Lemmings are small rodents who live in subarctic climates. Their populations are highly unstable, depending heavily on year to year vegetation growth and predation rates. This has led to the apocryphal belief that when overpopulated, lemmings become suicidal and swarm over the nearest cliff to their dooms.

This myth was knowingly perpetuated by the 1958 Disney "documentary" White Wilderness, which many children of appropriate generations saw when they were growing up. The film makers staged a "lemming suicide" by importing lemmings (which were not even native to the area they were filming) and dropping them in a river by a convenient cliff.

So remember, you can't believe everything you see, unless you see it in Irregular Webcomic!