Synchrony in short-term fluctuations of moose calf body mass and bank vole population density supports the mast depression hypothesis

Inter-annual variations in body mass of moose, Alces alces, in Norway and Sweden have been considered as most likely due to direct or indirect effects of weather, but so far predictions of autumn body mass of moose calves on the basis of weather data have given a poor fit to data. A striking, but hitherto unnoticed, feature of several time series on body mass of moose calves from south-eastern Norway is an apparently regular 3-4-year fluctuation pattern. This short-term fluctuation could be due to regular variations in forage quality, e.g. caused by a cyclic seed production of some important food plants, as envisaged by the "mast depression" hypothesis. One plant species important as food for moose calves in autumn is bilberry, Vaccinium myrtillus, which usually produces high seed crops (masts) at intervals of 3-4 years. Populations of the bank vole, Clethrionomys glareolus, which feeds on bilberry shoots in winter, are known to peak in bilberry post-mast years. In two study areas in Norway, there was a positive correlation between the autumn body mass of moose calves and the autumn population index of bank vole in the succeeding year. In the northern area there was an additional positive effect of summer precipitation, whereas in the southern area there was an additional negative effect of summer temperature. In both areas, however, the effect of weather was less pronounced than that of the bank vole index. These results support the mast depression hypothesis.