REPRODUCTIVE COSTS AND LITTER SIZE IN THE BANK VOLE

The potential reproductive costs for free-ranging bank vole (Clethrion omys glareolus) offspring and mothers were assessed by manipulating li tter size and by determining the effects of nursing varied numbers of offspring. Litter enlargement did not increase the number of weanlings per mother. The mass of juveniles was significantly lower in the enla rged litters and higher in the reduced litters, compared to the contro l group. However, the survival of juveniles from weaning aged three mo nths did not depend on their mass at weaning. Data from a previous stu dy (Mappes et al. 1995) indicated that a higher mass at weaning may in crease juveniles' abilities to maturate and breed during their summer of birth. Manipulation of litter size did not significantly affect the mass or survival of mothers or the success of subsequent breeding. Th e size of home ranges did correlate positively with the initial litter size. However, space use by females did not change with the degree of manipulation. Our results indicate that females nursing enlarged litt ers produce smaller offspring at weaning with no residual effects on f uture maternal survival or reproduction. Mothers did not seem to compe nsate the nursing costs with increased parental effort (which should b e reflected in the condition of mothers or in the use of resources), f or example, in an enlarged size of home range. Probably the possibilit y of obtaining a larger home range is constrained by the other breedin g females, in a saturated breeding population. These results may suppo rt the optimal investment hypothesis that a female will produce a part icular litter size which gives the best reproductive success in the pa rticular environment where offspring are nursed.