Lemmus lemmus

31.

Norway Brown Lemming

Lemmus lemmus

French: Lemming de Norvege / German: Berglemming / Spanish: Lemming de Noruega

Other common names: Norway Lemming, Norwegian Lemming

Taxonomy. Mus lemmus Linnaeus, 1758, Lappmark, Sweden.

Lemmus lemmus is closely related to L. sibiricus. The two lemmings easily hybridize under laboratory conditions. Recent phylogeographic analysis showed that L. lemmus did not colonize its current distribution from the south but survived the Last Glacial Maximum in an ice-free Fennoscandian refugium. Monotypic.

Distribution. Endemic to Fennoscandia, specifically to the Scandinavian Peninsula (Norway and N & W Sweden), N Finland, and Kola Peninsula (NW Russia); S border fluctuates ¢.200 km, shifting S during cyclic peaks.

Descriptive notes. Head-body 112-150 mm, tail 10-18 mm; weight 40-130 g. The Norway Brown Lemming is chunky, with relatively large, broad, and flattened head; small eyes; and short, tufted ears. Tail is slightly shorter than hindfoot, densely haired with long pencil. Palmar and plantar surfaces are densely covered with stiff hairs; pads are vestigial. Females have eight nipples, two pairs of inguinal and two pairs of pectoral. Fur is soft, dense, and long. Pelage is bright black on nose, forehead, shoulders, and proximal back; reddish brown on rest of back and between ears; and buff to yellow on venter and flanks. Black stripe of variable intensity is frequently present along spine. Color varies considerably among individuals, depending on season, age, and sexual status. It can be lighter or rustier, with conspicuous or nearly absent black markings. Three hypotheses were proposed to explain rich coloration that can be aposematic, protective, or a mix of both (i.e. aposematism and crypsis). In any case, Norway Brown Lemmings match the background in certain environments and at certain times (e.g. subalpine sphagnum bogs, birch woods in spring, and subalpine tundra in autumn). Skull is very broad and low, with powerful zygomatic arches that expand abruptly. Rostrum is relatively short; interobital region is narrow, with prominent medial crest; and postorbital process on squamosalis evident. Dorsal profile is flat except for nasals that bend downward abruptly. Mandible is rather slender, with short and blunt coronoid process. Upper incisors are relatively weak, and lower incisors are short. Molar are rootless, broad, and heavy, with cement in reentrant folds. Lingual salient folds are broadly truncate in all upper molars. M, has a simple structure.

Habitat. Barren alpine tundra of the Scandinavian mountain chain at elevations of normally 600-2450 m (usually above 1000 m). Norway Brown Lemmings temporarily occupy lowland coniferous forests during dispersal. In the taiga zone, they disappear after several population cycles. Permanent colonization may be prevented by predators. Winter habitats differ sharply from summer habitats. Winteris spent in snowbeds with diverse underlying vegetation, particularly mosses. After initial snowmelt in spring, snowbeds dry, and Norway Brown Lemmings move to wet habitats in mires and pine bogs and along creeks. Such habitats freeze in winter pushing them back into snowbeds.

Food and Feeding. Although 70 different plant species have been identified in the diet of the Norway Brown Lemming, it is somewhat of a feeding specialist. Staple diets contain mosses (Dicranum, Polytrichum, Pleurozium, and Hylocomium, but not Sphagnum) and monocotyledonous plants such as grasses and sedges (Deschampsia, Nardus, Festuca, Carex, and Eriophorum). Monocotyledones are more important in tundra, and their proportions in diets are higher in the wet tundra (85%) than in the dry tundra (58%). Mosses constitute more than 50% ofdiets in coniferous forests; in spring, their proportion can be as high as 88%. Lichens are not palatable to the Norway Brown Lemming. In winter habitats, they can exhaust food supplies by consuming 60% of all edible plants and 80-100% of preferred species. After years of peak density of Norway Brown Lemmings, vegetation may need 2-3 years to recover.

Breeding. The Norway Brown Lemming is able to breed in winter under snow. Winter breeding is typical of increasing populations and may start in February. Summer breeding resumes after a gap of up to two months in April-May and lasts deep into autumn. At high densities, summer reproduction ceases in summer, and nolitters are born in winter. Winter breeding is successful only under snow layers of 2-3 m. Gestation under laboratory conditionslasts 20-21 days, and litters have 1-16 young; majority of females (61%) have 3-5 young. Winter litters are smaller (three embryos) than summerlitters (mean 6-6-6 embryos). Age of maturation is c.5 weeks in the wild and c.3 weeks in the laboratory. Females sexually mature at younger ages than males and may become pregnant at two weeks old. Shortest interbirth interval is 17 days (mode 21 days). Young are born naked, blind, and deaf; body weight at birth is 3-9 g. Young are weaned at 12-14 days old. Litter survival is ¢.70%, and mortality is highest during first two days oflife and again at the time of weaning. Sex ratio in mature lemmingsis female-biased. Density of the Norway Brown Lemming periodically shows large oscillations peaks every 3—4 years.

Activity patterns. During midnight sun in June, Norway Brown Lemmings are active at any time of the day but tend toward nocturnal activity after mid-July. Diurnal activity during migrations may depend on photoperiod. Breeding females spend majority of active time feeding to compensate for high energy expenditure. Breeding males are highly mobile. Migrating Norway Brown Lemmings are able to move long distances, even more than 100 km. Their speed is ¢.3:6 km/h on the ground and c¢.5 km/h on ice; swimming speed is c.1 km/h. On average, they can travel more than 15 km/ day and can swim 2-3 km in still water. Within their home range, they use runways (width 5-5 cm) that are 2.5-5 cm undersurface of moss layer. Summer nests have diameters of 15-18 cm and are 12 cm high. Reproductive females relocate nest sites 3-5 times during summer to avoide predation. Nest sites are established several days prior to parturition and are abandoned after 21-23 days when young are weaned. After that, females search for new nest sites outside core areas of their home ranges. Winter nests are larger, with outer diameters of 20-30 cm and inner diameters of 8-10 cm. They are under snow.

Movements, Home range and Social organization. Movements of Norway Brown Lemmings are seasonal and multiannual. They move seasonally between summer and winter habitat. Spring movements are caused by snow melting and flooding of burrows, and they last for 1-2, rarely three, weeks. Autumn movements are density dependent. At high densities, movements start in early July and last into October. Early migrants are mature males, and juveniles start migrating at 1-5 months old. Multiannual migrations involve long distances and follow peaks in density. Such migrations can shift southern distributional border more than 200 km from northern mountains deep into boreal forests. Large-scale migration occurs only once in slightly more than 30 years and is not synchronized across the entire distribution. Lemmings move individually but congregate at obstacles. The Norway Brown Lemming is the only species of lemmings with such long-distance migrations. On average, males have larger home ranges (1-4 ha) than breeding females (0-2 ha). Females rise young in even smaller areas (0-09 ha), which are about the same size as home ranges of non-breeding females (0-05 ha). Male home range overlaps those of several females and other males. Breeding females have exclusive home ranges. Pregnant females shift home ranges before each parturition. Mating system is promiscuous. Captive individuals are frequently amicable toward juveniles. Adult females show little aggression but avoid contact. Migrating individuals are highly vocal and aggressive. Non-receptive mature females are particularly aggressive toward males and drive them from their home ranges.

Status and Conservation. Classified as Least Concern on The IUCN Red List.

Bibliography. Chernjavskij (2016b), Gower (1999), Lagerholm et al. (2014), Shenbrot & Krasnov (2005), Stenseth & Ims (1993), Tast (1982).