For centuries, people have puzzled over lemmings, the northern rodents whose populations surge and crash so quickly and so regularly that they inspired an enduring myth: that lemmings commit mass suicide when their numbers grow too large, eagerly pitching themselves off cliffs to their death 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, a team of European researchers report that the reason has nothing to...