When they're excited, scared or happy, elephants make the trumpeting sounds you're probably familiar with. They vocalize in other, more complicated ways, too — low-pitched rumbles outside our range of hearing. At less than 20 hertz, these infrasonic calls are often used to alert each other to danger. Low-frequency sounds travel much farther than high ones, perfect for long-distance communication.
Scientists have known for some time that elephants use a specific call to warn about bees. Adult elephants are vulnerable to stings around their eyes and in their tender trunks, and baby elephants' thin skins make them vulnerable all over, so the alarm provides an important signal. Researchers have noted that elephant anti-bee rumbles are usually accompanied by vigorous head shaking to fling off any insects that happened to land:
The human alarm call was discovered when researchers played back recorded voices of tribal people who share the same region as several groups of wild elephants. Elephants within earshot of the recording ran away, became more wary and made a specific rumble (click here for audio). The rumble was recorded and played back to a different group of elephants, who also ran off and behaved more warily. However, none shook their head as with the bee alarm signal. Elephants appear to use their rumbles to mean different things with different degrees of urgency. That isn't just noise; that's language.
Elephants can even tell the difference between human ethnic groups and classify them by their level of threat. Apparently, they distinguish less dangerous people from more dangerous by the color of their clothes and body odor.
So how do you know if an elephant is rumbling if you can't hear it? When elephants are listening, they hold their ears out. When they're rumbling, as they are in the clip below, their ears tend to flap.