Few mammals have inspired more fieldwork by ecologists than lemmings and voles. Who has not heard of the tale of lemmings jumping off cliffs or of hoards of voles attacking crops of wheat or barley? The base of ecologists’ interest in these small animals is that they contradict our assumed balance of nature idea, drilled into us from an early age. Lemmings of the tundra, and voles of grasslands and forests, often fluctuate in three–four-year cycles of abundance, and the challenge to the field biologist has enduringly been to understand why these cycles occur.