A lovely, intuitive idea
about why otters juggle rocks — that it helps them practice survival skills — might
not be correct, new tests show.
The term “juggling” is itself
overenthusiastic. Otters don’t keep stones flying around in some tall, aerial
circle. Instead, the animals shuffle rocks back and forth quickly between their
front paws. “It’s very close to the body,” says animal behaviorist Mari-Lisa
Allison, who studied the behavior as a graduate student at the University of
Exeter in England.
Such deft fiddling looks as
if it might make a great example of how animal play could serve as practice for
real-life challenges. In the wild, small-clawed otters need paw dexterity to tweak
shreds of seafood out of crustacean or mollusk shells. And yet, three kinds of tests
found no evidence that juggling builds otters’ food-picking
skills, Allison and her colleagues
report May 6 in Royal Society Open
The question of how play evolved (SN: 2/6/18) has long fascinated
biologists. According to the latest thinking, play behaviors serve no immediate
practical need. Yet even the mild tussles of puppies and kittens take energy
and carry some risk of injury, so it seems that some benefit must counterbalance
Asian small-clawed otters (Aonyx cinerea), the littlest of the 13
otter species, are “very playful,” Allison says. Smooth-coated otters (Lutrogale perspicillata) also juggle
rocks but eat fish that don’t require deft plucking of food tidbits from shells.
Because the otters eat in
different ways, Allison originally predicted that the active paws of the shellfish
eaters in three wildlife collections would surpass the simple fish grabbers in their
skill at pulling bits of meat out of crevices. Regardless of their differences
in feeding styles, however, both kinds of otters proved equally able to winkle
some meat out of challenging objects. Rock play didn’t seem to matter in
learning a life skill.
Allison and colleagues tested
both species by tucking minced meat into three kinds of inconvenient and
unfamiliar containers: plastic medicine bottles with screw-on caps, pierced
green tennis balls and toys resembling clamshells made of two oversized
Lego-like blocks. (Real Lego blocks, she worried, were small enough to become a
choking hazard for the otters.)
Especially frequent rock-twiddling
among the small-clawed otters, however, didn’t give individuals a noticeable
edge in working tidbits out of awkward containers. Instead, the study linked
surges of rock juggling to the approach of feeding time, with more juggling as
lunch drew nigh.
Also, it wasn’t just
youngsters that played with rocks: Elderly otters past reproductive age
likewise did a lot of paw fiddling. So, again, juggling may not be only a way
for youngsters to practice skills for feeding.
The possible disconnect
between play and real-life skills doesn’t startle Gordon Burghardt of the
University of Tennessee, Knoxville. Over decades, he has analyzed play
behavior, refining definitions and even reporting play in such unexpected animals
as a turtle romping with a basketball in a zoo. The thinking about the evolution of play
has by now expanded beyond simple notions of the benefits of instinctive practice,
Play is more likely to evolve,
Burghardt has pointed out, among animals with parental care that give
youngsters enough surplus food and a safe space for goofing around. He calls
this the “surplus resource” hypothesis, and otters are a good example of it (SN: 6/13/14).
Otters that juggle may be doing so “for pleasure, out of boredom, or both,” he
says. Either way, he says, they’re drawing on “part of their evolutionary
M-L. Allison et al. The drivers and functions of rock juggling in otters. Royal Society Open Science. Vol. 7, May 6, 2020, 200141. doi: 10.1098/rsos.200141
Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.
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