This thesis tested hypotheses related to the dispersal, behaviour patterns and response to forest fragmentation of two endangered colobus monkey species living in sympatry in Cantanhez National Park, Guinea Bissau. Western black-and-white colobus (BWC: Colobus polykomos) and Temminck’s red colobus (TRC: Procolobus badius temminckii) are two forest dwelling primates that share most of their ecological requirements but exhibit contrasting social systems, namely in dispersal, group size and social organization. By combining behavioural data obtained for one social group of each species and non-invasive genetic data (15 microsatellite loci and a fragment of the mitochondrial control region) from eight black and white colobus and six red colobus social groups, I examined: i) historical and current dispersal patterns; ii) the within-group distribution of social interactions among males and females, and iii) the effect of forest fragmentation on genetic structure. I found evidence for historical and/or long-range dispersal via males in BWC and via females in TRC. However, a change in the current dispersal pattern was detected for BWC, as both sexes seem to be dispersing. Behavioural analysis showed that TRC females exhibit stronger social bonding than BWC females. More interestingly, and contrary to what was described for the species, TRC females seemed to prefer to engage in grooming other females rather than males and males only rarely groomed other males. Finally, analysis of genetic structure indicates the existence of only one genetic unit for each species, although some fine-scale spatial genetic structure was found for TRC. Whereas BWC seemed to be able to use the available forest corridors to disperse between forest patches, TRC females tend to disperse to immediately adjacent groups showing some constraint in the ability to disperse throughout the park. I hypothesise that the detected changes in dispersal mode in BWC and social dynamics in TRC may constitute behavioural local responses to habitat degradation. Constraints in dispersal found for TRC support the evidence that forest fragmentation should be playing an important role shaping these colobus monkey social systems.