Genetic relationships in a western lowland gorilla population Sylvain Gatti,1 Florence Levréro,1 Eric Petit,1 Annie Gautier-Hion,1 and Nelly Ménard1 1Centre National de la Recherche Scientifique, Ethologie-Evolution-Ecologie, Station Biologique de Paimpont, Université de Rennes, France Abstract This study examines the genetic relationships within the western lowland gorilla population visiting the Lokoué clearing at Odzala National Park, Republic of Congo. During 17 months we have identified 37 breeding groups, 8 non-breeding groups (devoid of adult females) and 31 solitary males. Genetic relatedness analyses were conducted on adults and immatures from 22 breeding groups, 5 non-breeding groups and 5 solitary males using 8 microsatellites and a sex marker. Nuclear DNA was extracted from faeces. Kinship results are consistent with a scenario in which silverbacks monopolize the reproduction within their breeding groups during tenure of up to eleven years. Some females sire several offspring within a same group. Some subadult females were neither related to the silverback nor to the adult females of their group, which suggests that females may transfer early from their natal group. Most of the adult females that belong to a breeding group were unrelated while they can be related to females of other groups. Similarly, males from a group can be related to several other males of distinct units. Non-breeding groups that are mainly composed of immature males comprised both clusters of related and unrelated individuals. The pattern of dispersal that is characterized by an early natal dispersal of immatures and the dispersal of both sexes lead to the weak genetic structuring observed at this local population level.