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 r eproductive value of male and female offspring differentially. Here, I present data indicating that female Townsend's voles vary the sex rat io of their litters according to the nature of interactions between ph ilopatric females. 2. Most nestlings were marked and sexed before wean ing over three consecutive years in two intensively live-trapped natur al Townsend's vole populations near Vancouver. 3. Birth sex ratios cha nged seasonally when vole density was low (1989, 1991), with more daug hters than sons born in the spring. Similar numbers of males and femal es 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 bin omial distribution when average sex ratio was female biased. 5. Sex ra tio variation was not related to measures of maternal condition, nor t o local variation in vole density as measured by the distance to the n earest breeding female neighbour. 6. Seasonal and density-related chan ges in sex ratio closely tracked changes in the likelihood of young fe males 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 typicall y shared their natal home range with their mother and sisters. Coopera tion between related females born in spring may have enhanced their re productive value. 7. Females born to unbiased litters in the spring of high density or in summer were less likely to reproduce due to compet ition for space with other breeding females. Reduced natal dispersal d istances 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 densi ty. 9. I conclude that cooperation and local resource enhancement betw een related females favoured an increased production of females in spr ings of low density. Global competition for space between all females at high vole density reduced the benefit of producing female-biased li tters in the spring. No data on the reproductive value of sons produce d at different densities were available in this study such that no inf erence of the reproductive value of sons relative to daughters can be made.