Lemmings

The story of lemmings leaping off of cliffs to drown in the sea is kind of an exaggeration. They will do this is they are chased or herded (what creature wouldn't). They don't do it spontaniously.

Only when they swarm/migrate en masse and run into the ocean.

Yes.... It is a fallacy that lemmings commit mass suicide during migration. They do migrate in large groups when population density becomes too great. Lemmings can swim and will often choose to cross a body of water in search of a new habitat. On occasion, the lemmings are pushed into the sea by the pressure of more lemmings arriviving on the cliff-top or shoreline. Sometimes (particularly in the case of Norwegian lemmings) large migrating groups will reach a cliff overlooking the sea. They will stop until the urge to continue forces them to jump off the cliff and start swimming, sometimes the distance is too far, resulting in exhaustion and subsequent death.

Try Lemmings Universe.