Just to set the record straight, lemmings don't actually commit any kind of suicide. This is a very common misconception. The truth is that, they can swim. Due to this, when they have their migratory urges, they sometimes swim across bodies of water. Sometimes, they drown. That is the way of life. Lemmings are no more (as a matter of fact, probably less) suicidal than the human race.
Thanks! :)