The formation and maintenance of alliances is regarded as one of the most socially complex male mating strategies in mammals. The prevalence and complexity of these cooperative relationships, however, varies considerably among species as well as within and between populations living in different ecological and social environments. We assessed patterns of alliance formation for Indo-Pacific bottlenose dolphins, Tursiops aduncus, in Port Stephens, New South Wales, Australia, to investigate the stability of these alliances, the structure of associations, as well as variation in schooling patterns among males. Our results showed that association patterns among males within this population showed considerable variability. Males either formed strong and enduring alliances that lasted for at least 8 years with minimal partner switching, or less stable partnerships within a much larger male social network. Male alliances with the strongest levels of association within a given time period were significantly more likely to maintain their relationships over the long term compared with alliances with lower levels of association. Males in stable alliances also associated in significantly smaller schools than males who formed less stable alliance partnerships. Finally, we found that alliances consisting of more related males did not persist longer than alliances between unrelated individuals. Our study suggests that intrapopulation variation in male alliance formation in dolphins likely reflects different mating strategies adopted as individual responses to their complex fission–fusion social environment.