What is unusual about lemmings?

Lemmings - the strange thing about lemmings is the way they apparently commit mass suicide. These grey or reddish brown rodents look like mice but are slightly larger. They live in large areas of the northern world including Alaska, northwest Canada and Scandinavia. But in fact only the Scandinavian lemmings stampede in such a suicidal manner. Now and then thousands of them travel miles over land in panic to the sea. Many die on the way, but those that arrive leap immediately over the cliffs into the sea and swim until they become exhausted and drown. No one is certain what makes the lemmings behave so strangely. It may be because the weather becomes too warm for them or because their food supply changes or simply because they feel they are getting overcrowded.