I was often puzzled at the sight of a ring of river rocks encircling the central drain at the bottom of the pool. These smooth-surfaced eight- or nine-inch oval river rocks were stewn here and there on the bottom by divers so the dolphins could rub their bodies along them. … A quick investigation revealed that the divers weren’t responsible [for the rings.] How did the dolphins move these rocks?Did the dolphins pick the stones up with their mouths? No, says Reiss. “If they used their mouths, the dolphins would injure their teeth.”
Perhaps a dolphin would push the stone along the floor of the pool with its nose (or, as the “nose” is called in dolphins, the rostrum, there being no “nose” to the dolphin snout)? Reiss dismisses that guess, too. “Pushing with their snouts, the dolphins would injure their rostrums.”
The dolphin researchers had to keep a sharp eye out to catch the dolphins at their task.
It’s a clever and elegant solution.
They saw the dolphin, Stormy, with a stone. “Stormy turned upside dow, placed the top of her head on a rock, sucked it onto her blowhole, then turned right-side up and swam away with it atop her head.”
As to why the rings, the dolphins never told. Art?
source: The Dolphin in the Mirror: exploring dolphin minds and saving dolphin lives by Diana Reiss