An Immense World: How Animal Senses Reveal the Hidden Realms Around Us

Some might categorize surface vibrations as a form of sound, but airborne sound waves move in the direction of the force, whereas the movement of surface waves is perpendicular to the direction of the force. Throwing a stone in a lake illustrates this point: The stone sinks, but the waves move out across the water, perpendicular to the direction in which the stone fell. Surface waves move across both ground and water. The most important distinction that Yong makes between sound and surface waves is that scientists have ignored surface waves.

Plants easily carry surface waves, and around 200,000 insects communicate through vibrations that create surface waves on plants in “vibrational songs.” The vibrations can be converted to sound that is audible for humans with a vibrometer. Because surface waves travel only along flat planes, they do not lose their energy as quickly as sound waves do. Thus, they travel far, and these vibrational songs are not correlated to body size: Very small insects often sound very large when their songs are listened to with a vibrometer.

Listening to surface waves, for some animals, is as important as making surface waves. Scorpions, for example, have slits in their feet, called slit sensilla, that sense surface vibrations and allow them to find food.