The social behaviour of individuals within group living species is a topic of great interest for behavioural ecologists, as interactions between conspecifics can fluctuate between affiliative and aggressive in response to various social and ecologicalfactors.Aggressivebehaviourisprevalentacrosstheanimalkingdom, often arising as a result ofcompetition for valuable resources and can have a profoundimpactonsurvivalandreproduction.Examiningwhyconflictarisesand how it is mitigated is crucial for understanding how individuals gain the benefits associated with sociality. Intraspecific aggression is widely studied in terrestrial mammals; however, for cetacean species, gaps in our knowledge about the frequency and intensity of aggressive interactions remain,dueto the difficulty in directly observing social interactions in many marine mammals. Southern resident killer whales (Orcinus orca) are one of the most studied populations of killer whales in the world, with decades of individual-based data collected on them, providing a unique opportunity to study drivers of aggressive social interactions. In this thesis, using a longitudinal dataset spanning 37 years of photographic images and tooth rake marks as an indicator of social interactions, I investigate demographic, ecological and social drivers of socially acquired marksinsouthernresidentkillerwhales.Specifically,Iexamine patterns of tooth rake density across age and sex classes and find rake density to be highest in young whales, decreasing across the lifespan. I also investigate the influence of their primary food source, Chinooksalmon (Oncorhynchus tshawytscha), on rake density, and observe an increase in tooth rake density across the population as the abundance of salmon increases. Finally, I explore how the composition of the social environment impacts rake density and present an important finding which suggests that postreproductivefemales mayreducethe intensity of social contact 3events between individuals within their social group. Although this effect is seen in both sexes, a stronger effect is observed in males. Furthermore, I observe subadult males to exhibit a higher density of rakes when there aremore adult males present in their social environment, suggesting that adult males may be responsible for producing these marks. This thesis presents the first comprehensiveanalysisofsocialandecologicaldriversoftoothrakedensityina long-lived mammal, revealing the life-history stages at which individuals may face aggression from conspecifics, and how these whales might be organised to moderateconflict.Whilstthesefindingsaddressimportantquestionsaroundkiller whale social behaviour, I suggest that further research, utilising unmanned aerial vehicles would be pertinent to further elucidate the complex social lives of this endangeredpopulation.