FEW have glimpsed them in the wild and you won’t see any in captivity. Yet the elusive glacier mouse – small, green and fuzzy – suddenly found itself an A-list celebrity earlier this year when reports of its antics became the antidote to a blizzard of bad news.
You may recall the tales of ice-dancing mice that travel in troupes and move with a synchrony worthy of the corps de ballet. If so, you will know that the mice in question aren’t actually mice at all. Purists might call them unattached moss polsters, supraglacial globular moss cushions or just plain moss balls. But when the Icelandic glaciologist JÓn EythÓrsson first brought them to the world’s attention in 1951, he dubbed them ö-ý (glacier mice) and it stuck. “They genuinely look cute, like a small furry creature – at least from a distance,” says Scott Hotaling, a glacier biologist at Washington State University.
Hotaling is one of the scientists who served up the latest instalment in the long-running saga of the glacier mice, which remain a riddle wrapped in a mystery. Those lucky enough to have encountered a colony in one of their remote icy haunts confess that they find them puzzling in many ways, not least their curious movements. Yet thanks to some painstaking sub-zero surveillance, these mossy blobs are slowly giving up their secrets.
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Only in recent years have we begun to get the measure of glacier mice. That’s partly because they are rare, found only on select glaciers in Alaska, Iceland, Chile and on the Norwegian archipelago of Svalbard, and partly because glacier biologists are rarer still. But where there’s one mouse, there are usually many, and as glaciers became a hot topic, the scientists who explored them couldn’t help but notice when furry green balls dotted an otherwise white landscape. “Your first impression when you see them is how out of place they look – something so soft in an environment that is so harsh,” says Hotaling.
Their presence in locations often considered too cold and barren to support much more than microbes prompted many questions. How do they form, and why only on some glaciers? How do these living plants survive on ice? And, of course, what’s with those artfully choreographed migrations?
EythÓrsson had noticed that glacier mice usually contain a small stone. More recent investigations suggest they exist only where there are patches of stony debris on the ice, which helps explain their rarity. They seem to form when windblown moss spores from various local species settle on these rocky fragments: mosses are a major component of high-latitude habitats and are quick to colonise newly ice-free areas. However, for mosses to cloak a stone entirely, every surface must be exposed to the sun – and, if it is to stay covered, no moss can remain in contact with the ice for too long or it will die. This means glacier mice must keep turning over. EythÓrsson assumed that they roll downslope, helped by the flow of summer meltwater on the glacier.
Nothing so simple. In 2005, glaciologist Phil Porter at the University of Hertfordshire, UK, and his colleagues in southern Iceland. Most “mature” mice were ovoid, domed on top and flatter beneath – although many looked as if they had flipped over and were lying belly up. Porter noticed that upright balls were perched on pedestals of ice with tops that sloped in the same direction as the glacier’s surface. All the evidence suggested that the moss balls shield the ice beneath them from the summer sun and, as the surrounding ice melts, they are left atop a column of still solid ice – until they slide or topple off their perch back onto the surface of the ice and the whole process starts again.
As moss balls progress downslope, they trap windborne dust and soak up meltwater, encouraging an ever-thicker growth of moss. Mature glacier mice usually measure around 10 centimetres from nose to tail, but the occasional giant can be twice as long. “For mice to form, there must be debris with the right size stones, a source of moss spores, certain temperature characteristics – although we don’t know exactly what those are – and a slope so the balls will roll,” says Nicholas Midgley at Nottingham Trent University, UK.
In 2010, Midgley travelled to Iceland to study glacier mice movements more closely. He measure their speed and orientation, logging data every 30 seconds. Those sitting on the glacier surface don’t move, he found, but the ones on pedestals build up to a roll with a slow “creep”, rotating a few degrees before they plunge back onto the ice. Typically, the mice toppled every day or two and rotated between 30 and 60 degrees in the process. “Glacier mice rotate often and evenly enough to prevent any part staying in contact with the ice too long,” says Midgley. “So now we know how they maintain their covering of moss.”
Meanwhile, in the US, Tim Bartholomaus and Sophie Gilbert, both at the University of Idaho, had also caught the glacier mouse bug. In the summer of 2009 and the following three summers, they trekked to the Root glacier in Alaska’s Wrangell mountains to see how fast the mice travel and where their manoeuvres take them. The pair adopted the “capture-mark-recapture” technique used for tracking small mammals, “capturing” 30 mature glacier mice and marking them with wire bracelets threaded with a unique combination of coloured beads. That first summer, they returned eight times to “recapture” the tagged mice and plot their travels, calculating speed and direction from the locations they recorded.
“They travel in a herd-like fashion, performing a slow-motion ballet”
The mice travelled 2.5 centimetres a day on average. One positively sprinted, with a top speed of 8 centimetres a day, but generally they all moved at about the same pace. The big surprise, though, was that they but in a herd-like fashion, performing the strange slow-motion ballet that propelled them into the headlines earlier this year.
Bartholomaus and Gilbert monitored the mice for 54 days. For the first nine days, the whole population travelled south, moving at a sedate 2 centimetres a day. Then, they swerved slightly westwards and kept up a cracking 4 centimetres a day for a week before veering another 45 degrees west and slowing to 3 centimetres a day for the next five days. Finally, they turned another 10 to 15 degrees west and returned to their original pace for a further five days. For the final 28 days they continued on this track, progressively slowing as summer came to an end. The changing speed was linked, at least in part, to the rate at which the glacier was melting, but how to explain the herd-like migration? With Hotaling now on the team, the researchers considered three possibilities: the slope of the glacier surface, wind direction and local patterns of solar radiation. “We ruled out the obvious answers. None explained the patterns of movement, which left us shrugging our shoulders,” says Hotaling. “It’s most likely a complex mix of all those factors.”
Although they failed to crack that puzzle, the team did make an important discovery: glacier mice live a long time. Each summer, Bartholomaus and Gilbert hunted down their marked moss balls. Over the years, they lost a few – two fell into a crevasse and four shed their tags, escaping further scrutiny. On average, 86 per cent of mice survived each year, which translates to a lifespan of more than six years. This matters because there is more to glacier mice than stone and moss: they soak up water like a sponge and accumulate soil and organic debris, providing something very rare on a glacier – a habitat for animals.
Cosy haven
What we know about life inside glacier mice is thanks to Steve Coulson at the Swedish University of Agricultural Sciences in Uppsala. In 2008, he received a parcel of mice from Midgley and set about examining their contents. Each one harboured small soil invertebrates: there were two species of springtails along with large numbers of nematode worms and tardigrades. Coulson believes he might have found mites, rotifers and perhaps even spiders if his extraction methods had gathered in the dead as well as the living. “The mice spent a bit of time in the post, so there might have been other things that were dead on arrival,” he says.
For Hotaling, these teeming oases are yet another blow to the idea that glaciers are barren (see “The ice pack“, far left). Glacier mice provide warmth, food and water, creating a unique ecosystem supporting self-sustaining communities. “They are little islands of habitat that move around on the ice,” he says. “For the animals inside, it’s like being in a camper van, equipped with everything you need and buffered from the elements outside.”
The ice pack
Meet the sub-zero menagerie that calls glaciers home
Patagonian dragon
It may be the largest animal living on Patagonia’s glaciers, but at 2.5 centimetres long, the stonefly Andiperla willinki makes for a rather miniature dragon – and is flightless to boot. Its larvae develop in meltwater pools, while adults roam over the ice seeking food and a mate.
Glacier flea
Desoria saltans – which isn’t a flea but a springtail – hops about alpine glaciers, often in huge numbers. It is a tiny creature, just 1.5 to 2.5 millimetres long. And although it prefers a balmier 0°C, the glacier flea can survive temperatures as low as -15°C thanks to antifreeze in its blood.
Ice worm
A relative of earthworms, this is the only annelid adapted to life in ice. North American ice worms (Mesenchytraeus solifugus) are thin and up to 3 centimetres long, with dark blue, brown or black pigmentation that absorbs heat and protects them from high levels of UV light. At night, they migrate upwards through the ice crystals to feed on surface algae, retreating when it gets too warm. Hugely successful, they can reach densities of 100 per square metre. A second species lives in Tibet.
Glacial midge
The habitat of the flightless Himalayan glacial midge (Diamesa kohshima) is the coldest of any insect. Discovered on the Yala glacier in Nepal in 1984, the midge spends its entire life on ice and remains active at temperatures as low as -16°C. New Zealand’s Franz Josef and Fox glaciers are home to midges of another species, Zelandochlus latipalpis, confusingly known locally as ice worms.
Glacier finch
Though not strictly an ice-dweller, the South American white-winged diuca finch (Idiopsar speculifer) is the only bird known to nest and raise its young on glaciers. The finch’s glacier nests were discovered on Peru’s Quelccaya ice cap in 2008 at an altitude of 5700 metres.
