When threatened, an elephant will stomp its feet menacingly. And Stanford biologist Caitlin O'Connell-Rodwell says the tooth-rattling toe work isn't just for show; it is also a call for assistance. Her insight came while she was studying the penetrating low-frequency vocal rumblings elephants use to keep in touch with the rest of the herd. Whenever a listening elephant perked up its ears, it would also lift one foot off the ground. "It seemed peculiar, and I realized that their feet must be sensitive to something," she says.
O'Connell-Rodwell rigged up a system of sensors and microphones and learned that elephant calls travel better through earth than through air. "When an elephant rumbles a certain call, an exact replica of that pressure wave is created in the ground," she explains. Sound waves in air dissipate in three dimensions, like a widening sphere, but ground waves spread out in two dimensions, like ripples, so they remain detectable much farther away. For the best long-distance communication, elephants therefore send and receive vibrations with their feet. O'Connell-Rodwell predicts elephants should be able to sense underground signals at distances of up to 20 miles. "That is more than sufficient to pick up warning calls and stomping signals from other herd members or even neighboring herds," she says.