Animal Senses #7: Touch and Remote Touch

Instinctively, we think touch is based on contact. Not entirely true, explains Ed Yong in Immense World – there’s a more important aspect than contact:

“These incredible feats are possible through movement. If you rest a fingertip upon a surface, you can get only a limited idea of its features. But as soon you’re allowed to move, everything changes. Hardness becomes apparent with a press. Textures resolve at a stroke.”

As you run your fingers over a surface, they hit the tiny peaks and troughs and set off vibrations in the mechanoreceptors at their tips.

 

At the tip of its snout, the star-nosed mole has many pairs of hairless appendages. It uses them to touch things around itself. Alive or dead? Food or ignore? It does these movements at an insane speed. In the time it takes to blink our eyes, the star-nosed mole can probe, identify and swallow its food.

“Light may be the fastest thing in the universe, but light sensors have their limits, and the star-nosed mole’s sense of touch blows past them all.”

 

Many insects use touch via their antennae to find their way around. The whiskers on many mammals serve a similar purpose. There is a variety of wasp that uses touch in a grisly way. It will sting a cockroach twice, first to paralyze its legs, and the second time to the roach’s brain to make it lose the desire to move altogether. The wasp will then drag the roach by its antennae and lay her eggs on it. When the larvae hatch, they have a “docile source of fresh meat”. Macabre aspect aside, the second sting needs to be on the brain, and to identify the brain, the wasp uses its sense of touch (which is in the same organ that injects the venom).

 

The knot bill, an aquatic bird, pokes the sand in the water with its beak. A pressure wave is set off.

“If there’s a hard object in the way – say, a clam or a rock – the water must flow around it, which distorts the pattern of pressure.”

The knot bill can detect these patterns of changes, and use them to identify where objects lie, without having touched them. Scientists call this “remote touch”.

 

As a fish swims, it leaves behind a “hydrodynamic wake” – a trail of swirling water that continues to whirl for several minutes. A seal can detect this and use it to pursue the fish, even if the fish is no longer visible.

“To us, touch is rooted in the present… but to (other species), touch extends into the recent past.”

 

Back to fish. When a predator charges at them, fish scatter. How do they not collide with each other? How do they flow around their attacker? How do they pull of this “miraculous feat of coordination”? Sure, vision helps. But the bigger role is of the wake – the fish can detect if its wake bumped into something, an unexpected “increase in water resistance”. It uses this information to identify the location of other fish and the predator, and avoids bumping into them.