Coalitions influence the establishment and maintenance of social relationships among males in primate species. In this study, we compare the social behavior of males between two groups of Alouatta palliata: a group that was recently taken over by a coalition of two males (Mt), and a group that had a stable composition for at least 9 months (Rh). We predicted that coalition partners would be more cooperative and less competitive than dyads formed by immigrant and long-term resident males, and dyads formed by long-term resident males. Additionally, we predicted that these dyadic trends should be reflected in more competition and less cooperation in the group that was taken over. As predicted, the coalition partners of Mt showed the highest levels of cooperation among all dyads and the second lowest rate of agonism. Cooperation was higher in the group that had a stable composition. Results from this study suggest that the social relationships of male mantled howlers vary as a function of familiarity between males and that in the context of coalitionary takeovers, coalitionary males are highly cooperative. Cooperation is lower in groups recently taken over and competition is more intense, perhaps as a consequence of the process of establishment and reorganization of power relationships within some dyads. In the future, we must determine the frequency of coalitionary takeovers in this population and assess its ultimate consequences for male-male social relationships.