COST OF REPRODUCTION IN THE WILD: MANIPULATION OF REPRODUCTIVE EFFORT IN THE BANK VOLE

Abstract

For three years, we manipulated litter size by adding or subtracting pups in eight wild populations of the bank vole, Clethrionomys glareolus, to examine reproductive costs and allocation of reproductive effort between offspring number and size. In general, litter enlargements did not increase the number of weanlings per mother and significantly decreased the size of weanlings. Reproductive effort and the breeding success of individuals varied within breeding seasons, but time of breeding and litter manipulation did not interact to affect reproductive trade-offs. Our 3-yr field experiment revealed that litter enlargements also reduced survival and fecundity of mothers. Small mammals have been considered typical income breeders, in which potential reproductive costs may be masked due to their ability to compensate for increased energetic demands of reproduction. Our results provide evidence that, in the wild, females may be incapable of escaping the costs of reproduction. Corresponding Editor: E. J. Heske.