Animal Senses #8: Surface Vibrations

Surface vibrations. Vibrations that travel along the surface (obviously). Some question how they are different from sound – aren’t both vibrations? Yes, they do have their similarities. But there are also differences, which is why Ed Yong dedicates a chapter to surface vibrations in Immense World, albeit a small one. The first difference is that sound vibrations are in the same direction in which sound travels; whereas surface vibrations are perpendicular to the direction that the vibration travels.  The bigger difference is that, until recently, scientists focussed only on sound but not surface vibrations, so they didn’t know much of how surface vibrations are used by different creatures.

 

Take the spider web. We think of it as a trap. Yes, it is that, but it is also a “surveillance system”, extending the spider’s senses well beyond its body. The spider can sense vibrations in the web. They can differentiate the vibrations caused by rustling wind from a leaf that fell to a struggling prey. They can identify which part of the web the vibrations came from, and even the size of the prey. If a prey stops moving, they can find it by deliberately plucking the string and waiting for the returning vibration.

 

Then there are spiders that deliberately and violently “twang” the web of other spiders. Why? With the owner of the web trying to figure out the source of such a huge vibration, the intruder will charge down the web to kill the (web) spider, its charge masked by the violent vibrations it had initiated beforehand.

 

This is a sense that we haven’t studied much because the most common sources of surface vibrations – drumming, thumping, shaking and quivering of body parts – are assumed to be visual and/or audio signals, ones we understand instinctively, and thus we do not even realize there’s also a surface vibration component to things.

 

We humans barely use surface vibrations as signals. Until recently, says Yong tongue in cheek. When our smartphones started vibrating in our pockets, our Umwelt began to include vibrations as signals of breaking news and notifications!