A Rolling Moss Gathers Stones

    I spend my days keeping safe and wandering about the internet. I read the tables of contents of half a dozen or more scientific journals and check out the papers that have catchy titles or seem so off-beat that I wonder what they are about. I'm always drawn to things that are a mystery - unexplained phenomena. This week the award goes to a paper that appeared in the journal Polar Biology: Rolling stones gather moss: movement and longevity of moss balls on an Alaskan glacier.

    Moss balls are ovoid masses of moss, up to 8+ inches in longest dimension. Each moss ball has a center that is either living or dead moss, some gravel, a small rock, or a quantity of sand. They are known by other names: cushions, polsters, or the especially evocative "glacier mice."

Alaskan glacier with "glacier mice" scattered across its surface.(crop of Fig.1C, Hotaling et al., 2020;
Creative Commons 4.0

    Each ball is made of the same kind of moss, but many different moss species can form balls. They are an uncommon phenomenon, confined to temperate and, especially, polar regions of the world. They range from 2 to 8 inches in the longest diameter. They are not common, but have been observed on glaciers in Alaska, the Andes and Iceland. Glacier mouse is especially apt name because they move about the surface of the glacier.

    How the moss balls move is unknown. One explanation suggests that the moss ball protected the surface of the ice beneath it from wind erosion. As wind blows over the glacier it abrades the ice and, over time, the moss ball would be sitting on a pedestal of ice and would eventually slip off. This would suggest that movement of the moss balls would be in the downwind direction,

Moss ball with identification bracelet
(crop of Fig.1D, Hotaling et al., 2020;
Creative Commons 4.0

    Hotaling et al. studied the movement of moss balls on an Alaska glacier over a two month period of time. They marked 30 balls with a "bracelet" of colored beads, each ball receiving a unique combination of colors. The initial location of each ball was carefully determined and the color code made it possible to identify the balls and determine the distance and direction of their movement.  The researchers returned 7 times over two months, gathering data on the direction and distance of moss ball movement.

    The result was mystifying.  The moss balls did not move downhill (the glacier sloped east), nor did they move in the downwind direction (which was southeast). Neither did they move randomly. Instead, by the end of the study they headed, as a group, to the southwest.

    It's nice to know that there are still mysteries in the world.

References:

Hotaling, S., Bartholomaus, T.C., and Gilbert, S.L. (2020). Rolling stones gather moss: movement and longevity of moss balls on an Alaskan glacier. Polar Biol 43, 735-744.

Shacklette, H.T. (1966). Unattached Moss Polsters on Amchitka Island, Alaska. The Bryologist 69, 346-352.