Why do otters juggle?

Why do otters juggle? It sounds like the opening to a joke, but many otters are frequently seen shifting pebbles back and forth between their hands, an activity referred to by scientists as "juggling." While animal behaviorists have thought that the juggling is a way for the animals to practice pulling meat from crustaceans and mollusks, a task that requires fine motor skills and coordination. However, researcher Mari-Lisa Allison and colleagues from the University of Exeter found that otters who frequently juggled didn't exhibit any better food-picking skills. Turns out they're probably just doing it because it's fun. From Science News:

The possible disconnect between play and real-life skills doesn't startle Gordon Burghardt of the University of Tennessee, Knoxville. Over decades, he has analyzed play behavior, refining definitions and even reporting play in such unexpected animals as a turtle romping with a basketball in a zoo. The thinking about the evolution of play has by now expanded beyond simple notions of the benefits of instinctive practice, he says[...]

Otters that juggle may be doing so for pleasure, out of boredom, or both," he says.

"The drivers and functions of rock juggling in otters" (Royal Society Open Science)