CLAIM: Lemmings commit suicide by hurling themselves, en masse, off cliffs.

 FALSE:  Lemmings do not periodically commit suicide by hurling themselves off cliffs. The idea that they do is just a myth. Belief in this myth was strengthened by a 1958 Disney documentary, White Wilderness, in which the filmmakers herded some hapless lemmings off a cliff in order to show this supposedly natural behavior.

Where I learned that Disney circa 1958 was staffed by Lemming Haters:

The Gullibility Test

at

The Museum of Hoaxes