The sea otter is a member of the Mustelidae that is fully aquatic. Sea otters are known for their ability to use stones as anvils or hammers to facilitate access to hard-to-reach prey items.

It was found that orphaned juvenile sea otters in a research facility spontaneously developed stone tool use, without observing any models. Relatedly, stone handling behaviours were observed in 10 out of the 13 currently known subspecies of otters, including sea otters. Together, both studies suggest a genetic component to stone behaviours in otters – as this would explain both its occurrence without models and its cross-occurrence across species that do not observe each other.

Otters that use stones to open prey do not use stones every time they need to manipulate their prey. Crabs, for example, can be ripped apart by the forelimbs and then eaten. Otters will store a stone in the pouch of skin under the arm to eat prey with both forelimbs, such as crabs, and then retrieve the stone at a later instance. Otters have been observed to use a clam shell as a digging tool and as a pry tool as well. This seems to imply, "an anticipation of use that goes beyond the immediate situation". If a stone appears to be particularly good for opening one food item, it will be kept for several others.

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