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Otters ‘Juggle,’ but the Behavior’s Function Remains Mysterious

The animals seem to fidget more when they are hungry

Otters get fidgety when they're hungry. (JEAN-FRANCOIS MONIER/AFP via Getty Images)
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Otters are playful creatures. They tussle, slide and have been filmed playing piano. But researchers wanted to know why a few species are known to "juggle" rocks—swiftly passing one or more stones around between their chest, hands and mouth.

New research suggests the otters’ parlor trick may just pass the time between meals. Otters tend to fiddle with rocks more often when hungry, which made the researchers wonder: Do these wanton displays of dexterity make otters better at certain mealtime tasks, like picking crab meat from a shell? The team decided to test this theory by making the otters solve food puzzles, according to the study published in the journal Royal Society Open Science.

“While hunger is likely to drive rock juggling in the moment, the ultimate function of the behaviour is still a mystery,” says Mari-Lisa Allison, animal behavior researcher at the University of Exeter and lead author of the study, in a statement.

The study focused on two otter species known to engage in this kind of rock play: the Asian small-clawed otter (Aonyx cinerea) and the smooth-coated otter (Lutrogale perspicillata). The diminutive Asian small-clawed otter mainly eats shellfish and is “very playful,” Allison tells Susan Milius of Science News. The larger smooth-coated otter mostly eats fish.

Because the small-clawed otter’s shelled prey requires precise extraction, the researchers predicted the small otters might be more proficient at navigating the experimental food puzzles. While the shellfish specialists tumbled pebbles between their paws more frequently, they weren’t any better than the fish chompers at getting food out of complicated objects, including tennis balls, pill bottles and plastic bricks.

"We wanted to try and replicate foraging behaviors as best as we could," Allison tells Ashley Strickland of CNN. “All [the] puzzles required some form of manipulation to gain entry.”

The team’s analysis found frequent jugglers didn’t solve food puzzles any faster, but more research is needed to rule out longer term “practice makes perfect”-style skill building as an explanation for rock juggling, says Neeltje Boogert, behavioral ecologist and senior author on the paper, in a statement.

Hunger emerged as the main predictor of when the otters might start batting rocks about, but this explanation fails to tease out the behavior’s function.

“There, in that moment, they are rock juggling because they are hungry, they are anticipating food and they are getting excited,” Allison tells Nicola Davis of the Guardian.

The researchers also observed that juvenile and old otters juggle more than adults.

"As these [older] otters were no longer reproductively active, they didn't have offspring they needed to care for," Allison tells CNN. "As such, we thought that they might have more time and energy to be able to rock juggle when compared to mature otters whose time and energy was devoted to caring for young."

The apparent disconnect between play and beneficial life skills highlighted by the study isn’t surprising, Gordon Burghardt, an ethologist at the University of Tennessee who was not involved in the study, tells Science News. He says our current understanding of the evolution of play doesn’t require it to straightforwardly enhance the animal’s survival.

Instead, Burghardt tells Science News, play is most likely to evolve in species where parents provide their young with plenty of food and a safe, low-stress environment. Otters are a prime example of what he terms the “surplus resource” hypothesis. Burghardt tells Science News that otters may simply juggle rocks “for pleasure, out of boredom, or both.”

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