Breeding tactics and reproductive success in the bank vole

Abstract

The aim of this thesis was to study what kinds of breeding tactics bank vole Clethrionomys glareolus females will adopt in relation to the particular ecological environments where they live. We also studied how successful the adopted tactics were and what were the main mechanisms which determined the success. The determinants of optimal litter size were studied by manipulating litter size. Nursing different sized litters affected the success of the present reproduction, only. Increased litter size did not cause survival or fecundity costs for breeding females. Further, home range size seemed to determine the initial litter size of females, but the females did not compensate for the enlarged demand of effort by changing their space use. This may indicate that litter size is regulated by space competition of territorial females in the saturated breeding populations. Territorial behaviour was examined in relation to female reproductive state. Home range overlap decreased and territory size increased from non-pregnancy to nursing, though home range size did not change. Attacks against intruder females increased and amicable behaviour decreased as the time for delivery got closer. The territorial behaviour also seemed to correlate positively with home range and territory size of females. These results indicate that competition for space is most intensive in the late part of breeding, especially during nursing. The effects of kinship on spacing behaviour, breeding success and survival were studied with experimental populations established by related (R) and unrelated females (UR). The R populations grew twice as fast as the UR ones. The breeding success of UR females decreased with distance to the the nearest neighbour. The home ranges of UR females overlapped less and were smaller if they were situated close to each other. Further, juveniles seemed to avoid territories of adult females and their survival declined with dispersal distance from the natal territory in the UR populations. These results indicate that space competition is more intense among UR females decreasing their reproductive success. Infanticide towards UR nestlings and juveniles is one possible mechanism causing lower recruitment of young in this study. In an experiment where predation risk and the density of voles were manipulated, under high predation risk both old and young females suppressed breeding, while density did not affect overall breeding. Both risk factors appeared to increase litter sizes. In risky environments females seemed to choose between two opposite tactics: They suppressed breeding, which may increase their own survival to the next breeding or they continued to breed in spite of expected higher survival costs. Females seemed to compensate for the increased survival costs with a higher effort to the current reproduction.