The Norway lemming, also known as the Norwegian lemming (Lemmus lemmus) is a common species of lemming found in northern Fennoscandia, where it is the only vertebrate species endemic to the region. The Norway lemming dwells in tundra and fells, and prefers to live near water. Adults feed primarily on sedges, grasses and moss. They are active at both day and night, alternating naps with periods of activity. Distribution It has a wide distribution from western Norway east to the Kola Peninsula. A 2021 study suggests that the lemmings of Novaya Zemlya may also belong to this species; this would extend its range east to Novaya Zemlya island, but these results have not been fully accepted. Description The mainland Norway lemming has a bold pattern of black and yellow-brown, which is variable between individuals. In contrast, the Novaya Zemlya lemming has a cryptic gray coloration (hence why it was previously thought to be a population of the Siberian brown lemming). The species grows to a size of 155 mm. The tail is very short (10 – 19 mm). It weighs up to 130 g. The dental formula is 1/1, 0/0, 3/3. Behaviour The Norway lemming has a dramatic three- to four-year population cycle, in which the species' population periodically rises to unsustainable levels, leading to high mortality, which causes the population to crash again. The Norway lemming spends the winter in nests under the snow. When the spring thaws begin and the snow starts to collapse, they must migrate to higher ground, where the snow is still firm enough for safety, or, more commonly, to lower ground, where they spend the summer months. In autumn, they must time their movement back to sheltered higher ground carefully, leaving after alpine snow cover is available for their burrows and nests, and before the lowlands are made uninhabitable by frost and ice. When the seasons are particularly good (short winters without unexpected thaws or freezes, and long summers), the Norway lemming population can increase exponentially; they reach sexual maturity less than a month after birth, and breed year-round if conditions are right, producing a litter of six to eight young every three to four weeks. Being solitary creatures by nature, the stronger lemmings drive the weaker and younger ones off long before a food shortage occurs. The young lemmings disperse in random directions looking for vacant territory. Where geographical features constrain their movements