that these crests might have acted as resonating devices for calls is based on the discovery they work well in this way."
A study in the 1990s suggested the acoustics of the Parasaurolophus crest acted like a musical instrument on top of the dinosaur's head. Even if it had no vocal organs, this three-ton plant-eater may have been able to produce deep, low-frequency sounds using resonating air cavities.
"It's the only reasonable explanation to explain the complex internal passages," Naish said.
The structure of fossilized ear bones also point to dinosaurs being able to hear frequencies much lower than those detectable by humans.
Dinosaur crests may have played a similar role in receiving these low calls. Again, the cassowary's casque could hold the key to understanding how this would have worked.
Mack said: "It could work like a boundary-layer microphone because the casque has a softish keratinous sheath of one density and a fluid-filled center of a different density. So as sound passes through these, they will vibrate differently to the incoming wavelength and the differential in their response could tell the bird about the sound."
Mack and his team are currently collecting fresh anatomical material and developing new models to test this theory.
Meanwhile, that unsettling sensation produced by the cassowary's call could be the closest we'll ever get to the experience of hearing the sound of a dinosaur.
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