Die Sommerkrise der Erdmaus (Microtus agrestis L.) Teil I. Die Entwicklung einer freilebenden Population

An analysis of the seasonal cycle of growth and reproduction of the field vole in southern Bavaria in the years 1972, 74 and 75 showed a turnover of development in midsummer indicated by reduced weight increase, restricted reproduction and low fertility. Highest weight increase of 436 marked and recaptured specimens in the field was observed during May and June with 269 mg (335 mg males, 188 mg females) per day and individual (Abb. 1). During August and September a mean weight decrease of –82 mg per animal was measured. The daily weight increase per individual reached 67 mg (74 mg males, 54 mg females) in the average of all observed years.An autopsy of 220 snap-trapped specimens showed a significantly reduced weight of testes (–86%), of stomach content (–22%) and of liver (–18%) in late summer compared to the weight in spring when growth and reproduction were highest and these organ weights reached the maximum level. In the summer crisis also some blood parameters showed significant changes. The erythrocyte counts were reduced by –34% hemoglobin content by –24%, hematocrit by –7%, glucose concentration by –80% and lipid content by –33% in comparison to the mean values observed in December and January when these parameters reached the maximum level. However the spleen size was increased by 140% and the leucocyte counts were augmented by 234% in late summer.A complex of factors influencing the summer crisis of the field vole such as detioration of food supply, increased parasite attack especially of Babesiosis and hormonal changes of growth potencie during the seasonal cycle of development were discussed.

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