ROOT VOLES - LITTER SEX-RATIO VARIATION IN FRAGMENTED HABITAT

1. Litter sex ratio variation in enclosed populations of two strains of root voles originating from northern and southern Norway was studied in experimentally fragmented habitats during 2. years (1990 and 1991) in the period early July-mid October. 2. Three fragmentation patterns differing with respect to habitat fragment size and interfragment connectivity (corridors) were colonized by voles from either strain in each of the 9 years; yielding two population replicates per strain and fragmentation pattern. Litter affiliation of weanlings was decided from a combination of trapping data, marking of lactating mothers with fluorescent powder and marking of nestlings. 3. Habitat fragmentation had no consistent influence on weanling sex ratio at the population level. However, within populations of both strains, mothers inhabiting a habitat fragment alone produced more female-biased litters than those sharing a fragment with other reproductive females. 4. For the south Norwegian strain, large mothers produced more female-biased litters than small mothers. The southern strain had an overall female-biased weanling sex ratio the first year, probably because reproductive females were generally heavier in 1990 than in 1991. 5. The results are consistent with hypotheses predicting that females should adjust the litter sex ratio according to resource availability and social environment at a local scale, as well as to their own competitive ability. It is argued that adaptive sex ratio variation in Microtus is more probably due to spatial variability of fitness returns from daughters than from sons, because females are the philopatric sex and compete at a smaller spatial scale than males.