Social network correlates of food availability in an endangered population of killerwhales, Orcinus orca

Keywords: association chinook salmon clustering foraging group living killer whale Orcinus orca prey social dynamics social organization For the majority of social species, group composition is dynamic, and individuals are interconnected in a heterogeneous social network. Social network structure has far-reaching implications for the ecology of individuals and populations. However, we have little understanding of how ecological variables shape this structure. We used a long-term data set (1984e2007) to examine the relationship between food availability and social network structure in the endangered southern resident killer whales. During the summer months individuals in this population feed primarily on chinook salmon, Oncorhynchus tshawytscha, which show annual variation in abundance. We tested the hypothesis that temporal variation in chinook salmon will correlate with variation in social network structure. Using a null model that controlled for population demography, group size and sampling effort, we found a significant relationship between the connectivity of the social network and salmon abundance, with a more interconnected social network in years of high salmon abundance. Our results demonstrate that resource availability may be an important determinant of social network structure. Given the central importance of the social network for population processes such as the maintenance of cooperation and the transmission of information and disease, a change in social network structure caused by a change in food availability may have significant ecological and evolutionary consequences. For the vast majority of social species, group composition is dynamic and individuals move between social groups leading to a highly interconnected social network. Who interacts with whom and the local and global population social structures have implications for central issues in ecology and evolution (Krause & Ruxton 2002; Croft et al. 2008). For example population social structure is central to the way animals exploit their environment Cross et al. 2004). Describing the social structure of populations and unravelling the mechanisms and ecological factors underpinning this is therefore a key research focus in ecology and evolution. Insights into the evolution of sociality have been gained by analysing social structure based on group size and composition, particularly by comparing traits among populations (or species) living under different ecological conditions (Crook 1965; Jarman 1974; Seghers 1974). This body of work has demonstrated that an important determinant of population social structure is the distribution and availability of resources. For example, the size of the group is often limited by the quality and quantity of food available (Caraco & …