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.