The otters’ playful juggling of rocks has been reported many times. Whether it’s in the wild, at a zoo, or an animal sanctuary, the little creatures have often been seen playing with rocks — throwing them into the air and skillfully catching them, or rolling the rocks around their necks.

Now, researchers believe they know why otters do this: they’re excited about food.

Researchers from the University of Exeter did what we all secretly crave to do: they spent a lot of time watching otters playing. Of course, it wasn’t for the fun of it — they were looking to see why captive otters tend to play with stones, commonly referred to as “rock juggling”.

Otters are social creatures, learning by copying and mimicking each other. It makes sense that such behavior would have been passed from generation to generation, but why did it emerge in the first place? Researchers suspected that it might help otters practice their dexterity, enabling them to improve their foraging skills (such as cracking mussels and clams), but this doesn’t seem to really be the case.

When researchers gave otters a series of puzzles to solve (which were rewarded with food, of course), otters that spent more time playing with rocks did not seem to fare any better.

Instead, it seems that otters tend to juggle with rocks more when it’s close to their feeding time, which led researchers to believe that they’re just excited about food — and let’s face it, who isn’t?

However, while hunger is a factor in this behavior, it’s still not clear just what drove its emergence in the first place. Mari-Lisa Allison, lead author of the study said:

“Zoo visitors are often enthralled by the otters’ playfulness. Surprisingly, very few studies have investigated why otters are so keen to juggle stones. Our study provides a glimpse into this fascinating behaviour. While hunger is likely to drive rock juggling in the moment, the ultimate function of the behaviour is still a mystery.”

Overall, rock juggling frequency appeared to increase with age, which may be a way to keep their brains active and to maintain the skills they need to survive in the wild. It could also be that older otters don’t have any parental responsibilities, which leaves them with more free time to be playful. The frequency of rock pay also depends on context, sex, and species, researchers note.

Juggle rate did not predict an otter’s ability to solve food extraction puzzles, suggesting that rock juggling does not enhance food extraction ability. Senior author Dr. Neeltje Boogert added:

“While it did not appear that frequent jugglers solved food puzzles faster, more research is needed to exclude the ‘practice makes perfect’ hypothesis to explain rock juggling in otters.”

Journal Reference: The drivers and functions of rock juggling in otters, Royal Society Open Science, royalsocietypublishing.org/doi/10.1098/rsos.200141