Pity the cassowary. The flightless bird tries to lead a secretive life in the dense forests of New Guinea, but it's hardly inconspicuous. Thought to have diverged from other avian lines in the Late Cretaceous Period, the colorful animal can measure five feet tall from its helmetlike casque to its daggerlike foot spurs, and it weighs up to 125 pounds.
Now the camera-shy creature may draw even more attention, thanks to a newly discovered talent. Biologist Andrew Mack of the Wildlife Conservation Society was studying a captive specimen last year when "it made several gulping motions, possibly inflating internal air sacs. It then opened its bill wide, raised its body upward, inhaling deeply, then threw its head down between its legs and began booming."
That's right, booming. Mack's recordings, reported in the ornithological journal The Auk, show that the birds' calls approach the lower limit of human hearing, around 20 hertz. These low-frequency cries might be ideally adapted for communication over long distances, without being attenuated by surrounding vegetation.
It's been known since the 1930s, with the discovery of bat echolocation, that animals can produce pitches too high for human hearing. But the only common instances of animal infrasound—inaudibly low frequencies—were the 20 hertz songs of fin and blue whales, and these proved fiendishly hard to investigate. "It's not easy to study infrasonic communication," says Katy Payne, a research associate in the Bioacoustics Research Program in Cornell's Laboratory of Ornithology. "You've got to identify and analyze sounds you cannot hear and determine their effects on other individuals at unknown distances."
It was during a zoo visit in 1984 that Payne discovered low-frequency communication among land animals. Standing near the elephant cages, she felt a throbbing in the air—she compares it to "being in a car with the windows rolled up wrong"—and recalled her experience singing in a choir near a large pipe organ, where she could barely hear the sound in the lower registers but "the pressure was huge."
She suspected that the elephants were communicating with low-frequency vocalizations, and investigation proved that the animals were indeed rumbling at between 5 and 30 hertz. Subsequent field research revealed that infrasound plays a significant role in coordinating complex elephant societies over great distances. In the evening on the Namibian savanna, with a sound-reflecting temperature inversion overhead, a loud elephant call may fill an area as large as 300 square kilometers.
Payne's infrasound discovery helped solve some longstanding pachyderm mysteries. It explained how females attract distant males during their brief estrus, and it showed how dispersed groups coordinate their movements without converging on the same scarce resources. It has also opened scientists' ears to an unsuspected channel of animal communication.
Biologist William Barklow of Framingham State College has found that hippo calls are partly infrasonic and are transmitted both above and below water. Elizabeth von Muggenthaler of North Carolina's Fauna Communications Research Institute has identified low-frequency vocalizations in okapi and giraffes, and has found intriguing twin vocalizations in Sumatran rhinos and humpback whales. Infrasound also has other uses, von Muggenthaler notes: A tiger's roar contains an 18 hertz component that induces feelings of terror in humans and can paralyze prey for up to 10 seconds.
Payne is now working with the five-year-old Elephant Listening Project, which monitors hidden forest-dwelling elephant populations in central and west Africa. Instead of counting dunghills, researchers have set up an array of microphones that record calls for three months at a time, yielding rich data about the animals' movements and communication.
How do animals make such low-frequency sounds? Elephants and tigers appear to produce their calls vocally, like human speech. Von Muggenthaler believes giraffes may use Helmholtz resonance, causing the air in their long windpipes to vibrate at a low pitch. And she notes that rhinos might use their horns to detect infrasound. "They could essentially feel the sound resonating through their skulls."
This last possibility could shed a new light on the cassowaries and their curious casques, which resemble those found in many dinosaur fossils. If the birds' crests are shown to play a role in communication, then investigators may have a clue as to how ancient casque-bearing dinosaurs interacted.
"I know there are people who think the casque might have served as a resonating chamber in dinosaurs," Mack says. "The fact that it is largely fluid-filled might argue against this. But who knows for sure?" He's working now with physicists and 3-D computer modelers to investigate the crest's response to low-frequency sound. Stay tuned.—Greg Ross