A team of researchers from the University of Exeter has studied potential drivers of ‘rock juggling’ in two species of otters in zoo environments.
Asian small-clawed otters (Aonyx cinereus). Image credit: Neil McIntosh / CC BY 2.0.
Although elusive in the wild, otters are noted to be very playful and inquisitive animals based on observations in captivity.
The animals are often seen lying on their backs and batting the stones into the air, catching them and rolling them around their chests and necks.
Researchers had suggested the behavior might help otters practice the foraging skills they need to extract foods from complex prey such as mussels and clams.
However, while this new study 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.
“Zoo visitors are often enthralled by the otters’ playfulness,” said lead author Mari-Lisa Allison, a researcher in the Centre for Ecology and Conservation at the University of Exeter.
“Surprisingly, very few studies have investigated why otters are so keen to juggle stones.”
“Our study provides a glimpse into this fascinating behavior. While hunger is likely to drive rock juggling in the moment, the ultimate function of the behavior is still a mystery.”
In the study, Allison and colleagues studied two otter species commonly found in zoos and wildlife centers and reported to show rock juggling behavior: Asian small-clawed otters (Aonyx cinereus) and smooth-coated otters (Lutrogale perspicillata).
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 authors 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 function of the behavior may change over an otter’s lifetime — aiding development in juveniles while potentially keeping the brain active in seniors,” Allison and co-authors said.
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 behavior in anticipation of feeding time.
“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,” said senior author Dr. Neeltje Boogert, also from the Centre for Ecology and Conservation at the University of Exeter.
The study was published in the journal Royal Society Open Science.
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Mari-Lisa Allison et al. 2020. The drivers and functions of rock juggling in otters. R. Soc. open sci 7: 200141; doi: 10.1098/rsos.200141
