Common marmosets are members of the family Callitrichidae, South American primates
characterized by highly social group living and cooperative breeding. In this study we analysed
1112 base pairs (bp) of the mitochondrial control region in 59 Callithrix jacchus
individuals, sampled mainly from two geographically distinct field sites in N.E. Brazil.
Analysis of molecular variation revealed a highly significant genetic structuring of haplotypes
between social groups and between populations. Examination of matrilineal genetic
structure within social groups revealed that seven of nine recorded breeding pairs were
from different maternal lineages, indicating assortative mating and outbreeding. In addition
to the breeders, at least six of 10 groups contained adult individuals from different
matrilines, with five haplotypes present in one social group of nine animals. Groups of mixed
lineages raise questions about potential reproductive conflicts of interest, and the extent of
kin-selected altruism in the evolution and maintenance of cooperative breeding in this species.