Part of the crocodilian reptile order, alligators have some 4,000 of tiny, raised black spots, or domes, on their heads, particularly along their jaws, inside their mouths, and between their teeth. Crocodiles have a similar setup, plus a liberal sprinkling over the rest of their bodies, bringing their total to about 9,000.

Scientists have known about these bumps—called integumentary sensory organs, or ISOs—for more than a century. But for a long time their purpose was a mystery.

Perhaps, some biologists suggested, the domes have a waterproofing purpose. Or maybe they detect faint electrical fields given off by prey—or salt, to alert crocs to unsafe drinking water.

In 2002 an alligator study seemed to crack the mystery. Croc dots, it revealed, could detect