Matrilineage and allelic sorting within an expanding population

Genetic analyses, demographic data, and climatic indices were used to examine kin and group structure of wedge-capped capuchin monkeys, Cebus olivaceus, in the Venezuelan llanos. One large and one small group were followed for 23 and 7 years, respectively. Variables included rates of birth, death, male and female individual transfer, and group fission and fusion. Capuchins in both groups and across the region were genotyped at autosomal microsatellite loci, and sequenced at a 485-base pair segment of the mitochondrial control region. The large group's birth rate and matrilineal infant death rate were influenced by annual fluctuation in rainfall levels, which in turn were signalled in part by ENSO cycles. Some low-ranking matrilines went extinct and others fissioned from the group. Of eight initial matrilines in 1977, only the three highest-ranking persisted in 1999. Unique alleles in the group were lost with each matriline. Thus, matrilineage loss and allelic erosion were nonrandom with respect to female rank. Multiple group fissions followed El Nino events. In 1999, members of two matrilines from the large group fused with the small group, introducing new microsatellite alleles. The new females also assumed higher ranks than natal females. Based on the empirical evidence above, matrilineage sorting in the small group likely will remove natal members and unique alleles, and replace them with those of the new females. Across the region, depauperate mitochondrial variability and deficient heterozygosity in autosomal genotypes revealed a recent population expansion. Supported by NSF 9908455, NSF DBI 9602234, and the Wildlife Conservation Society.