For centuries, people have puzzled over lemmings, the northern rodents whose populations surge and crash so quickly and so regularly they inspired an enduring myth: that lemmings commit mass suicide when their numbers grow large, eagerly pitching themselves off cliffs to die in a foamy sea.
Scientists debunked that notion decades ago. But they have never been certain what causes the rapid boom-and-bust cycles that gave rise to it. Now, in a study of collared lemmings in Greenland, being published today in the journal Science, European researchers report the real reason has nothing to do with self-annihilation and everything to do with hungry predators.
After 15 years of research, the scientists discovered the combined actions of four predator species _ snowy owls, seabirds called long-tailed skuas, arctic foxes and weasel-like creatures known as stoats _ create the four-year cycles during which lemming populations explode and nearly disappear.
Scientists say such cycles have been one of the most enduring, and hotly debated, mysteries in ecology.
"There have been several dozen hypotheses and sometimes everybody was almost killing each other, they were sticking so close to their hypothesis," said Dr. Olivier Gilg, an ecologist at the University of Helsinki in Finland who is an author of the paper.
Dr. Peter Hudson, a population ecologist at Penn State University who was not involved with the work but who wrote a commentary for Science, called it a textbook case, noting that population cycles are found in birds, insects and larger mammals, like lynx.
"These animals show this lovely clockwork change in numbers," he said, "and yet we haven't been able to nail it down. This paper reveals the mechanism. That's why this study is particularly important."
The population cycles of the brown 6-inch rodents have captured human imagination for hundreds of years. In Scandinavia, ancient sagas describe lemming outbreaks, and as early as the 1500s there were writings attempting to explain why lemmings would periodically overrun regions, some suggesting the animals rained down from the sky.
Recently, scientists have tested more plausible explanations, including climate change and the idea that the quality of plants eaten by lemmings might vary cyclically or that high densities might stress lemmings, decreasing their ability to reproduce and causing populations to crash. Even sunspots had been proposed as a possible cause.
In the new study, researchers took advantage of Greenland's never-ending daylight in summer. The open tundra environment also allowed the small, skittering rodents to be seen and counted easily.