In fact, they ask: do lemming populations boom and bust because the availability of food changes? Or does the rate at which other things eat them fluctuate? To separate the two possibilities, the researchers built a mathematical model describing how lemmings and the moss upon which they 'prey' interact, and compared several long-term records of the populations of lemmings and voles.

In northern Scandinavia, the populations of both Norwegian lemmings (Lemmus lemmus) and voles (mostly Microtus agrestis) fluctuate dramatically but approximately regularly. Vole population graphs have rounded peaks, whereas those for lemmings have a saw-toothed pattern. This shows that the vole cycle is predator-driven, Turchin and colleagues explain in Nature1. Lemming numbers, on the other hand, crash when they exhaust their plant food.

To understand the reasoning behind this, one needs to think through the population cycles. When a prey species is rare, food and space are plentiful, and its population increases rapidly. Meanwhile, the population of predators that feeds on it grows only once the prey population is big enough to fuel expansion. When this happens, numbers shoot up.

Predators could never eat enough to counteract the frenetic pace of rodent reproduction. Instead, the ceiling for prey is set by something called 'density dependence': the tendency for crowded populations to stop growing. For example, says Turchin, female voles mature more slowly in crowded conditions.

Eventually, there are so many predators that the prey population crashes. But by then, the number of prey has spent several years at or near its peak -- leading to a rounded curve.

Faced with this dearth of prey, the predators must emigrate or starve -- either way, their numbers see a sharp decline. For lemmings, says Turchin, "as soon as peak density is reached, food begins to run out, and the lemming population begins to crash" -- leading to a population trajectory with jagged peaks.

But why is the ecology of these superficially similar species so different? Diet, say Turchin's group. Voles eat grass, which recovers from damage quickly, so they don't run out of food. Their population cycles are driven by the predatory attentions of the least weasel (Mustela nivalis), which in turn shows a 'predator'-type cycle.

Although lots of lemmings fall prey to owls, arctic foxes and suchlike, this isn't what makes their numbers oscillate. Rather, this happens, say the researchers, because lemmings 'prey' upon moss. Moss regrows slowly, and so a horde of hungry lemmings can empty the larder in short order. And then hit the road in search of food.

But -- contrary to folklore -- neither hunger nor any population-control mechanism drives them over precipices. Allegedly the cameraman on the legendary 1958 nature documentary White Wilderness was able to film this phenomenon because he had a friend standing atop the cliff lobbing lemmings over the edge.