sea otters commonly exhibit swimming behavior where they swim on their backs. the features which facilitate swimming on the back also facilitate tool use. otters that do not use tools still feed on their backs. this habit of feeding on the surface with the chest up facilitates a flat surface for resting rocks and pounding items together (1). once discovered and consistently performed by one or some otters, the tool use behavior may have spread through a population of nearby otters by horizontal social learning.