Lemmings Aren't Suicidal

Oct. 31, 2003 — One of the oddest phenomena in the natural world — the sudden mass death of lemmings — has been resolved, according to a trio of European biologists.

Unexplained population crashes of this rodent of the high northern latitudes have bred the myth that, whenever they become too numerous for the available food, the creatures band together and fling themselves off cliffs in a crazed suicidal rush.

But, said the researchers, the truth is even more complex.

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Lemming populations, they said, surge spectacularly and fall just as quickly, thanks to the combined feasting of four predators: the stoat, arctic fox, snowy owl and a seabird called the long-tailed skua.

The researchers trawled over data they collected over the past 15 years from a 75-square-kilometer (30-square-mile) valley in eastern Greenland where the hamster-like lemming is the plat du jour for these four predators.

They found that the population of lemmings and their cousins, the vole, could explode by 100 or even 1,000 times their original size.

That, in turn, boosts the predator numbers, which become so numerous and gorge so much on the lemmings that the rodent numbers plummet dramatically. The next phase is that the lack of lemming drives down the predator numbers.

Predator-prey cycles such as this are familiar to biologists, but what is interesting in this case is that, with the lemmings, the pattern is almost like clockwork.

It is a four-year "boom and bust" cycle whose key is the stoat, a specialist predator whose only source of food is the lemming. The three other predators are "generalists" — they like to tuck into lemmings but also have alternative nutrition.

The statistical clue to understanding the cycle is stoat numbers, which take a delay of about a year before falling in response to a lemming crash.

The "generalist" predators are still affected by a fall in lemming numbers but their population decline takes longer to kick in because they can immediately find other food when the crash takes effect.

The study was published in Thursday's issue of the weekly U.S. journal Science by Olivier Gilg and Ilkka Hanski of the University of Finland, and Benoit Sittler of the University of Freiburg in Germany.

"This question of lemming cycles has been open for almost a century," said Gilg, who is also a researcher at the Center for Biology and Management of Populations in Montferrier, France. "Different schools have argued about this. It has been a very, very hot issue."