This case study summarises some findings about the diet of the wood lemming (Myopus schisticolor). The wood lemming is found through a large part of northern Eurasia. The studies reported here were carried out in eastern Finland during the years 1997 and 1998. The wood lemming's ideal habitat is old, spruce-dominated forest with a thick, continuous moss carpet below. There should also be an abundance of rotting trees or stumps and holes. These holes provide storage places and protection from enemies. The wood lemming gathers winter food stores during autumn and caches them in the holes. The wood lemming feeds mainly on moss. It also eats grasses and the stems and leaves of shrubs in the genus Vaccinium, but each of those to a lesser degree.
The mosses encountered in this study were: Aulacomnium palustre, Hylocomium splendens, Pleurozium schreberi, Ptilium crista-castrensis, Rhytidiadelphus triquetrus and unidentified species in the genera Dicranum, Polytrichum and Sphagnum. Since there will be no possibility of confusion only the genus names will be used from now on, for the sake of brevity.
The wood lemming has slightly different habitat preferences during the year, preferring drier sites in winter and wetter ones in summer. However, apart from that consideration the winter and summer habitats were fairly similar in the eastern Finland study sites. There seemed to be no differences in the abundances of Hylocomium and Pleurozium between the winter and summer habitats. Ptilium was more abundant in winter habitats and Dicranum, Polytrichum and Sphagnum in summer habitats.
Evidence of a feeding area was clear. The tops of mosses were missing and there were feeding marks on the Vaccinium stems and leaves. As well, fragments of all these plants could be seen in the feeding areas. Lemmings graze for a long time on the same moss species within the same plot. From the colour of the faeces it is even possible to distinguish the different mosses eaten:
Aulacomnium was greatly utilized in just one area of the winter habitat during the study reported here and no other studies of wood lemming diet have ever reported consumption of this moss. Rhytidiadelphus occurred sparsely in the study area and there was no evidence that wood lemmings ate it.
The following table summarizes the wood lemming's feeding habits in the summer habitats. For each moss the first numerical column shows an estimate of the percentage of ground covered by that moss. The second is an estimate of the percentage of each moss that was grazed by the lemmings. The third column is the ratio of the two. I have rounded the percentages to whole numbers.
The data are derived from a number of survey plots within the researcher's study site and the percentages are the means over those plots. Being statistical results there is some uncertainty associated with them. You can read more about the statistical side in the first reference given in the reference button at the end of this page.
Without worrying about the statistical niceties for the moment, you can use that table to get an idea of wood lemming preferences. Look at Dicranum and Sphagnum. The percentage of ground covered by each is almost the same, yet the wood lemmings barely touch Sphagnum but happily eat Dicranum. You could look at the ratio in the third column as a "desirability index", the higher this ratio the more the wood lemming prefers the moss. Looked at this way, Dicranum is the most desirable while even a starving wood lemming might baulk at nibbling Sphagnum. From the table the two most sought-after mosses were Dicranum and Polytrichum, even though these were not the commonest two. That award belong ed to Pleurozium and Hylocomium.
Near the end of autumn the researcher also investigated the food stores that the wood lemmings had built up for winter. The majority of the stores were composed mostly (about 85%) of Dicranum, even though it had an abundance of only about 15% around the storage holes. Pleurozium accounted for about 11% of the winter stores, even though it accounted for almost half the moss cover in the surroundings of the holes. Hylocomium accounted for about 30% of the coverage in the wintering areas, but only 3% of the winter stores. Ptilium was scarce, both in the surrounds and in the stores. Polytrichum and Sphagnum were scarce in the surrounds of the wintering holes and did not occur in the stores.
The wood lemmings would still forage during winter, with Dicranum still the preferred moss. Ptilium had the second highest percentage of damaged tops in the winter sites, though it occurred in only 30% of the winter study plots. Polytrichum was found in 60% of the winter plots but overall its mean cover was less than that of Ptilium. Moreover, despite the relatively high abundance of Polytrichum it was less favoured than was Ptilium in the winter habitats. Pleurozium was common in the winter habitats but, unless Dicranum was absent from a particular area, Pleurozium did not feature highly in the diet. Aulacomnium was abundant in one study plot and in that one plot about 30% of the Aulacomnium cover was eaten. The other mosses were either untouched or barely touched.
Dicranum was clearly the most preferred moss, regardless of season, and the presence of Dicranum is one of the most important factors in habitat selection by the wood lemming. Chemical analysis of nitrogen content in the mosses in the study site showed Dicranum and Polytrichum to have the highest nitrogen percentages, Polytrichum's higher than Dicranum's. Nitrogen content is an indicator of protein content and it may be that higher protein content is the reason for the desirability of these mosses. The preference for Dicranum over Polytrichum may be due to the greater abundance of Dicranum and perhaps that moss is more easily digestible. Other wood lemming studies have also found this preference for Dicranum. The researcher did not know why there was such a difference between the consumption of Pleurozium and Hylocomium. It is known that many mosses have various secondary compounds, such as phenols, which make them unpalatable. This could explain the Pleurozium / Hylocomium difference but, in the absence of any chemical analysis of the two mosses, that remains a hypothesis.
The percentage of the moss cover eaten by wood lemmings in the study plots ranged from 2% to 30%, with an average of 13% overall. A higher percentage was eaten in summer than in winter, the overall summer average being 21% and the winter one 10%. Wood lemmings can survive on mosses alone but young wood lemmings grow faster if they supplement their diets with other plants.
By comparing the carbon/nitrogen ratios in the mosses and the corresponding faeces, the researcher found that efficiency of the wood lemming's absorption of nitrogen was highest on a diet of Dicranum, Polytrichum, Aulacomnium and Ptilium. As noted earlier Aulacomnium was greatly utilized in just one area in the winter habitat. The researcher also pointed out an earlier study that had found Pleurozium as the most abundant moss in winter stores in northern Finland. Given these seemingly anomalous facts about Aulacomnium and Pleurozium it's likely that there's still more to the story about the diet of the wood lemming.