Few sights in nature are as awesome as a 6-ton elephant guarding her baby from a hungry predator.
Rather than retreat, the threatened mother is likely to launch a mock charge - a terrifying display of ground stomping, ear flapping and frantic screaming designed to frighten off lions and hyenas.
But elephant researchers have discovered that there is more to a mock charge than meets the eye or the ear, for that matter. It turns out that foot stomping and low-frequency rumbling also generate seismic waves in the ground that can travel nearly 20 miles along the surface of the Earth, according to a new study in the Journal of the Acoustical Society of America (JASA).
More astonishing is the discovery that elephants may be able to sense these vibrations through their feet and interpret them as warning signals of a distant danger.
"Elephants may be able to detect stress from a herd many miles away," says Caitlin O'Connell-Rodwell, an affiliate of the Stanford Center for Conservation Biology and a postdoctoral fellow in the Department of Pediatrics.
"They may be communicating at much farther distances than we thought," adds O'Connell-Rodwell, author of the JASA study.
To test the theory that elephants transmit and receive underground messages, O'Connell-Rodwell and her colleagues previously conducted several novel experiments with pachyderms in Africa, India and at a captive elephant facility in Texas.
"We went to Etosha National Park in Namibia and recorded three acoustic calls commonly made by wild African elephants," she says. "One is a warning call, another is a greeting and the third is the elephant equivalent of 'Let's go!'"
The researchers wanted to find out if elephants would respond to recordings played through the ground, so they installed seismic transmitters at a tourist facility in Zimbabwe where eight trained, young elephants were housed.
The idea was to convert audible "Greetings," "Warning!" and "Let's go!" calls into underground seismic waves that an elephant could feel but not hear directly through the air.
"We used a mix of elephant calls, synthesized low-frequency tones, rock music and silence for comparison," says O'Connell-Rodwell.
"When the 'Warning!' calls were played, one female got so agitated she bent down and bit the ground," she notes. "That's very unusual behavior for an elephant, but it has been observed in the wild under conditions of extreme agitation."
The young female had the same agitated response each time the experiment was repeated. Researchers also played recorded calls to seven captive males.
"The bulls reacted, too, but their response was much more subtle," notes O'Connell-Rodwell.
"We think they're sensing these underground vibrations through their feet," she adds. "Seismic waves could travel from their toenails to the ear via bone conduction, or through somatosensory receptors in the foot similar to ones found in the trunk. We think it may be a combination of both."
Other creatures produce seismic signals -- among them the golden mole, the elephant seal (no relation to terrestrial elephants) and a variety of insects, amphibians, reptiles and fish.
"Many organisms use vibrations in the ground to find mates, locate prey or establish territories," says O'Connell-Rodwell.
The ability of large mammals to communicate long distances also is well established. Fin whales, for example, produce calls that carry hundreds of miles underwater at a frequency of 20 hertz -- a range so low that it's barely audible to the human ear.
In the late 1980s, researchers discovered that elephants also produce strong, low-frequency 20 hertz rumbles that can travel up to six miles through the air under ideal weather conditions. Later studies indicated that elephants use these low-frequency vocalizations to coordinate movements with other far-off herds.
In the early '90s, O'Connell-Rodwell began to suspect there was more to long-distance elephant communication than airborne rumblings alone.
"I started working with elephants in Etosha National Park in 1992," she recalls. "I was observing them at a drinking hole when I noticed this strange set of behaviors. They would lean forward, pick up one leg and freeze for no apparent reason."
She theorized that the elephants were responding to vibrations in the ground from approaching herds.
"When I returned to the University of California at Davis, I teamed up with my Ph.D. adviser, Lynette Hart, and geophysicist Byron Aranson to find out if there really are seismic communications among elephants," she says.
The result was a series of geophysical experiments with captive elephants. The first one in 1997 confirmed O'Connell-Rodwell's suspicion that acoustic rumbles are accompanied by vibrations in the Earth.
"When an elephant transmits airborne low-frequency (20 hertz) vocalizations," she wrote, "a corresponding seismic wave is transmitted in the ground."
To determine how far these seismic waves propagate, O'Connell-Rodwell and her coworkers conducted experiments with two female Asian elephants at a private training facility near Jefferson, Texas.
They placed two microphones outside the elephants' enclosure one about 30 feet away, the other about 130 feet. The researchers also buried a geophone directly below each microphone to measure underground vibrations, so whenever the elephants vocalized or launched into a mock charge, the geophone/microphone pairs made simultaneous acoustic and seismic recordings of the event.
"Our results show that elephant rumbles travel separately through the air and the ground," writes O'Connell-Rodwell in the December issue of JASA.
She points out that mock charges generate airborne and seismic signals with frequencies of about 20 hertz -- ideal for long-distance communication.
"Based on our mathematical models, we estimate that seismic signals produced by elephants can travel between 10 and 20 miles in the ground, while acoustic signals have the potential of traveling only about six miles through the air," O'Connell-Rodwell observes.
"If elephants were capable of detecting the seismic information we measured, the maximum range of their airborne communication would be enhanced considerably," she concludes.
Mock charges and mates
The implications of O'Connell-Rodwell's research are far-reaching.
"Elephants may be communicating at much larger ranges than we realized both within and between herds," she says.
The Texas experiments demonstrated that vibrations produced during low frequency vocalizations and mock charges may have the indirect effect of alerting other elephants of potential predators and other threats.
"Seismic signals may also play a role in elephant reproduction," O'Connell-Rodwell observes. "We know that airborne vocal communication is an important component of mate finding. Perhaps they're also sending out seismic signals to potential mates far away.
"Elephants may be able to sense the environment better than we realize," O'Connell-Rodwell contends, pointing to studies showing that elephants can detect and move toward thunderstorms from great distances.
"When it rains in Angola, elephants 100 miles away in Etosha start to move north in search of water," she says. "It could be that they are sensing underground vibrations generated by thunder."
O'Connell-Rodwell's findings also have implications for the way humans and elephants interact -- particularly in countries like South Africa, where declining habitat has led wildlife managers to shoot elephants from helicopters as a way of thinning overpopulated herds.
"There's anecdotal evidence that culling in the northern end of Kruger National Park causes nervousness among elephants in the southern end many miles away," she says. "Helicopter noise is traumatic, so perhaps these animals are picking up low-frequency signals from the rotating blades or from the distress calls and running of the distant herd that's under attack."
Her research also has implications for how we treat captive elephants in urban zoos.
"If elephants are really that sensitive to seismic noise," she argues, "then more could be done to protect them from loud city noises."
O'Connell-Rodwell hopes to return to Namibia's Etosha National Park this year to record and play back more seismically transmitted vocalizations in the wild. Her goal is determine how far seismic signals actually carry through open grasslands and how wild elephants respond to those underground vibrations.
"If elephants could detect the seismic properties of low frequency vocalizations, movements of other herds and weather patterns," she concludes, "then seismic signals could expand the range of elephants' long-distance communication capabilities, adding a powerful component to their sensory perception."
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