Reproductive trade-offs in the bank vole

The aim of this thesis was to develop an understanding of reproductive tradeoffs using the bank vole Clethrionomys glareolus as a model organism. The studies were mainly carried out as experiments in large outdoor enclosures and a laboratory colony of bank voles was used in quantitative genetic analyses of life-history traits. Litter size manipulations were performed to study the tradeoff between number and quality of offspring and reproductive costs. Neither enlarging nor reducing litter size increased the number of offspring entering the breeding population, and the litter enlargements decreased offspring body mass at weaning. In male offspring lower body mass was maintained over winter until next spring. Future fecundity or survival of mothers was not affected by litter size manipulations. To study ecological factors affecting reproductive success, food resources and breeding density during nursing were manipulated together with litter size in two experiments. The weaning success of females was improved both in lower density and with supplemented food. However, when offspring in enlarged litters suffered from lower mass independent of density, with supplemented food litter enlargements did not result in lower weight of weanlings. Spacing behaviour of females was dependent on the phase of reproductive cycle, food availability and breeding density. The results from both laboratory and field experiments indicate that size at birth predicts breeding success: larger female offspring maturated earlier than smaller ones. Heritability estimates were high both for litter size (h²±se: 0.68±0.22) and body mass at birth (0.77±0.21) indicating high amount of genetic variance in these traits. Both phenotypic and genetic correlations were negative between the number and size of offspring indicating antagonistic pleiotropy between the two traits. The results suggest that resource allocation between the number and size of offspring is an important fitness component for bank vole females. This allocation seems to be determined mainly during pregnancy. In addition, the amount of food available is an important factor determining the reproductive success of bank vole females