The impact of population kin-structure on nestling survival in Townsend's voles, Microtus townsendii

1. In many small rodent species, females are philopatric and may reproduce in proximity to their female relatives. The prevalence of clusters of relatives in populations reflects previous patterns of immigration, local recruitment and mortality. Kin-structuring among females will be most prevalent in populations which have experienced successful reproduction and philopatric recruitment. If population kin-structure influenced key demographic parameters this would introduce a potential time-delay in the regulation of population size by social factors.

2. We tested the influence of population kin structure among female Townsend's voles on nestling survival, using data from a replicated experiment with two non-enclosed populations. Most nestlings were marked before weaning, so that we knew which recruits were related to one another and to which adult female. The average size of matrilineal groups was experimentally manipulated by removing immigrants, and preventing predation by birds, on the high kinship treatments (Kin), and by removing members of family groups on the low kinship treatments (Non-kin). We compared preweaning survival of pups marked in the maternal nest between high and low kinship treatments using the recent generalisations of the Cormack-Jolly-Seber statistical models as implemented in the software SURGE.

3. The manipulation was effective in that the number of breeding females with relatives among the six nearest breeding female neighbour was higher on the high-kinship treatment grids than on the low-kinship treatment grids. Most first degree related females were nearest neighbours and the prevalence of any relatedness relationship among the six nearest neighbours declined with neighbourhood class. The degree of matrilineal relatedness influenced the distance between the nests (and pups) of breeding females; first degree relatives and second degree relatives were, respectively, 1.32 and 1.19 times closer to each other than unrelated females. Unrelated females and more distant relatives had similar internest distances. The relationship between matrilineal relatedness and distance was similar for the six nearest breeding female neighbours and was not influenced by year, season or treatment.

4. The parsimonious capture-recapture models selected included a gradual increase of trappability during the first 5-7 weeks of life and three age-specific survival rates. Survival over the first 2 weeks of life was estimated as a single parameter and was relatively low. Weekly survival probabilities were very high (0.96, SE = 0.012) from week 3-5 and decreased later in life (week 6-15, 0.85, SE = 0.012) possibly as a result of emigration.

5. The experimental manipulation of female kin-structure had a significant influence on the weekly survival probabilities of nestlings during their first 2 weeks of life. Pup weekly survival was higher on the high-kinship treatment (0.76, SE = 0.018) than on the low-kinship treatment (0.67, SE = 0.020).

6. Pairs of first degree relatives among the six nearest neighbours were present, although in different proportion, within both treatments and we asked whether this within-treatment variability in relatedness was reflected in within-treatment variability in early age survival. Estimates of pup survival for the three replicates with the largest sample size suggest that this was the case in 1990 (on both treatments) but not in 1991 (high kinship treatment). This suggests that between-treatment differences probably reflect differences in the prevalence of related and unrelated females rather than the presence of immigrant females on either treatments.

7. A simple age-structured demographic model, using values of juvenile and adult survival measured for populations with high and low kinship, indicate that this factor may have a substantial impact on the rate of growth of microtine populations (predicted monthly multiplication rate lambda = 1.48 vs. 1.72 for low and high kinship, respectively). Thus, the impact of kinship on demographic parameters may introduce powerful time lags in the social regulation on vole populations.