Composition of cyclic and non-cyclic vole populations: On the causes of variation in individual quality among Clethrionomys glareolus in Sweden.

A cyclic Clethrionomys glareolus population from north Sweden was compared with a non-cyclic population from south Sweden regarding age structure, body weight, reproductive development and suppression, and sex ratios.The non-cyclic population, after years with winter breeding, was characterized by especially early and intense breeding in spring and many post-reproductive animals in autumn. Non-cyclic populations with no winter reproduction were similar to increase years in cyclic populations. In peak and especially in decline phases of cyclic populations reproduction decreased clearly below the level in non-cyclic populations. Animal weights varied insignificantly between years in the non-cyclic population but changed greatly in the cyclic population.A unifying hypothesis for explaining variations in animal quality between different populations is proposed: Maternal conditions related to food supply are assumed to affect body growth and sexual maturation of the young. Predictions from this hypotheses are outlined and compared with those from competing ideas.