On the area of 3,100 ha of cultivated fields the density of the following species of rodents was estimated: M. arvalis in fields and C. glareolus, A. flavicollis and A. agrarius in woods. At the same time the numbers of predatory nammals and birds were estimated. During the whole period of study in area under examination 3,400,000 individuals of M. arvalis and 43,000 of forest rodents were present. Between the depression and the peak of numbers the density of M. arvalis increased over one hundred-times. At the density of 44 individuals/ha the voles consumed in a year 79 kg d.w. of alfa-alfa/ha, and at 774 indiv./ha as much as 1,153 kg d.w./ha. It was demonstrated that the ratio of plant weight destroyed to consumed amounted in alfa-alfa cultures to 1.8:1. At a low density c\f M. arvalis considerable losses of winter rye during winter are compensated by the regrowth of plants in spring. Food requirement of predators was determined and their diet in field conditions was investigated. The highest number of M. arvalis was consumed by foxes, lower by cats and weasels. The predators hunted mainly in areas showing the highest rodent density. With the increased numbers of M. arvalis its share in the diet of all predators rose up to 80%. Despite this the pressure of predators on the population of M. arvalis decreased, e.g. in the year of outbreak the predators removed barely 10.8% of all present individuals. With the increased proportion of M. arvalis in the diet of fox the proportion of consumed hares decreases. A scheme of trophic relationships of M. arvalis is presented. INTRODUCTION AND STUDY AREAComprehensive understanding of the importance of the common vole, Microtus arvalis (Pallas, 1779), in the trophic relationship of cultivated fields is still incomplete despite a large number of ecological studies concerning this species, as manifested by the publication of two monographs (Kratochvil, 1959; B a s e n i n a, 1962). Existing historical notes indicate that since ancient times M. arvalis has been recognized * Praca została wykonana w ramach problemu węzłowego 09.1.7., koordynowanego przez Instytut Ekologii PAN. Acta theriol. 9 r. 2 si

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The progress of moults in the annual cycle of the mole was investigated. Material (N = 800) was collected for the most part in one habitat in the grasslands of Krakow. Changes in hair were estimated by the planimetric measuring of pigmented areas on the skin. The spring moults of males and females resemble each other in pattern but differ in time. Unfertilized females moult at the same time as males. The moles from the Tatra Mts. finished the spring moult 10 days later than those of the Krakow region. The summer moult occurs at the same time in all moles. Adult females and young males moult more intensely. Males begin the autumn moult first, but in females the moult proceeds faster. Changes in hair were also ascertained in the winter, especially during the spells of warmer weather. The intensest pigmentation was found in moles from areas with poor edaphic conditions. There were considerable changes in the times of moults of the captive moles.

Gross body composition was studied in 465 captive bank voles during postnatal development (from 1-180 days of life). It was shown that voles from small litters (1-3 young) do not differ from individuals from large litters (4-6 young) either as to degree of adiposis or the average rate of accumulation of the fat-free components of the body mass (FFB). The animals reached chemical maturity at the age of 55-60 days. The level of FFB composition at the stage of chemical maturity is not a species-specific feature and only characterizes local populations. During the first 6 months of development the calorific value of a unit by weight of the protein and fat in bank voles is constant, being respectively 4.583 and 9.240 cal/g. I. INTRODUCTIONThe large number of studies of Clethrionomys glareolus (S c h r e b e r, 1780), one of the dominant rodents in forest ecosystems in Europe, have contributed to what is known of the features of development and growth of this species in captivity (Sviridenko, 1959; Petrov & Airapet'jans, 1961; Mazak, 1962;Drożdż, 1965) and under natural conditions (Pearson, 1962;N e w s o n, 1963;Bergstedt, 1965;Kubik, 1965; Bujalska & Gliwic z, 1968;Claude, 1970;Crawley, 1970;Zejda, 1971; K a i k u s a 1 o, 1972). These data, however, relate to the biology of the species or to morphological parameters. An exception to this is the study by Sawicka-Kapusta (1974) on the quantitative relations between the main body components in bank voles during the process of growth.Growth is an increase in the mass of the organism, consisting of the growth of tissues of which it is made, and should therefore be considered *) Praca została zrealizowana w ramach problemu węzłowego 09.1.7., koordynowanego przez Instytut Ekologii PAN.[381]