Sea otters keep a favourite rock tucked in a loose pouch of skin under each forearm, carry it between dives, and use it as a personal anvil to crack open shellfish, making them one of the few animals known to keep and reuse a single tool

A cute sea otter floating peacefully in the waters of Moss Landing, California.

Fewer than one percent of mammal species are known to use tools at all, and the number that retain a specific tool across multiple uses drops to a handful. Chimpanzees make the list. So do a few cr...