Do otters actually keep rocks?

Yes, sea otters do keep rocks. They often have a favorite stone that they use as a tool to crack open shellfish like clams, mussels, and crabs.

Because they need these tools to feed effectively, they will tuck their preferred rock into a specialized loose fold of skin under their forearm—essentially a built-in pocket. They will hold onto the same rock for days or even weeks, carrying it with them whenever they dive for food.