Effects of Climatic Factors on the Field Vole Microtus agrestis

Abstract

The influence of climatic factors on the field vole Microtus agrests (L.) was investigated in the vicinity of Oulu (640 55'N, 250 30'E) in northern Finland. Monthly trapping from 1967 to 1972 gave a total of 1033 individuals. Climatic data from the same time was used for the analysis. The length/weight ratio (B/W) was used as an indicator of the animals' condition (the smaller the ratio the better the condition). Sexual status was indicated by the length of testes in males, and reproductive status in females. Both indices were compared to monthly mean air temperature, temperature at soil surface, coldest night temperature, and precipitation. Multiple regression and correlation analyses (polynomial curves) were used to illustrate possible parallel changes in the parameters. The voles were divided into five size groups. Most individuals fell into the two groups 95. 1-1 10.0, and 110.1-125.0 mm. There was a correlation between B/W ratio of the mean population and changes in climatic factors during late winter and spring (January-May) but no correlation was found during other seasons. The most striking effects on the voles was found in April. The smaller of the two dominating size groups was more sensitive to changes than the larger group. The condition of the mean population was weaker when temperature was lower. Increasing precipitation seemed to have unfavourable effects in April and later. No correlation was found between climatic factors and sexual status of females. The growth of male testes in the smaller size group was more sensitive to climatic changes than in the larger size group. The rate of reaching sexual maturity in males was not significantly correlated with climatic factors.