Animal facts for kids

For about as long as anybody can remember, mice and men have been constant companions, though the relationship has not always been a happy one.

The word “mouse,” for example, illustrates both the antiquity and the adversarial relationship between man and mouse. It comes from the ancient Indo-European language, Sanskrit, and means “thief.”

Mice are some of the smaller members of the rodent family, which also includes rats, gerbils, lemmings and others. And while there are several types of mice, including harvest mice, deer mice and grasshopper mice, the most commonly seen is the house mouse, which grows to about two-and-a-half to three-and-a-half inches long and lives almost everywhere in the world where there are people.

Living up to the name of “thief,” these little rodents steal their meals from people, eating grain, vegetables and meat, or even leather, glue and soap.

But if house mice make their living from people, people also take advantage of mice, sometimes keeping them as pets or using them as subjects to test drugs or study behavior.

Lemming…common name for several species of small, mouselike arctic rodents constituting four genera, Lemmus (true or brown lemmings), Dicrostonyx (Arctic or collared lemmings), Synaptomys (bog lemmings), and Myopus (wood lemmings), of the subfamily Microtinae, which also contains the voles, family Muridae.

The four species of true lemmings are small, short-bodied animals, about 5 inches long, with very short tails. They are tan above and light gray, mixed with tan, below. The head is round, with small ears concealed by fur and with a stubby, hairy snout. The legs are short. Lemmings live in extensive burrows near the water, feed on vegetation, and build nests out of hair, grass, moss, and lichen. The female produces several broods a year, each of which contains about five young.

The Norwegian lemming (Lemmus lemmus, pictured at top) lives at 2,500-3,300 ft above sea level, above the line of willows. (see habitat map) In summer lemmings occupy moist stony ground partly covered by sedges, willow shrubs and dwarf birch. They make paths through the carpet of lichens and rest in natural hollows or cavities in the vegetation. In autumn they move into drier areas, at or about the same level as the summer quarters. In winter they usually live under the snow, protected from cold and from enemies, building rounded nests of grass that are sometimes left hanging on twigs when the snow has melted. They make extensive tunnels under the snow. Because their food is lichens, mosses and grasses wintry conditions do not interrupt their feeding and in an ordinary winter they continue to breed.

Female lemmings produce several litters per year. The gestation period is 20-22 days and each litter has 3-9 young, born in a spherical nest of shredded fibers, moss and lichens, made under cover of a rock or in a burrow.

Stoats, weasels, rough-legged buzzards, ravens, longtailed skuas, various members of the crow family, and snowy owls make up the lemming’s “enemies list.” In winter lemmings are safe under their covering of snow from all but stoats and weasels, and even they are present in fewer numbers, so there is a marked difference in predation between summer and winter.

emmings, like most small rodents, are subject to fluctuations in numbers from year to year. The populations build up over a period of years to abnormal numbers and then comes a crash fall and numbers are reduced to normal. The interest lies in the causes of the rises and falls and in what actually happens when their numbers are abnormally high. The scientific explanation in the past has been that in years of abnormal numbers the lemmings migrate down the mountainsides into the fertile valleys in search of food (see migration diagram). This is near the truth. The popular stories, aided by artsits’ impressions in the form of pictures, based on local hearsay, is of columns of lemmings in headlong dash down to the sea, where they are drowned in a sort of mass suicide.

Years of study have found three main reasons for the population explosions. First, an early spring and a late autumn produce favorable climatic conditions that not only yield an abundance of food but give the lemmings a longer period in which to take advantage of it. Secondly, mild winters with thaws and also severe winters are damaging to winter breeding. In winters that are between these extremes there is a high rate of breeding and of survival among the young. The third is that because of the lack of enemies during winter there is no brake on the mounting increases in numbers resulting from the first two.

Mass migrations of lemmings do actually occur, almost always as the result of explosive population growth. Such migrations are most noticeable mainly in places where there are obstacles to the horde spreading out evenly, for example, a long lake or where two rivers meet making a kind of funnel. Then there comes an accumulation of lemmings followed by a kind of panic in which they march recklessly but not in any special direction. They may go up the mountains as well as down into valleys. They may go to any point of the compass. They may go over glaciers or swim across rivers or lakes or, as in Norway where the mountains run down to the sea, into the sea. Lemmings are excellent swimmers and their fur is waterproof, so they take no harm from getting wet. If the water is calm they can cross a river or lake. If it is choppy, however, as in the sea, or on lakes in windy weather, many are drowned.