Biologists have put paid to a folk myth while resolving the mystery of one of the world's most puzzling boom-and-bust cycles - the four year population explosion of the lemming.

Every few years, the numbers of the vole, clinging to a precarious existence in the Arctic tundra, expands 1,000-fold. The gregarious little rodents feed off berries, moss and lichen; gestate for 20 days; and produce up to nine offspring at a time.

Their name is from the Norwegian for "destroyer". When the food runs out, they migrate. They do not, however, commit mass suicide by leaping off cliffs, a myth compounded 45 years ago by Walt Disney's White Wilderness film - which showed lemmings apparently going to their doom.

But the puzzle remained: why do lemming numbers explode, and then crash?

The answer, according to German and Finnish researchers in Science today, has more to do with their enemies than any self-destructive urges.

The collared lemming in the high arctic is supper for four predators - the stoat, the arctic fox, the snowy owl, and a seabird called the long-tailed skua.

Olivier Gilg of the University of Finland in Helsinki and colleagues spent 15 summers in a valley in Greenland, counting lemmings and the creatures that lunch upon them.

They found stoat numbers tended to increase in cycles a step or two behind lemmings. This was expected, because the stoat dines exclusively on lemmings, shares the same valley all year round, and reproduces more slowly.

Fox, owl and skua numbers, on the other hand, increased in step with the lemmings because they pounced only when nature was lavish with the voles. In a peak year, the three kept the lemmings in check while stoats caught up. The four together then practically wiped out the surplus.

"This question of lemming cycles has been open for almost a century," Mr Gilg said. "Different schools have argued about this. It has been a very, very hot issue."