An isolated population of Clethrionomys glareolus (Schreber, 1780) was investigated over the course of one year. Marking of all trappable individuals and knowledge of the time in which the various individuals were born made it possible to distinguish 5 age groups (cohorts). The maximum participation in the current year's population mass, expressed by the number of individual-days, was that of the early summer cohort (56%), the maximum participation in natality -(53.6%) -was that of the early spring cohort. It was also found that individuals from the early summer cohort were the most numerous survivors to the following year (they formed 75% of the population in the spring). The disappearance rate of the young animals is far higher than that of trappable individuals in the summer. Disappearance of the young of spring and early summer cohorts is lower than that of the late summer and autumn cohorts. Disappearance of the trappable part of the population is lower in the winter than during reproduction.
In a free-living isolated population of Clethrionomys glareolus, in relation to which a very accurate knowledge was obtained of the numbers of the whole population, pregnant females (N p ) and thus also f -Np-.Nę, and the reproduction period (T), while the size of the litter (L) and length of pregnancy (t D ) were taken from literature, the number of young born were defined by formula:and the formula proposed by the authors:The result which was obtained by the first formula was about twice lower than that obtained by the second. In two cases the first formula gives unreliable results. The number born calculated from two censuses per year (»Standard-Minimum« method) gives results 3°/o lower than those based on full information.
Calculation was made, on the basis of individual growth and empirical survivorship curves, of the productivity achieved by a population of C. glareolus during the course of one year (P = 11,429 g live weight/ /year/population). Production achieved during the reproduction season was P = 10.263 g and during the winter P = 1166 g. During the reproduction period the value was determined of the production of young achieved at the mother's expense (up to the 21st day of life) -P r = 5848 g and value of production due to growth -P 2 = 5581 g.
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