Norway lemmings emigrate en mass in search of food once their population size reaches 40-100 individuals per acre.
"One much smaller species of herbivorous mammal that still undergoes periodic swarming on a spectacular scale is the lemming. Displaying a formidable reproductive rate - more than 100 offspring can be born to a single pair within six months - a population of Norwegian lemmings (Lemmus lemmus) can expand very dramatically. In doing so, the lemmings deplete their food supply within a given area of the Scandinavian tundra and scrub that comprises their normal habitat. Once the population size reaches 40-100 individuals per acre (100-250 individuals per hectare), which tends to occur every three to five years, emigration ensues, whereby a sizable horde of these volelike rodents travels southward in search of food, expanding their population's range by 120 miles (200 km) or more as they go." (Shuker 2001:78-79)
Learn more about this functional adaptation.
- Shuker, KPN. 2001. The Hidden Powers of Animals: Uncovering the Secrets of Nature. London: Marshall Editions Ltd. 240 p.