The most common mammal on Severnaya Zemlya is the collared lemming ("Dicrostonyx torquatus"), also known as Arctic lemming, which is present on all of the large islands and, in some places has been recorded to reach a density of 500 per km2 (1,300 per sq mi). The Arctic fox ("Alopex lagopus") has been known to den on the islands, with several hundred observed in the 1980s. Other mammals occasionally observed include the wolf ("Canis lupus"), the polar bear ("Ursus maritimus"), ermine ("Mustela erminea"), walrus ("Odobenus rosmarus"), Arctic hare ("Lepus timidus"), and reindeer ("Rangifer tarandus").