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Does Rain Forest Bird "Boom" Like a Dinosaur?

James Owen
for National Geographic News
November 4, 2003
 
From deep in the rain forest comes a low, booming sound. It rumbles on for several minutes; enough time for any listeners to wonder whether they've stumbled into some real-life Jurassic Park. Far-fetched? Not according to scientists who've been investigating the noise. They say it could hark back to the time of the dinosaurs.


Biologists from the New York-based Wildlife Conservation Society recorded these rumbles in the jungle during a recent study of the cassowary. The ostrich-like bird—which grows up to five feet (1.5 meters) tall, weighs upwards of 125 pounds (57 kilograms), and can reach running speeds up to 30 miles per hour (50 kilometers per hour)—is the world's largest forest bird.

Analysis of the bird's vocalizations revealed that it has the world's lowest known birdcall. Furthermore, the research team says the cassowary could hold the key to understanding how dinosaurs communicated.

The study took place in the remote rain forests of Papua New Guinea, where all three cassowary species live. The researchers' findings, now published in the scientific journal The Auk, reveal that the southern cassowary (Casuarius casuarius) and dwarf cassowary (Casuarius bennetti) produce sounds at the limit of human hearing. (The third species is the northern cassowary, Casuarius unappendiculatus.)

The southern cassowary produced frequencies down to 32 hertz, while its smaller cousin, the dwarf cassowary, went even lower, dropping to a scarcely audible 23 hertz. (Humans can hear sounds with a frequency from 20 to 20,000 hertz.)

"When close to the bird, these calls can be heard or felt as an unsettling sensation," said Andrew Mack, a Wildlife Conservation Society conservation biologist who led the team. "One student thought there was an Earth tremor, when in fact it was a cassowary."

Mack said that while humans can hear much of the bird's call, portions of it fall below the bottom range of human hearing. "It's hard to describe, but somehow you sort of feel it," he said. "Your internal organs vibrate a bit."

Mack said animals must be of a certain size to make such sounds. "There are structural problems that arise for small organisms," he added. "It's much easier in large animals like cassowaries, whales and elephants."

Scientists believe cassowaries emit these low frequency vocalizations in order to communicate in dense rain forests. The distinctive call carries great distances as its long wavelength can penetrate vegetation. Yet the cassowary's closest relative, the emu, has a very different call.

Solitary Birds

Mack said emus prefer habitats that are more open and that the birds—unlike the widely dispersed, solitary cassowaries—are more social and travel in groups, avoiding the need for long-distance communication.

How cassowaries produce their deep "boom" is unclear, though Mack and his team speculate that cassowary communication is linked to the tall casques, or horn-like crests, that rise from the bird's head.

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.

Resonating Crests

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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