Elephants know the difference between good vibrations and bad, according to new research into the big animals' low, rumbling alarm calls. They pay attention to seismic waves made by elephants they know and ignore those of strangers.

Behavioral ecologist Caitlin O'Connell-Rodwell of Stanford University in Palo Alto, California, discovered in 2004 that African elephants communicate with each other from kilometers away through ground vibrations. Although they make the calls with their trunks, the sounds also travel several kilometers along the surface of the ground, about as far as airborne sounds. O'Connell-Rodwell witnessed groups of Namibian elephants stopping in their tracks, leaning forward onto their toes, and pressing their trunks to the ground. The animals often adopted this listening posture before the arrival of another group of elephants. O'Connell-Rodwell recorded various elephant calls and found that wild elephants responded to ground vibrations alone. Researchers aren't sure how elephants detect the waves, but they have vibration-sensitive cells in their feet and trunks.

In the new study, O'Connell-Rodwell asked whether the elephants can tell who is making the alarm calls. So the team recorded alarm calls made by elephants encountering lions in Kenya and Namibia. Then they converted the sounds into seismic waves and played them back to Namibian elephants visiting a water hole. The elephants responded to the Namibian vibrations by freezing, huddling, and leaving the area sooner. The elephants appeared to detect the Kenyan calls--they sometimes paused and looked more alert, for instance--but did not react dramatically. The Namibian elephants also ignored control recordings of synthesized sounds that had similar frequency and duration. The research is slated to appear in the August Journal of the Acoustical Society of America.

The scientists don't know why elephants respond differently to the alarm calls, but O'Connell-Rodwell suspects it is not due to dialect differences. The calls from the two countries are similar in frequency and duration. More likely, she says, is that the elephants trust the calls from animals they know but not those of strangers.

Behavioral ecologist Jan Randall of San Francisco State University in California, who studies kangaroo rats that use foot drumming vibrations to communicate, agrees that the elephants may be gauging the trustworthiness of the calls and heeding only the ones from reliable sources. That might help them avoid expending unnecessary energy responding to bogus calls. But alarm calls are hard to capture in the wild, and the researchers need to test more samples, Randall says. "It's an exciting result and it's really suggestive, but it needs some of the follow-up work to really pin it down."