Elephants aren't bashful about speaking their minds. Anyone who has ever watched a National Geographic television special knows all about those long trumpeting blasts.
Big animals, big sounds -- sounds meant mainly for other elephants' big ears, or so wildlife specialists long assumed.
But a Stanford University scientist has discovered that elephants actually have two distinct ways of communicating: by ordinary soundwaves rippling through the air, and by vibrations transmitted through the ground to exquisitely sensitive elephant toes.
The seismic waves are set in motion by the same "low-frequency vocalizations" that famously rumble across African savannas, said Caitlin O'Connell-Rodwell. The ground sounds travel a greater distance than airborne calls, and may help keep herd members in touch with one another across a dozen or more miles.
Through these ground vibrations, O'Connell-Rodwell suggested, elephants are able to raise long-distance alarms, offer advice, advertise love yearnings, or just swap greetings: Lions aprowl! Succulent forage over here! Mate wanted!
"They are talking through the ground," O'Connell-Rodwell said. "It's not just elephant-to-elephant noise. It's a richer system of communication than we'd thought. They can discriminate very subtle vibrations through their feet."
In O'Connell-Rodwell's latest study, scheduled to be published next month, the elephant specialist reports these seismic signals are so sophisticated that elephants can distinguish whether the messaging comes from trusted fellow herd members or strangers -- a sort of "caller ID of the wild," according to O'Connell-Rodwell. The next step in her research is to figure out how the elephants make these distinctions.
"With elephants, listening is not just what is picked up by their ears," she said by phone from Namibia, the southern African nation where for 15 summers she has studied migrating elephants at Etosha National Park. She and her colleagues are also studying elephant communication at California's Oakland Zoo.
In close quarters, those floppy ears work just fine. To reach elephants miles away, the low-frequency vibrations channel through soil, and clay represents the more reliable long-distance service.
The "listening" elephant catches the vibrations with its toes, behind which lie pads of acoustically sensitive fat. Similar tissue is in the heels of the elephant's feet and in its trunk, which augment the toe sensors. The vibrations speed along bones to the elephant's middle ear. They are processed in the auditory cortex area of the brain, just like regular sound.
The Stanford work in Namibia marks the first time scientists have studied seismic communication in large land animals. Several small mammals are known to be acutely sensitive to ground vibrations, including the kangaroo rat, which recognizes its siblings through their signature foot drummings, according to O'Connell-Rodwell.
Eventually, according to audiology researchers, O'Connell-Rodwell's work could yield a human pay off by contributing to techniques for helping the deaf "hear" through vibrations.
"Understanding a communication mode in nonhuman species has implications for developing rehabilitation strategies in humans with sensory defects," said Gerald Popelka, chief of audiology at Stanford. A patient who cannot be helped by conventional means, such as surgery or hearing aids, "may be directed toward developing communication through the sense of touch," using vibrations much as elephants do, he said.
Before O'Connell-Rodwell started her work in Namibia, researchers had already established that elephants produce powerful, low-frequency calls that can travel through the air to herd members up to 6 miles away.
Now O'Connell-Rodwell's research suggests elephants may be able to at least double that communication distance with toe talk.
"We haven't been able to measure how far the vibrations can be understood, but we think the seismic messages are travelling farther than the airborne waves," O'Connell-Rodwell said, estimating that 12 or 13 miles might be a realistic range, but that the seismic waves -- unlike air sound, which quickly dissipates -- theoretically "have no outer limit." Part of this summer's research will involve trying to determine how far elephants can actually "hear" through the ground -- it could be a much larger distance, she said.
Meanwhile, elephants not only can decipher the meaning of the low-frequency transmissions, O'Connell-Rodwell said, they can tell exactly who is sending them.
In experiments, O'Connell-Rodwell recorded seismic "danger signals"' -- specifically, warnings about lions -- one trumpeted by elephants from another herd in Namibia and nearly identical calls from elephants in Kenya, on the opposite side of the continent.
Using electronic equipment, her team then transmitted the rumblings into the ground near Etosha's Mushara water hole.
The Etosha elephants reacted instantly to the warning sent by the fellow Namibian herd, tightening into defensive groups, putting pressure on their toes to "fine-tune" the rumbles, and, finally, shuffling nervously from the water hole, according to O'Connell-Rodwell.
The warning cries from the Kenyan elephants, by contrast, elicited almost no response -- the elephant equivalent of a shrug of the shoulders.
Apparently, elephants don't heed advice from strangers. That may be smart or foolish, depending on the circumstances. But the experiment suggests that elephants know exactly who they are listening to with their tootsies.
"They will react to an individual they consider worth paying attention to," said O'Connell-Rodwell. "When the warnings come from elephants they know, they take the message seriously. But they don't respond to callers whose identities are a mystery."
Another possibility is that elephants in Namibia and Kenya "speak" in mutually unintelligible dialects, like Spanish and Portuguese, but O'Connell-Rodwell suspects not. The tones of the warning cries are too similar in duration and frequency modulation.
"It seems more a matter of trust," O'Connell-Rodwell said, offering her interpretation of the data. "A strange elephant isn't a reliable source of information, even about lions."
As a control, O'Connell-Rodwell and her team, using technology devised by Stanford mechanical engineer Sunil Puria, also sent a series of meaningless sounds into the ground. Again, no response from the Etosha elephants.
Sensitivity to vibrations may also explain the extraordinary ability of elephants to suddenly start heading toward regions of rain, hundreds of miles, in search of green fodder. It may be they are detecting the faint vibration from distant thunder and understand it signals a good feed.
O'Connell-Rodwell, a researcher for the department of otolaryngology at Stanford's School of Medicine, will provide scientific details of her most recent experiments in the August issue of the Journal of the Acoustical Society of America. In March, she published a well-reviewed book, "The Elephant's Secret Sense: The Hidden Life of the Wild Herds of Africa," explaining her ideas of elephant communication to general readers and offering rousing, often poetic accounts of her adventures in the bush.
O'Connell-Rodwell started out as a bug expert, trained as an entomologist. In a tiny, soundproof chamber she spent days on end recording the love serenades of Hawaiian planthoppers, an insect that communicates through vibrations carried through its limbs, not sound.
O'Connell-Rodwell switched to bigger beasts during a 1992 trip to Africa, where she snared a job advising Namibian farmers how to protect their crops against elephants without harming the creatures. She became intrigued by how they communicate.
"I didn't see it then, but I see now that I was embarking on my life's journey," said O'Connell-Rodwell, who with her husband, Tim Rodwell, a physician and epidemiologist, directs a nonprofit organization, Utopia Scientific (www.utopiascientific.org), which raises funding for elephant conservation and science.
"We think what we're learning could be used to help babies born with hearing impairments, but very sensitive to vibrations for the first 11 months," she said. "For now, though, we've got so very much more to learn about how elephants communicate. They are smart, subtle creatures, and not always easy to understand."