Lemmings are like guinea pigs.  When the first few lemmings in a group are born, there is plenty of food.  Then more and more lemmings are born.  Food is scarce.  Some of the tribe leave.  They march and march.  Nothing can stop their march.  Though some are eaten,  more are born and they keep on marching.  Eventually, they reach the ocean.  Then they march right into the ocean and drown themselves.  Back at the tribe, more are born.  The saga starts again.