At a glance, round animals might not seem that out of place on land. After all, small mammals like rabbits and pikas often look a bit spherical when their fur is puffed out and they're sitting. But Chris Law, an evolutionary biologist at the University of Washington in Seattle, said this shape is mostly an illusion.
A sphere has the lowest surface-area-to-volume ratio of any shape, which is ideal for heat conservation. Law said the ability to curl or scrunch into a sphere can be essential to the survival of small animals, which lose heat more quickly than larger animals do.
Some small and furry mammals like the cotton-tail rabbit, Ezo flying squirrel and northern pika make look spherical when they're scrunched up and sitting down, but in reality they're not as physically round as they appear to be. (Image credit: Chris Rogers/Satoru S/feathercollector via Getty Images)Sign up for our newsletter(Image credit: Marilyn Perkins / Future)
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"As a round animal, you're probably going to lose mobility, and you're going to be picked off by predators pretty easily," Law told Live Science.
If we're counting animals that are only round sometimes, then pill bugs (also known as rolly pollies) might take first place for the roundest land animal. But if not, rain frogs, and beetles like ladybugs, are the top contenders for the roundest land animal. But even these animals aren't perfectly round; ladybugs have flat bottoms to make it easier to move around, and rain frogs are less spherical when not puffed up.
Karly E. Cohen, a biomechanist at Friday Harbor Labs in Washington, studies lumpsuckers, a type of fish she describes as "quite round." These bulbous fish have suction cups made of enamel on their stomachs, which they use to latch on to surfaces at the bottom of the ocean. Lumpsuckers have armor, also made of enamel, covering their round shape. According to Cohen, lumpsuckers' roundness plays a huge part in keeping them attached to the seafloor or other surface. The curve of their bodies, coupled with the tooth armor, modifies the drag around the animal, creating a force that pushes them down rather than shooting them into the water column.
A lumpsucker sits on a rock. These animals use their enamel armor and their round shape to stay anchored to the ocean floor. (Image credit: MWCPhoto via Getty Images)"There's no real good way to eat an apple whole; you have to take a bite," Cohen told Live Science. "And then this thing is armored, so it's very hard to take a bite."
All about symmetry
Echinoderms — which include sea stars, sea urchins and sand dollars — have pentaradial symmetry, which means they have five planes of symmetry arranged around a central point. According to Laurent Formery, a developmental biologist at the Oceanological Observatory of Banyuls-sur-Mer in France who studies echinoderms, scientists aren't entirely sure of the benefits of pentaradial symmetry or why echinoderms are the only modern animals that have it.
"They are kind of like a big crawling eye and brain, so they are receiving information from everywhere," he told Live Science. "They have a completely different way, that is difficult for us to imagine, of interacting with their environment."
An urchin in the genus Histocidaris. Underneath their spines, sea urchins are almost perfectly spherical. (Image credit: Volgi archive via Alamy)Discounting their spikes, certain species of sea urchin are almost perfectly spherical. Like lumpsuckers, sea urchins' spherical shape, coupled with their spines, makes them difficult meals for predators, Formery said. Sponges lack any symmetry, so they can develop all sorts of body shapes. One species, called the death ball sponge, looks like a Sputnik lamp, with many hook-covered spheres attached to its spindly "arms."'
Related mysteriesUnder the sea, urchins in the genus Histocidaris, like Histocidaris purpurata and Histocidaris formosa, are the closest to perfect spheres, making them the top candidate for roundest animal.
"As somebody that's interested in the tools and ways organisms work in their environments," she said, "they're a real treasure trove of evolution."
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