Studies on small rodents from Central Europe show that almost all animals stop breeding each winter and have then undeveloped gonads. In northern Scandinavia it is difficult to obtain the animals from beneath the snow but the situation has been supposed to be similar. However, some cases of winter-breeding have been observed, e.g. in lemmings (Lemmus lemmus L. and Myopus schisticolor Lillj.). Trapping in localities rather regularly spaced over Sweden (Fig. 1) at peak winter conditions 1972-73 revealed that a considerable proportion of the small rodents were sexually mature (males according to Jame-
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Larsson et al. (1973) studied this question.