Why would an animal kill itself? It seems a strange question, and yet it is one that has【B1】___some people for a long time. The lemming (旅鼠) is one such animal. Lemmings periodically commit mass【B2】___, and no one knows just why!

The small【B3】___, which inhabit the Scandinavian mountains, sustain themselves on a diet of roots and live in nests they make underground. When their food supply is【B4】___large, the lemmings live a normal, undisturbed life.

However, when the lemmings’ food supply becomes too low to support the population, a singular【B5】___commences. The lemmings leave their nests all together at the same time, forming huge crowds. Great numbers of the lemmings begin a long and hard journey across the Scandinavian plains, a journey that may last weeks. The lemmings eat everything in their path, continuing their【B6】___march until they reach the sea.

The reason for what follows remains a mystery for zoologists and naturalists. Upon reaching the coast, the lemmings do not stop but swim by the thousands into the surf. Most【B7】______only a short time before they tire, sink, and drown.

A common theory for this unusual phenomenon is that the lemmings do not realize that the ocean is such【B8】___water. In their cross-country journey, the animals must traverse many smaller bodies of water, such as rivers and small lakes. They may【B9】that the sea is just another such swimmable【B10】___. But no final answer has been found to the mystery.

【B4】