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Wood Lemming

Myopus schisticolor

French: Lemming des bois / German: Waldlemming / Spanish: Lemming de bosque

Taxonomy. Myodes schisticolor Lilljeborg, 1844, near Lillehammer, Mjosen, Gudbrandsdal, Norway.

Mpyopus schisticoloris the only extant species of Myodes and is closely related to brown lemmings ( Lemmus). Five subspecies, as tentatively recognized, do not match evolutionary structuring of the species. Phylogeographical assessment retrieved two major lineages: one restricted to a small area in the Far East and the other one widespread. Monotypic.

Distribution. Fennoscandian Peninsula across N & C Russia, as far E as Pacific coast of Russian Far East and Sakhalin I, S to N Mongolia and NE China (Heilongjiang).

Descriptive notes. Head—body 90-115 mm, tail 12-19-7 mm; weight 20-38 g (males) and 20-40 g (females). Externally, the Wood Lemming resembles voles rather than brown lemmings. It is small and robust, with large and blunt head, well-developed ears concealed in long fur, small eyes, and very short tail. Feet are slender and normally sized, with naked plantar and palmar surfaces; there are five plantar pads of considerable size. Females have eight nipples, two pectoral and inguinal pairs each. Pelage is slate-gray, with dull rusty brown patch of variable size; in one extreme, patch extends from eyes to rump but is frequently restricted to posterior back. Feet and tail are blackish slate. Skull is broad and depressed with nearly flat dorsal profile; nasals bend downward. Zygomatic arches are heavy. Supraorbital ridges merge into medial crest. Mandible shows no peculiarities, except a small coronoid process. Molar are rootless, broad, and heavy, with cement in reentrant folds. Enamel pattern is very similar to that in brown lemmings.

Habitat. Taiga-like habitats with abundant moss at elevations of 600-2450 m. Wood Lemmings occupy coniferous and mixed forests, forest edge, and open areas, preferring old spruce ( Picea, Pinaceae) stands with plenty of decaying wood and continuous thick moss layer. Snow cover provides protection against cold; Wood Lemmings do not occupy dense spruce forests with no snow. During migration, Wood Lemmings temporarily colonize various types of meadows and willow thickets.

Food and Feeding. Staple diet of Wood Lemmings consists of stems and leaves of mosses (more than 90% of plants eaten). Dicranium mosses (Dicranium), hair mosses (Polytrichum), knight plum moss (Ptilium crista-castrensis), and stair-step mosses (Hylocomnium) are highly preferred. Other moss species ( Pleurozium, Aulacomnium, Sphagnum) are eaten to much lesser degrees, and goose neck moss ( Rhytidiadelphus) is entirely avoided. Nitrogen content may be the main reason for such dietary preferences. In Finland, 2-30% (mean 13%) of moss cover was eaten by Wood Lemmings, and proportions were higher in summer (21%) than winter (10%). Hairgrasses ( Deschampsia, Poaceae), blueberry ( Vaccinium myrtillus), and lingonberry (V. vitis-idaea), both Ericaceae, are also eaten. Daily consumption is 5-6 g of dry weight. Captive individuals spent ¢.100 minutes/ day feeding, mainly during daylight. Some studies reported food stores containing 2-3 deciliters (exceptionally 1-2 liters) of moss, usually dicranium.

Breeding. Breeding season of the Wood Lemming is between early May and August— September and starts under snow. Gestation lasts 20-22 days, number of embryos per female is 2-9, and litters have 3-7 young. Mean number of embryos is higher in old females and also varies geographically and seasonally (means 4-1-7 young). Proportion of resorbed embryosis high (17:4-20-6%). A female can mate 3-4 days after parturition, and intervals between successive litters can be as short as 23-28 days. Young are born naked, blind, and with sealed ears. Body weights are 1-9-2-3 g at birth and 15-7 g at weaning on day 20. Young open eyes at 12-14 days old. Females mature at younger ages (22-40 days) than males (over 44 days). Individuals seldom live more than one year. Due to an unusual genetic system, Wood Lemmings produce c.3 times as many female as male offspring. Females are of two types, depending on their X sex chromosomes. The X-chromosome can be either orthodox (X) or with a mutation that inhibits male-determining effect of the Y-chromosome (X*). Only one of the possible combinations of sex chromosomes will produce males (XY), while all remaining combinations are females (XX, X*X, X*Y). X*Y females produce only daughters and have higher reproductive potential. Such a system may buffer against inbreeding during population lows. Densities of the Wood Lemming periodically have large oscillations, but peaks are rarely regular.

Activity patterns. Daily activity is polyphasic and mainly nocturnal. There are 5-6 active phases, each of 90-120 minutes, giving a total of ¢.750 active minutes/ day . Body temperature (38-38-8°C) and basal metabolic rate are high throughout the day . Wood Lemmings move 10-20 cm below the surface of the moss layer. Runways are 3—4 cm wide and spread 5-10 m around the nest. Oval nests (14-16 cm in diameter) of dry grass, mosses, and horsetail ( Equisetum, Equisetaceae) are hidden in rotten wood stumps, between tree roots, within moss hummocks, or among stones overgrown with mosses.

Movements, Home range and Social organization. Wood LLemmings are mainly solitary and non-aggressive. Daily movements of captive individuals are c.700 m. They disperse from overpopulated habitats to other areas, and such movements are usually called migrations. Migrating Wood Lemmings are frequently killed on roads or drown in canals and while swimming across water obstacles or after falling from precipices and bridges. Migrations are most common in late August and early September and end with low temperatures. Migrations have no direction and cover several kilometers at most. Females have smaller home ranges (285 m?) than males (2144 m?).

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

Bibliography. Eskelinen (2002, 2004), Fedorov et al. (2008), Fredga et al. (2000), Mironov (2016a), Niethammer & Henttonen (1982), Saarela & Hissa (1993), Shenbrot & Krasnov (2005), Stenseth & Ims (1993).