Reproductive effort of female bank voles in a risky environment

In cyclically fluctuating vole populations, strong intraspecific competition and intense predation simultaneously or separately increase the costs of reproduction and so may set the framework for the optimal breeding tactic of voles. In a factorial experiment, we manipulated two factors in the breeding environment of bank vole (Clethrionomys glareolus) pairs, each with two treatment levels: no predation risk or high risk of specialist predators and low or high density of voles. In the manipulation, we used odours of conspecific voles and/or mustelid predators. Both over-wintered and young, summer-born, wild-caught bank vole females and males in breeding condition were used. Each female-male pair of voles was placed in a cage and the cages were distributed in large outdoor enclosures. All animals were fed an libitum. Under predation risk, both old and young females suppressed breeding significantly. The density of conspecific voles did not affect overall breeding. However, there was some evidence that population density stimulated breeding of old females but suppressed breeding of young ones. Both risk factors appeared to increase litter sizes of those individuals who 'decided' to breed. Our results indicate that the risk of predation may be an important factor determining reproductive tactics of bank vole females. In risky environments, females seemed to choose between two totally opposite tactics: they suppressed breeding, which may increase their own survival to the next breeding event, or they continued to breed in spite of expected high survival costs. Females seemed to compensate the latter costs with a higher effort to the current and probably the last reproduction.