DETERMINANTS OF GEOGRAPHIC VARIATION IN GROWTH AND REPRODUCTIVE TRAITS IN THE ROOT VOLE

I studied growth rate and reproduction in two geographically distinct strains of root voles, Microtus oeconomus, in the laboratory to unravel the mechanisms underlying the observed increase in body size from south to north among European microtine rodents. In the field, adult northern root voles are 51% heavier than southern. Many proximate and ultimate mechanisms underlying this geographic pattern have been suggested, but none has been adequately tested. I measured offspring growth rates in one northern and one southern strain over three consecutive laboratory generations, employing two separate experimental protocols (cross-mating and cross-fostering) to control for differential maternal investment. Paternal investment in the young was evaluated in separate tests. Northern root voles were larger at birth and grew faster than southern voles. Litter size and litter mass at birth were largest in the southern strain, when maternal body mass was adjusted for, indicating a larger prenatal reproductive expenditure in southern females. Cross-fostering and cross-breeding experiments demonstrated that the north-south difference in body mass is of genetic origin. Postnatal growth rate of cross-bred litters was intermediate between the two purebred strains, thus being consistent with an additive genetic mechanism. In contrast, neonatal mass of offspring from cross-bred pairs was similar to that of the southern strain, consistent with southern genetic dominance. Cross-fostering between the two strains showed that parental investment had no discernible effects on postnatal growth rate, although southern fathers showed more parental care than did northern fathers. Together, the cross-fostering and the cross-breeding experiments suggest that the growth rate of southern root voles was con- strained by their own genotype and not by the characteristics of their mothers. Low en- vironmental predictability in southern habitats and high productivity of northern habitats probably result in different selection pressures on growth rates and reproductive effort.