Herds of Moss Balls Mysteriously Roam the Arctic Together

In parts of Alaska and Iceland, glacier mice roam wild.

While glacier mice look small and fluffy, they aren’t rodents or even animals—they’re lumps of moss about the size of a flattened softball. Their name comes from a 1951 report in the Journal of Glaciology, when an Icelandic researcher referred to them as jökla-mýs, glacier mice. Now, new research takes a close look at moss ball locomotion.

The study, published last month in the journal Polar Biology, shows that glacier mice can live for years and that herds of them move together in sync. But the researchers aren’t sure yet why that is, as NPR’s Nell Greenfieldboyce reports.

"Most people who would look at them would immediately wonder, 'Well, I wonder if they roll around out here in some way,'" says University of Idaho wildlife biologist Sophie Gilbert to NPR. "Tumbleweeds come to mind, which are obviously totally different, but also round and roll around."

Gilbert and her husband Tim Bartholomaus, a glaciologist at the University of Idaho, first stumbled on a gathering of glacier mice in 2006 when hiking on Root Glacier in Alaska, Candice Wang reports for Popular Science. Then in the summers between 2009 and 2012, the two experts tracked the movement of 30 glacier mice, which were identifiable by unique beaded bands. Then, Gilbert and Bartholomaus met Washington State University glaciologist Scott Hotaling in 2018.

“We just needed to push this project across the finish line but we didn’t have the time and energy to do that ourselves,” Bartholomaus tells Popular Science.

Together the team analyzed the data of the moss balls’ movement and found that they roll about an inch each day. The moss balls seem to insulate the ice below them, so as the glacier surface melts, each ball is sitting on a small pedestal. Eventually, it tumbles off.

"The whole colony of moss balls, this whole grouping, moves at about the same speeds and in the same directions," Bartholomaus tells NPR. "Those speeds and directions can change over the course of weeks."

He explains that herd of 30 moss mice that they observed first moved slowly southward before accelerating west, and then losing speed. Previous research using accelerometers had shown that the balls roll, and that the balls are green all over suggests that all sides get sunlight at some point. The new data show that the moss balls don’t move randomly—but the researchers couldn’t yet deduce what’s driving them.

The glacier mice didn’t follow any pattern that the researchers checked. The moss wasn’t rolling down a slope, getting pushed by the wind, or following the sun. Since the NPR story was published on May 22, readers have started sending their own theories to the research team, who are collecting them in a Google Doc, per Popular Science.

“It’s been pretty charming. So many people are keying in on this mystery.” says Bartholomaus to Popular Science. “Some plants like sunflowers turn their faces toward the sun over the course of the day. Folks are curious if moss balls might do that, and lean towards the sun until they tumble.”

Bartholomaus thinks that the glacier mice are too heavy and slow-growing for that theory to pan out. But he and Gilbert expect that time lapse footage of glacier mice herds taken alongside weather data is the next step in their research. They’re also interested in the possibility that volcanic ash might have something to do with moss ball formation.

For now, the glacier mice are still full of mystery, and a charming sight in a stark landscape.

"They're not attached to anything and they're just resting there on ice," Bartholomaus tells NPR. "They're bright green in a world of white."

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