题干
Only humans speak using words. But all kinds in the animal world can communicate in one way or another. Maybe you have heard about the way bees dance around to send messages to each other and the way dogs bark in different ways to give warnings, to be friendly or to be playful. But did you know about infrasonic communication used by elephants? This is how it works:
Humans hear low sounds like thunder rumbling in the sky. But we don't hear sounds lower than this. However, animals such as elephants and hippos can hear much lower sounds than humans can. And what's more, they can make those low sounds as well, and they use them to communicate with each other. This is known as infrasound.
The amazing thing about infrasound is that it travels over several kilometers. Higher-level sounds like the one people can hear, don't travel well through walls, leaves, trees and so on, which is why we can't hear sounds from more than 100 meters away. But infrasound is much stronger and things like grass and trees have no influence on it. Therefore, it can travel much further. Elephants can hear infrasonic calls from four kilometers away!
There have been reports of people watching groups of elephants feeding or resting and then the elephants suddenly all stopped what they were doing for no reason at all. They obviously heard a warning call from a long way away, but the people didn't hear a sound. In places like a zoo or wildlife park where you can get nearer to animals, it is a bit easier to sense when infrasonic sounds are made. When you stand near mother elephants with their babies in a zoo, you may notice a slight rumbling in the air every few minutes—not loud or strong, but clearly noticed. This is infrasonic communication—the mother elephants “talking” to their babies.
