Tummy rumbles? Otters juggle pebbles when hungry, study finds
Asian otters’ playfulness with rocks a mystery but Exeter study links juggling to hunger
Nicola Davis Wed 6 May 2020 THE GUARDIAN
Whether tossing pebbles between their paws or rolling stones on their chest and even into their mouth, otters are experts at rock juggling. Now researchers say the behaviour largely appears to be linked to a rumbling tummy.
Many species of otters are known to toss pebbles around, often while lying on their backs, in what appears to be an example of animals playing with inanimate objects. Mari-Lisa Allison, of the University of Exeter, said: “[I have seen] an otter at the fence putting a pebble through the mesh and then catching it underneath and rolling it round, round the fence”.
But quite why otters rock-juggle has been something of a conundrum. Now Allison and colleagues say their experiments back up the view that otters rock-juggle more when they are hungry.
“This seems to be the immediate driver of the behaviour. There, in that moment, they are rock juggling because they are hungry, they are anticipating food and they are getting excited,” said Allison. But, she noted, further research is needed. “The ultimate function is still quite a mystery,” she said.
Writing in the journal Royal Society Open Science, Allison and colleagues told of examining the behaviour of otters at three wildlife parks and zoos in the UK. The team recorded about 12 hours of observations for each of 48 otters, six of which were smooth-coated otters and 42 of which were Asian small-clawed otters.
The team note the small-clawed otters forage for shellfish and crabs, while the smooth-coated feeds on fish. This difference suggested the latter might juggle rocks more frequently as they relied on dexterous movements to a greater degree. However the team found no such difference, although they said that could be due to the small number of smooth-coated otters observed, adding that the two species did show different juggling actions.
Further analysis revealed that among otters aged 11 years or below, juggling was more frequent the younger the animal, suggesting, the team said, that the movements could help with the development of motor skills.
However, the behaviour was also common among senior otters. Perhaps, the team said, the juggling had a role in preventing cognitive decline, although more work is needed to explore the idea.
The researchers also found otters juggled more frequently if they had not been fed for two or more hours. The team said that that could support previous suggestions that rock juggling was a type of “misdirected foraging” – in other words the hungry otters could be performing the type of dexterous movements they would use when extracting meat from shellfish but on rocks or pebbles instead.
Yet rock juggling does not seem to be a way by which the otters practise or improve their skills. The team presented the otters with minced meat inside medicine bottles, tennis balls and between two plastic bricks, but those experiments gave no sign that otters who juggled more frequently had greater success with solving puzzles.
But the team say the experiment had limitations; many of the older otters were arthritic, and the animals could have been put off by the unfamiliar nature of the task or might not have understood the puzzle.
Prof Hannah Dugdale, of the University of Groningen, in the Netherlands, who was not involved in the work, welcomed the study. “The reason why otters juggle rocks has been difficult to study as there are many different variables that need to be considered at the same time, such as the age of individuals and how hungry they are,” she said, adding that the new study helped tease apart such factors.
But, she added, more work is needed to explore factors involved in rock juggling in wild otters, and its frequency, to fully understand the evolution of such behaviour.
Dr Elizabeth Chadwick, of Cardiff University, who is principal investigator of the Cardiff University Otter Project, also said questions remained. “Otters are absolutely captivating and rock juggling is one of their most intriguing behaviours,” she said. “The study helps explore and describe some of the differences in rock juggling between individuals, but is unable to provide the elusive answer as to why otters juggle.”
Otters juggle stones when hungry, research shows
by University of Exeter
Credit: CC0 Public Domain
Hunger is likely to be the main driver of stone juggling in otters, new research has shown.
Researchers from the University of Exeter have carried out an extensive study into why captive otters tend to play with stones, commonly referred to as "rock juggling".
The dextrous mammals are often seen lying on their backs and batting the stones into the air, catching them and rolling them around their chests and necks.
Experts had suggested the behaviour might help the animals practise the foraging skills they need to extract foods from complex prey such as mussels and clams.
However, while the new research showed that young and older otters tended to juggle more than reproductive adults, these frequent jugglers were no better at solving food puzzles—suggesting the practice does not hone foraging skills.
Instead, the study showed that otters juggle more when hungry, in anticipation of feeding times in captivity—suggesting that it could be excitement for food.
The study is published in Royal Society Open Science on Wednesday, May 6th 2020.
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."
For the new study, the research team analysed rock juggling in Asian small-clawed (Aonyx cinereus) and smooth-coated (Lutrogale perspicillata) otters found in zoo environments.
While the species are closely related, Asian small-clawed otters perform extractive foraging movements to exploit crabs and shellfish, while smooth-coated otters forage on fish.
The team set a series of puzzles for the otters to solve in order to gain access to a reward of food, to see whether the otters that juggled more frequently were faster at solving the puzzles. They also investigated whether sex, age and hunger levels could explain why some otters juggled more than others.
The team found that both juvenile and senior otters juggled more than adults. The authors suggested that the function of the behaviour may change over an otter's lifetime—aiding development in juveniles while potentially keeping the brain active in seniors. Otter parents may have been juggling less as they did not have the time while looking after their pups. However, rock-juggling frequency did not differ between species or sexes.
Crucially, otters juggled more when "hungry", indicating that juggling may be a misdirected behaviour in anticipation of feeding time.
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 "practise makes perfect" hypothesis to explain rock juggling in otters.
Explore further Otters learn by copying each other
Hunger is likely to be the main driver of stone juggling in otters, new research has shown.
Researchers from the University of Exeter have carried out an extensive study into why captive otters tend to play with stones, commonly referred to as "rock juggling".
The dextrous mammals are often seen lying on their backs and batting the stones into the air, catching them and rolling them around their chests and necks.
Experts had suggested the behaviour might help the animals practise the foraging skills they need to extract foods from complex prey such as mussels and clams.
However, while the new research showed that young and older otters tended to juggle more than reproductive adults, these frequent jugglers were no better at solving food puzzles—suggesting the practice does not hone foraging skills.
Instead, the study showed that otters juggle more when hungry, in anticipation of feeding times in captivity—suggesting that it could be excitement for food.
The study is published in Royal Society Open Science on Wednesday, May 6th 2020.
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."
For the new study, the research team analysed rock juggling in Asian small-clawed (Aonyx cinereus) and smooth-coated (Lutrogale perspicillata) otters found in zoo environments.
While the species are closely related, Asian small-clawed otters perform extractive foraging movements to exploit crabs and shellfish, while smooth-coated otters forage on fish.
The team set a series of puzzles for the otters to solve in order to gain access to a reward of food, to see whether the otters that juggled more frequently were faster at solving the puzzles. They also investigated whether sex, age and hunger levels could explain why some otters juggled more than others.
The team found that both juvenile and senior otters juggled more than adults. The authors suggested that the function of the behaviour may change over an otter's lifetime—aiding development in juveniles while potentially keeping the brain active in seniors. Otter parents may have been juggling less as they did not have the time while looking after their pups. However, rock-juggling frequency did not differ between species or sexes.
Crucially, otters juggled more when "hungry", indicating that juggling may be a misdirected behaviour in anticipation of feeding time.
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 "practise makes perfect" hypothesis to explain rock juggling in otters.
Explore further Otters learn by copying each other
More information: The drivers and functions of rock juggling in otters, Royal Society Open Science, royalsocietypublishing.org/doi/10.1098/rsos.200141
Journal information: Royal Society Open Science
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