Zoo otters are often seen tossing stones in the air while standing or lying on their backs.
Now scientists believe they have found out why – the animals juggle stones when they are hungry because they could be excited about food.
The rock juggling might help younger otters to learn the skills they will need to get food from prey such as mussels and clams, according to scientists from the University of Exeter.
For older otters it could just be a way to keep their brains active.
Mari-Lisa Allison, of the University of Exeter and lead author of the study, said that while hunger may be a key driver, the ultimate reason for the behaviour remains a mystery.
She told the PA news agency: Our strongest finding is that otters juggled more frequently before being fed, indicating that the immediate driver of the behaviour is hunger.
More research is needed to investigate the ultimate function of the behaviour.
The study, published in Royal Society Open Science, looked at the behaviour of 44 Asian small-clawed otters (Aonyx cinereus) and six smooth-coated otters (Lutrogale perspicillata) in captive environments.
The Asian small-clawed otters forage on crabs and shellfish and the smooth-coated otters hunt for fish.
The researchers used food puzzles to examine the foraging behaviour – tennis balls with holes to allow the otters to reach inside for food, medicine bottles with the lid on loosely, and two stacked Duplo bricks with meat inside.
Ms Allison said these were designed to imitate foraging behaviour – snapping apart the bricks to get the food is similar to breaking mussels and clams open for food, for example.
The animals juggled more when hungry, and both juvenile and senior otters juggled more than adults with offspring.
Ms Allison said: We hypothesised that juveniles may rock juggle to develop those food extracting skills.
When they reach maturity and begin reproducing, their time and energy is devoted to raising their offspring. As such, they may not have the time or energy to play.
She added: In senior otters, they no longer have those parental responsibilities so may have more time to rock juggle.
In a similar way to how humans stave off Alzheimer’s by reading and doing puzzles, we hypothesised that the senior otters may be performing the behaviour to engage their brains to prevent cognitive decline.
(c) Sky News 2020: Otters juggle stones because they are excited about food – study