The mole experiment that left two explanations open

The beetle ran around the pail, then tucked itself beside a clod. The moment it stopped, a red snout poked up directly beneath it. The beetle vanished.

The naturalist Richard Kearton had put a mole in a zinc pail full of soil. It burrowed out of sight. At dusk he heard its claws against the metal and dropped the beetle onto the surface. What stayed with him was the precision. He tried again and again, and the snout kept appearing exactly where the prey had stopped.^[1]

How did the mole find it? Kearton leaned toward sound. He had heard the mole scratching, but the pail gave him no way to tell what the mole heard. The beetle might also have left a scent. The setup never tested sound and scent separately.

Every time Kearton saw the snout come up, he learned the same thing: the mole could pinpoint its prey from below. To learn how it did it, he would need a test that separated the two cues.