What is an otter thinking?
Keeps a favourite stone in a fold of loose skin. Uses that one, and only that one.
Sea otters have a loose pouch of skin under each foreleg, and they keep a chosen stone in it. They use it to crack shellfish on their chest, and individuals keep the same stone across many dives rather than picking up a new one each time.
It is genuine tool use, and rare. Very few non-primates carry a tool around rather than finding one when needed, which is what makes the pocket the interesting part rather than the rock.
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