Glacier Mice: Moss Balls That Roll Across Ice in Coordinated Herds

Glacier mice are golf-ball-to-fist-sized clumps of moss that form on glacier surfaces in places as scattered as Alaska, Iceland, Greenland, Svalbard, Uganda, and Venezuela. Beyond the moss itself, each ball becomes a tiny mobile habitat, sheltering nematodes, springtails, and tardigrades because it traps heat and moisture that bare ice cannot. In the harsh glacial environment, the moss often abandons normal reproduction and propagates asexually. The balls are surprisingly durable, apparently persisting for six years or more. Icelandic meteorologist Jón Eyþórsson first documented them in 1950, calling them jökla-mýs.

What makes them scientifically notable is movement. The balls travel across the ice at roughly 2.5 cm per day, and accelerometer tracking shows they genuinely rotate and roll rather than slide, which exposes all sides over time and keeps the whole surface alive. More striking is that a cluster tends to move together, like a herd, in a non-random direction that wind and slope alone don’t explain.

The leading hypothesis, from work by Hotaling, Bartholomaus, and Gilbert, is thermal: the dark moss absorbs sunlight and shades the ice on its shaded side, so the ice melts only on the sunlit side. The ball settles into the resulting depression and the cycle repeats, producing steady migration toward the sun—southward in the Northern Hemisphere, northward in the Southern. The coordinated herd behavior, however, remains unexplained.