Why would an animal kill itself? It seems a strange question, and yet it is one that has made some scientists curious for a long time. For there is a kind of animal called lemming , that periodically killed themselves together, and no one knows just why! The small creatures which live in the Scandinavian mountains, feed on a diet of roots and special insects and live in nests they dig underground. When their food supply is large, the lemmings live a normal and undisturbed life. However, when the lemmings' food supply becomes too low to support the population, a _ migration begins. The lemmings leave their nests in groups. Great numbers of them begin to travel across the Scandinavian plains, a journey that may last weeks. The lemmings eat everything in their path, continuing their long march until they reach the sea. The reason for what follows remains unbelievable to naturalists. Upon reaching the coast, the lemmings do not stop but swim by the thousands into the sea. Most stay afloat only a short time before they tire, sink and drown. A common theory for this mass self-killing is that the lemmings do not realize that the ocean is such a huge body of water in their cross-country journey, the animals must cross many smaller bodies of water, such as rivers and small lakes. They may think that the sea is just another such swimmable path to go through. But no final answer has been found to the strange happening. The passage is mainly about  _  .
how lemmings find food why animals kill themselves the lemmings' self-killing the food supply in the Scandinavian mountains