for National Geographic News
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 birdwhich 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.
Mack said emus prefer habitats that are more open and that the birdsunlike the widely dispersed, solitary cassowariesare 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.
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