California ? Elephants use vibrations to make long distance calls through the ground, and now scientists say the big animals can tell who’s calling and may even screen their calls.

The low-frequency rumbles elephants make, some below the range of human hearing, travel through the ground as seismic waves that other elephants can feel, and understand. In fact, they know if the caller is a neighbor or a stranger.

“It turns out that they can discriminate very well in the ground just as they can in the air, and that was a surprise,” said Stanford behavioral ecologist Caitlin O’Connell-Rodwell. She led the study, which will appear in the Journal of the Acoustical Society of America in August.

O’Connell-Rodwell first discovered the ground communication three years ago after seeing elephants in Africa suddenly stand still, lean their weight forward onto their toes and rest their trunks on the ground without spreading their ears to listen. Often, other elephants would arrive soon afterward.