Glacier Moss Mice: Rolling stones CAN gather moss!

Glacier mice in Iceland. Hotaling et al.

In the 1950's an Icelandic researcher, Jon Eythorsson, described the features shown in the photo to the left as "rolling stones [that] CAN gather moss," and dubbed them "glacier mice." These balls of mass are not attached to anything and just rest on the ice...but they move around in a "coordinated herd-like fashion" (Hotaling et al., 2020 as reported here by NPR).
   Hotaling et al., describe these as "soft, wet, squishy pillow(s) of moss" that seem to form out of different moss species nucleated around an impurity such as a rock or a bit of dust (but some moss balls do not have a seed kernel and so it appears that they can form without a seed nucleus).  The impurities, possibly with fine-grained sediment adhering to them, provide a growth substrate for moss spores brought in by wind.  They can grow up to ~10 cm (rarely, up to 18 cm). Other authors have suggested that the size is limited by the tensile strength of moss stems.
     Because moss would die if not exposed to sunlight, the mice must roll around to change the orientation of the surfaces to the sun. The mice sometimes teeter on a pedestal of ice, and the authors speculate that the moss insulates the underlying ice from sunlight until eventually they fall off the pedestal and roll away.
     Hotaling and colleagues tagged 30 moss balls with a loop of wire that had a sequence of colored beads on it, and then tracked them from 2009-2012. When the mice fell off the pedestals, they did not move simply downhill, nor in an obvious wind direction, nor with the dominant direction of solar radiation.  But, they moved about 1"/day like a choreographed formation of "birds or a herd of wildebeests."
    Although glacier scientists have long observed these and "dote" on them, finding them "extremely engaging," they have no explanation other than that "the explanation is somewhere in the physics of the energy and the heat around the surface of the glacier" (Ruth Mottram, Danish Meteorological Institute).

REFERENCE: Hotaling, S., Bartholomaus, T.C., Gilbert, S.L., Rolling stones gather moss: movement and longevity of moss balls on an Alaskan glacier, Polar Biology, https://doi.org/10.1007/s00300-020-02675-6. Published online May 14, 2020.