A mole locates food by the difference between its two nostrils

The common mole's nostrils sample slightly different air. Block one, or cross the scent with tubes, and the animal can no longer turn toward the smell. It was using the difference.

Kenneth Catania set eastern American moles in an arena with a food odour and filmed the path. With both nostrils open they turned toward the source. With one nostril blocked they were poor at it. Then he crossed the inputs, a small tube from the left nostril's air to the right and the reverse, so the stronger smell arrived on the wrong side. The moles turned the wrong way. The turn followed the nostrils, not the room. That is a stereo system: two sensors, a comparison, a direction.

The separation between a mole's nostrils is tiny, and the world underground does not offer a clean breeze. The gradient still exists, at the scale of a snout, and the animal is built to read it quickly, sniff after sniff, while it digs. People can be trained to notice a difference between the nostrils. They do not steer by it. The mole does, in the dark, as the ordinary way of finding a meal it will never see.