Reputation: Norwegian lemmings are stupid. Like other lemmings, they show none of the cunning of other rodents. When overcrowding becomes an issue, they will run for the sea, throwing themselves off cliffs for the good of the species. Lemmings can also explode. It's true, honest.
Reality: No, no and no. Norwegian lemmings are not stupid. They are masterful burrowers and, like other rodents, score highly when it comes to reproduction. When the population becomes too dense, lemmings will seek pastures new - but they do not commit suicide and they do not explode.
It's often said that in a city you are never more than 6ft (2m) from a rat. In the north of Scandinavia, in the middle of an Arctic winter, the same might be said of Norwegian lemmings.
It is said they are marching to the sea and committing suicide
Standing on the surface of the empty snow-covered tundra, it would be easy to assume you are alone. Yet every three to four years, when the conditions are right, you could be surrounded by furry mammals, hundreds of little lemmings scurrying around in a network of branching tunnels beneath your feet.
When the population is at its lowest, lemmings will be so thin on the ground that there might be just one in 100,000 sq m (1 million sq ft). However, in good times the same area could be home to as many as 3000. This dramatic cycling of the population helps account for an enduring myth about lemmings: that they emerge from these burrows and run, en masse, to their death.
"It is said they are marching to the sea and committing suicide," says Nils Christian Stenseth of the University of Olso in Norway, and co-author of The Biology of Lemmings. Does this fable annoy him? "Yes, indeed."
Accounts of lemming migrations go back hundreds of years. In 1823, for instance, one explorer wrote of seeing "such inconceivable numbers" in his Scandinavian travels "that the country is literally covered with them".
An army of lemmings advanced with extraordinary purpose, "never suffering itself to be diverted from its course by any opposing obstacles," not even when confronted by rivers, or even the branches of narrow fjords. "They are good at swimming," says Stenseth. "They can easily go across small bodies of water, across small lakes," he says.
It is a nice film, but there are some bits and pieces that are wrong with it
Given such sudden and apparently reckless behaviour, it is perhaps inevitable that local people in bygone centuries came to see the lemming as a crazed creature, and a swarm as "the forerunner of war and disaster". But we have Walt Disney to thank for really embedding this stereotype in the public consciousness.
On the back of the animated classic Bambi, Disney undertook a series of ground-breaking, feature-length nature documentaries known as The True-Life Adventures. In one of these, White Wilderness, he dramatised the lemming mass suicide.
Stenseth is generous about the movie. "It is a nice film actually," he says. "But there are some bits and pieces that are wrong with it. That [the lemming segment] is one of them."
For a start, White Wilderness – filmed in Canada rather than Scandinavia – depicts the wrong species. Although all lemmings experience population highs and lows, the accounts of mass movements were all based on observations of Norwegian lemmings, not the brown lemmings that Disney used. He paid Eskimos "$1 a live lemming," says Stenseth.
They didn't march to the sea. They were tipped into it from the truck
But that's just the start. In an infamous sequence, the lemmings reach the edge of a precipitous cliff, and the voiceover tells us that "this is the last chance to turn back, yet over they go, casting themselves bodily out into space."
It certainly looks like suicide. "Only they didn't march to the sea," says Stenseth. "They were tipped into it from the truck."
Once you know the sequence has been faked, it makes for rather awkward viewing. Several of the brown lemmings pause at the edge. One or two look like they are trying to turn back. They don't want to be there at all. They don't want to jump. It looks less like suicide and more like murder.
So if lemmings do not commit suicide, what is going on? What are the causes and the consequences of these wild fluctuations in lemming numbers? Based on data collected between 1970 and 1997, Stenseth and his colleagues were able to demonstrate in 2008 that what lemmings really need to thrive is the right kind of snow.
"If the snow is soft and dry then a space under the snow builds up within which the lemmings can survive very well during the winter and reproduce very well," says Stenseth. If there are a couple of consecutive winters like this, vast numbers of lemmings can emerge in the spring, as if from nowhere.
That's when the trouble starts. With too many hungry lemmings about, the vegetation quickly gets overgrazed and the animals are forced to seek pastures new. It is in these circumstances, as they move from higher to lower ground, that they can occasionally tumble down a slope. "By way of gravitation they tend to move downwards," says Stenseth.
About to go pop
At its peak, the lemmings also become noticeably more aggressive. "A person walking across the meadow would cause the lemmings, several metres away, to give themselves away by unexpectedly shrieking and jumping about," noted one group of researchers. "Even the farm tractor was greeted in this way, leaving a trail of infuriated lemmings behind."
No one has seen a lemming explode
These displays of aggression may have fuelled another age-old myth about lemmings: that they get so furious they explode. But this fanciful notion probably has a simple explanation.
In the months after a reproductive boom, lemming predators will have a field day, slaying but not eating their victims. Once ravens have pecked their way through these killing fields, the eviscerated lemmings do look like they might have burst with anger.
What's more, as Stenseth points out, "no one has seen a lemming explode."
Tweetable truths about Norwegian lemmings
If Norwegian lemmings are to thrive in an Arctic winter, they need "the right kind of snow" #TTAA http://bbc.in/1xYG6qn @WayOfThePanda
Walt Disney Productions paid $1 per live lemming for the suicide sequence in White Wilderness #TTAA http://bbc.in/1xYG6qn @WayOfThePanda
The fable that Norwegian lemmings explode may be explained by ravens pecking at intestines #TTAA http://bbc.in/1xYG6qn @WayOfThePanda