Moving moss

In glacial landscapes across the world, small balls of moss form, oval in shape, and tumble simultaneously as the glaciers melt, as if moving in a herd.

Known as “glacier mice”, these moss balls are understudied, but recently researchers have taken notice of them and their weird, herd-like behaviour^[1]. This has led to all sorts of questions and a couple of published papers on the phenomenon, such as...

How do they form?

Researchers have theorised that the moss balls form through “nucleation” at rough points on the glacier surface – just as crystals start growing on impurities in their containers. First, one crystal or drifting moss fragment attaches, and then others attach onto that, gradually coming together to make the shape of the final structure. It’s not clear how this always leads to oval balls, and none of them are round, but it does generally make sense as a theory.

How long do they “live”?

After “tagging” 30 moss balls with coloured bracelets like animal trackers, researchers came back and refound their migrating balls for 6 consecutive years^[1]. By the end, 18 were still findable and intact. They theorise the reason the moss balls live so long is partly because they move and partly because they are habitats. The movement may help keep the balls together because it means both sides get the benefit of sunlight. The communities of bugs, beasts, and microorganisms that inhabit them may also contribute to their structural integrity^[2].

How do they move?

In winter, the glacier mice are in hibernation, but in summer, they get on the go. This happens at the glacier melts, creating unmelted peaks under the shade of the moss balls from which they eventually tumble and roll. The glacier melts fast, and the balls can get moving again as quickly as every other day.

How fast do they travel?

By mapping where they found the travelling balls every summer, researchers were able to calculate that the balls in one site moved ~2.5 cm/day during the summer. Not terribly fast, but definitely noticeable!

Why do they move together?

Glacier mouse. dration via Flickr.

This is where much of the mystery comes into it – because scientists don’t really know. They’ve suggested that the balls share subtleties in shape we haven’t picked up, like a slightly fatter nose than tail, that causes them to come together, or that the conditions pulled them together – but if so, they can't say how.

Weirdly, the moss balls don’t bundle on top of each other: they align, keep their distance, and move like a coordinated herd, even changing direction in unison. They don’t move with the wind and they don’t even travel exclusively downhill! They don’t follow the ice flow either. It’s bewildering. Almost like they have a life of their own.

The last research was carried out on the mice in 2020, but perhaps the researchers will be going back soon to address these questions? Watch this space – or you might miss it moving!

References
why don't all references have links?

[1] S. Hotaling et al. Rolling stones gather moss: movement and longevity of moss balls on an Alaskan glacier. Polar Biology. Vol. 43, June 6, 2020, p. 735. doi: 10.1007/s00300-020-02675-6.
[2] S.J. Coulson and N.G. Midgley. The role of glacier mice in the invertebrate colonisation of glacial surfaces: the moss balls of the Falljökull, Iceland. Polar Biology. Vol. 35, p. 1651, November 2012. doi: 10.1007/s00300-012-1205-4.