Experimental manipulation of breeding density and litter size: effects on reproductive success in the bank vole

1. Reproductive success of individual females may be determined by density- dependent effects, especially in species where territory provides the resou rces for a reproducing female and territory size is inversely density-depen dent. 2, We manipulated simultaneously the reproductive effort (litter size manip ulation: +/- 0 and + 2 pups) and breeding density (low and high) of nursing female bank voles Clethrionomys glareolus in outdoor enclosures. We studie d whether the reproductive success (number and quality of offspring) of ind ividual females is density-dependent, and whether females can compensate fo r increased reproductive effort when not limited by saturated breeding dens ity. 3, The females nursing their young in the low density weaned significantly more offspring than females in the high density, independent of litter mani pulation. 4, Litter enlargements did not increase the number of weanlings per female, but offspring from enlarged litters had lower weight than control litters. 5, In the reduced density females increased the size of their home range, b ut litter manipulation had no significant effect on spacing behaviour of fe males. Increased home range size did not result in heavier weanlings. 6, Mother's failure to successfully wean any offspring was more common in t he high density treatment, whereas litter manipulation or mother's weight d id not affect weaning success. 7, We conclude that reproductive success of bank vole females is negatively density-dependent in terms of number, but not in the quality of weanlings. 8, The nursing effort of females (i.e. the ability to provide enough food f or pups) seems not to be limited by density-dependent factors.