I was horrified to find out that the whole thing about Lemmings jumping off cliffs in suicide/migration attempts is a big fake that began with a Walt Disney documentary called White Wilderness. Basically, Lemmings don't jump off cliffs unless they are herded over the edge by Disney film makers!!! There is more information on here and here. The clip below was extracted from the movie. Looking at it having read the info in the two links above, it does look really fake (tight shots and lots of lemmings trying to get back up the cliff).