SEX-RATIO VARIATION IN RELATION TO FEMALE PHILOPATRY IN TOWNSEND VOLES

1. The over-production of either sex may be favoured when competitive or cooperative social interactions between individuals influence the reproductive value of male and female offspring differentially. Here, I present data indicating that female Townsend's voles vary the sex ratio of their litters according to the nature of interactions between philopatric females.

2. Most nestlings were marked and sexed before weaning over three consecutive years in two intensively live-trapped natural Townsend's vole populations near Vancouver.

3. Birth sex ratios changed seasonally when vole density was low (1989, 1991), with more daughters than sons born in the spring. Similar numbers of males and females were produced at all times when density was relatively high (1990).

4. Female voles produced litters with 'precise' sex ratio in springs of low vole density without reducing their total reproductive output. Litters were more consistently female-biased than expected under a binomial distribution when average sex ratio was female biased.

5. Sex ratio variation was not related to measures of maternal condition, nor to local variation in vole density as measured by the distance to the nearest breeding female neighbour.

6. Seasonal and density-related changes in sex ratio closely tracked changes in the likelihood of young females reproducing in their natal range. Females born to female-biased litters in spring experienced little competition for space, they were most likely to reproduce when their mother was alive and they typically shared their natal home range with their mother and sisters. Cooperation between related females born in spring may have enhanced their reproductive value.

7. Females born to unbiased litters in the spring of high density or in summer were less likely to reproduce due to competition for space with other breeding females. Reduced natal dispersal distances in the spring of high density may have increased competition among related females.

8. Female-biased litters produced in springs of low density yielded twice as many philopatric-breeding daughters per litter than unbiased litters produced in the spring of high vole density.

9. I conclude that cooperation and local resource enhancement between related females favoured an increased production of females in springs of low density. Global competition for space between all females at high vole density reduced the benefit of producing female-biased litters in the spring. No data on the reproductive value of sons produced at different densities were available in this study such that no inference of the reproductive value of sons relative to daughters can be made.