A lemming is a small rodent type animal, not dissimilar to the hamster, and they live in colder climates.
People used to believe that lemmings would commit suicide jumping off cliff sides in order to reduce their population.
The truth is because lemmings reproduce quickly they sometimes cannot live in one habitat as it cannot support them all, so the lemmings go in search for new surroundings to populate, and in doing so sometimes they fall over cliff sides or get run over by cars.