Web edition: August 2, 2012
Print edition: September 8, 2012; Vol.182 #5 (p. 11)
Elephants don’t purr so much as sing when they unleash low-frequency rumblings at friends and foes kilometers away.
Too low for humans to hear, the infrasonic components of elephants’ calls have at times been attributed to a process similar to a cat’s contented thrum. But new measurements made by blowing air through the voice box, or larynx, of a deceased zoo elephant suggest that the mechanism is actually a (much bigger) analog to a person speaking or singing.
Cascades of fast, active muscle contraction give cats their purr. Biologists have speculated that some similar muscle twitching creates the deep throbbing of elephant infrasound. But elephants can get their rumbles going just by exhaling air through their vocal tracts, says Christian Herbst of the University of Vienna. With the new demonstration of air power, Herbst says, “there’s no need to go into the purring hypothesis.” He and his colleagues make their case for superlow elephant song in the Aug. 3 Science.
The part of an elephant’s call audible to humans travels only about 800 meters through air, but the infrasonic tones can reach up to 10 kilometers under ideal conditions. The noise can vibrate via the ground too, theoretically going much farther.
“We really do not know that much about the specific characteristics of elephant sound production,” says Peter Wrege of the Cornell Lab of Ornithology, who directs a project monitoring hard-to-see forest elephants in Central Africa by listening for infrasound.
Too true, Herbst says. He’s recorded his own larynx in action by threading equipment into his mouth, but he’s not going to try that with any big mammal.
“The elephant would just close his mouth and say, ‘Thank you for the snack,’” Herbst says. “If you’re lucky.”
He and his colleagues made arrangements with a zoo to extract the larynx of an elephant within just a few hours of its death. An elephant larynx has strips of tissue lying across the airway called vocal folds, just as humans and other mammals do. As air rushes up out of the lungs, the folds separate and then touch again, creating puffs that set the whole voice mechanism into action. To visualize their vibrations, Herbst says, “think of a flag in the wind.”
Elephants have vocal folds about eight times the size of a human’s, and greater size lowers the sound of the vibrations. If people had elephantine vocal folds, humankind might be speaking infrasound too.
The disembodied larynx in the lab could be re-creating sounds similar to an elephant’s infrasonic blast in the lowest-pitched vibration, called the fundamental frequency. But the test didn’t capture all of the complexities of a real call from a living animal with an intact head.
“Elephants have these really interesting anomalies,” says Caitlin O’Connell-Rodwell of Stanford School of Medicine’s otolaryngology department. A large cavity opens in the front of an elephant skull, with some deposits of dense fat a bit like the acoustic structures in marine mammals. So vocal folds flapping in the wind might be only the beginning of the story, she says.
C.T. Herbst. How low can you go? Physical production mechanism of elephant infrasonic vocalizations. Science, Vol. 337, August 3, 2012, p. 595. doi: 10.1126/science.1219712
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