Could otters who juggle rocks be hungry and excited to eat?
Zoo otters juggle stones when they are hungry because they may be excited about food, scientists believe.
The mammals can often be seen playfully tossing rocks in the air while standing or lying on their backs.
Scientists from the University of Exeter who conducted the study believe this behaviour might help juvenile otters practise the foraging skills they need to extract foods from complex prey such as mussels and clams.
In older otters, experts suggest the act of juggling rocks could be a way to pass the time and keep their brains active, similar to how humans do puzzles to keep their minds engaged.
But Mari-Lisa Allison, of the University of Exeter and lead author of the study, said that while hunger may be a key driver of rock juggling in otters, the ultimate function of the behaviour is still 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 researchers studied 44 Asian small-clawed otters (Aonyx cinereus) and six smooth-coated otters (Lutrogale perspicillata) in captive environments.
While the species are closely related, Asian small-clawed otters forage on crabs and shellfish while smooth-coated otters hunt for fish.
The team used three different types of man-made food puzzles to analyse the animals' foraging behaviour: tennis balls with holes to allow the otters to reach inside for food, plastic medicine bottles with the lid loosely screwed on, and two stacked Duplo bricks with the meat placed inside.
Ms Allison said the puzzles were designed to mimic foraging behaviour, like for example, snapping apart the bricks requires the otters to have skills that would allow them to break into mussels and clams to get their food.
The team found the creatures juggled more when hungry, and that 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."
The study is published in Royal Society Open Science.