The Wood Lemming (Myopus schisticolor) is a species of rodent in the family Cricetidae. It belongs into the Arvicolinae subfamily of rodents therefore is a relative of the voles, lemmings, and muskrats. It is found in the taiga biome of China, Finland, Mongolia, Norway, Russia, and Sweden.^[1]

Sex determination

Wood Lemmings produce about 3 times as many females as male offspring. This is due to an unusual genetic system where they have two different types of X chromosomes, the normal X and a mutated X*. Females with that mutation on their X*-chromosome inhibit the male determining effect of the Y chromosome. This leads three genetic types of females XX, X*X and X*Y and one genetic type of males XY. The X*Y females are fertile but only produce X* ova, which means they only produce female offspring. Its not fully known the extent to which nature balances out the differences in the adult population, but sampling studies seem to suggest that males do make up as little as 25% of the population at equilibrium. Females with abnormal genotype (XO, XXY, X*YY) occur regularly. The high female sex ratio may be an adaptation against local mate competition and inbreeding during population low points.^[2][3]

Population Dynamics

The population density of lemmings shows extreme peaks interspersed by years of very low densities. In peak years wood lemmings migrate from overpopulated areas to areas of low population density. The migration usually begins around the end of July and end in early October. The migratory distances are typically quite short, ranging a few kilometers at most, with no specific direction.^[4] If geographical features do not allow the animals to disperse evenly during their migration, then thousands of lemmings can be seen on their migration. This migratory behavior was exaggerated in popular stories about lemmings that in innumerable numbers dash down the hillsides and fall down over rocks into the sea, only to drown. However, such stories of mass suicides in lemmings are mere legends.^[5][6] The size of home range between male and female wood lemmings varies greatly. Male home ranges (>2000m²) are 5 to 7 times larger than those of females.^[6]

Diet

The diet of the wood lemming consists mostly of moss.^[7] The most commonly consumed species of moss are Dicranum, Polytrichum, Ptilium, and Pleurozium, with Dicranum being the moss of choice. It was found that in Northern Finland, Dicranum spp. made up the majority of the lemming’s winter food supply.^[4] It is thought that these mosses are preferred because of their usability and relatively high nitrogen content.^[4] Lemmings will also eat some grass species such as Deschampsia, as well as the leaves and stems of Vaccinium spp.^[4]

Habitat

Wood Lemmings can be found in areas from southern Norway to Kamchatka (taiga area of Eurasia).^[4] The ideal habitat for the wood lemming is a spruce forest with thick, copious moss cover.^[7] Lemmings gravitate towards areas that have abundant areas of cover from predators i.e. holes provided by decomposed trees, stumps, and mossy rock. In the winter, wood lemmings seek out drier areas than summer.^[7]

Activity

Metabolism

The wood lemming is well adapted to live in cold climates due to its high thermogenic capacity, thermal insulation, and non-shivering thermogenesis.^[8] Compared to other voles, lemmings drink greater amounts of water, which may be why they seek moister areas in the spring and summer.^[4] As far as average distances traveled by lemmings, males travel distances anywhere from 4-12 times longer than females.^[9]

Reproduction

Breeding and reproduction usually happen in the winter months. Females reach sexual maturity at the age of 22–40 days, while male reach sexual maturity beginning around 44 days old and not before they reach a body weight of 20 grams. Females are able to have 2-3 litters per year. Litter sizes usually range from 3-7 young. The approximate life expectancy of a wood lemming is less than one year.^[4]

References

1. ^ Musser, G. G. and M. D. Carleton. 2005. Superfamily Muroidea. Pp. 894-1531 in Mammal Species of the World a Taxonomic and Geographic Reference. D. E. Wilson and D. M. Reeder eds. Johns Hopkins University Press, Baltimore
2. ^ Gippoliti, S. 2002. Myopus schisticolor. 2006 IUCN Red List of Threatened Species. Downloaded on 09 July 2007
3. ^ Winking H et al "Sex Determination and Phenotype in Wood Lemmings with XXY and Related Karyotypic Anomalies" HUMAN GENETICS 58, pp 98-104, 1981
4. ^ ^{a b c d e f g} Eskelinen, Olavi (2004). "Studies on the ecology of the wood lemming, Myopus schisticolor". University of Joensuu, PhD Dissertations in Biology (Joensuu) 24
5. ^ Burton M and Burton R "International Wildlife Encyclopedia, 3rd ed." Lemmings pp 1432-1434, Marschall Cavendish Corp. Tarrytown, NY, 2002
6. ^ ^{a b} Eskelinen, Olavi (2004). "Studies on the ecology of the wood lemming, Myopus schisticolor". University of Joensuu, PhD Dissertations in Biology (Joensuu)
7. ^ ^{a b c} Eskelinen, Olavi (2002). "Diet of the wood lemming Myopus schisticolor". Annales Zoologici Fennici (Helsinki, Finland: Finnish Zoological Botanical Publishing Board) 39 (1): 49-57
8. ^ Saarela, S.; Hissa, R. (Dec. 1993). "Metabolism, Thermogenesis and Daily Rhythm of Body Temperature in the Wood Lemming, Myopus Schisticolor". Journal of Comparative Physiology & Biochemical Systemic and Environmental Physiology (New York, NY: Springer Verlag) 163 (7): 546-555
9. ^ Andreassen,, HP.; Bondrupneilsen, S. (1991). "Home Range Size and Activity of the Wood Lemming, Myopus Schisticolor". Holarctic Ecology (Copenhagen, Denmark: Munksgaard) 14 (2): 138-141

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