LITTER SEX-RATIO VARIATION IN LABORATORY COLONIES OF 2 GEOGRAPHICALLYDISTINCT STRAINS OF THE ROOT VOLE MICROTUS-OECONOMUS

Litter sex ratio at birth was studied in two laboratory colonies of ro ot voles Microtus oeconomus originating from northern and southern Nor way, respectively. Sex ratio was female biased in the northern (57% fe males), but not in the southern colony. Sex seemed to be binomially di stributed within litters of both colonies, and did not show consistent relationships with mother weight, litter size, parity or number of ge nerations bred in the laboratory. Both colonies originated from popula tions exhibiting cyclic population dynamics in the wild. Thus differen t demography as such is unable to explain the difference in sex ratio between the two laboratory colonies. Social organization has been show n to differ between the two strains of root voles both in the field an d in the laboratory, but it is yet unclear whether this factor is rela ted to the sex ratio difference documented in this laboratory study.