Douglas Vincent, age 11, of Pittsfield, Mass., for his question

WHAT IS A LEMMING?

Lemming is the common name for four species of small arctic rodents. One species, the Norway lemming, appears in the cultivated fields of Norway and Sweden during the course of the periodic mass migrations for which it is famous. Two species can be found in arctic North America.

True lemmings are about five inches long. They have stout bodies and very short tails. The head is round, with small ears concealed by fur that is tan above and light gray, mixed with tan, below. It has short legs and a hairy snout.

The animals live in extensive burrows near the water and feed on vegetation. They build nests out of hair, grass, moss and lichen. The female produces several broods a year, each of which contains about five young.

At intervals of about four years when overpopulation of Norway lemmings has led to a scarcity of food and overcrowding of habitat, many thousands of the animals band together and migrate in search of food. The migrators swim lakes and rivers, cross mountains and eat all vegetation in their path. Eventually some reach the sea attempting to swim it as if it were a river. They are drowned.. Other lemming species do not migrate in this way.