The social dynamics of southern resident killer whales and conservation implications for this endangered population K.M. Parsons a, b, * , K.C. Balcomb, III b,1 , J.K.B. Ford c, 2 , J.W. Durban d, 3 a Northwest Fisheries Science Center, National Marine Fisheries Service, Seattle, WA, U.S.A. b Center for Whale Research, Friday Harbor, WA, U.S.A. c Fisheries & Oceans Canada, Pacific Biological Station, Nanaimo, BC d NMML, Alaska Fisheries Science Center, National Marine Fisheries Service, Seattle, WA, U.S.A. article info Article history: Received 1 October 2007 Initial acceptance 31 January 2008 Final acceptance 29 December 2008 Available online xxx MS. number: A10870 Keywords: association conservation endangered killer whale mammal Orcinus orca social dynamic social structure Quantitatively characterizing the social structure of a population provides important insight into the forces shaping key population processes. Moreover, long-term social dynamics provide an avenue for understanding population-level responses to changes in socioecological conditions. This is particularly true for species that show natal philopatry and highly stable hierarchically structured social units, such as the piscivorous resident killer whales of the northeast Pacific. The southern resident killer whale population is a small, demographically closed population, comprising three commonly recognized pods (J, K and L pods), that has recently been listed as endangered throughout its range in both Canadian and U.S.A. waters. In this study, we quantitatively assessed social structure in this population from 29 years of photo-identification data to characterize significant temporal changes in sociality. Preferential affiliation among killer whales within both genealogical matrilines and pods was supported by two different analytical methods and, despite interannual variability, these social clusters persisted throughout the study. All three pods experienced fluctuations in social cohesion over time, but the overall rate of intrapod affiliation was consistently lowest within L pod, the largest of the southern resident pods. The most recent increase in fluidity within social units, occurring in the mid to late 1990s, was coincident with a significant decline in population size, suggesting a possible common response to external stressors. Quantifying these trends in social structure is the first step towards understanding the causes and consequences of long-term changes in killer whale social structure. Ó 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Sociality reflects the balance between the selective forces causing and maintaining social groups, and the detriments asso- ciated with group living (Crook & Gartlan 1966; Alexander 1974; Clutton-Brock & Harvey 1977; Bertram 1978; Lott 1984; Packer et al. 1990; Gowans et al. 2007). The costs of group living include such factors as increased competition and aggression, increased para- sitism or disease transfer, while the benefits include social foraging, group vigilance and cooperative care of young. Social groups develop and persist when the driving forces enhance the fitness of individuals in groups above and beyond that of solitary individuals, despite the inherent costs of living in larger groups. However, sociality evolves under the influence of ecological constraints, which vary across both time and space and can affect both the size and persistence of social units (Crook 1970; Emlen & Oring 1977; Wrangham 1980; Wrangham et al. 1993; Chapman et al. 1995; Wittemyer et al. 2005). The organization of individuals into social groups can be characterized by the empirical assessment of social structure within a population. Social structure can be defined by the pattern of relationships between individuals (Hinde 1976; Whitehead & Dufault 1999; Kappeler & van Schaik 2002). At the most basic level, these rela- tionships are defined by the day-to-day interactions among indi- viduals and, as such, they shape many key population processes. This class of interactions not only affects competition for resources (including mating opportunities) and information transmission (e.g. Whiten 2000; Rendell & Whitehead 2001), learned behaviours (Giraldeau et al. 1994) and disease (Altizer et al. 2003; Cross et al. 2004; Keeling & Eames 2005), but also shapes patterns of gene flow and opportunities for cooperative behaviour and reciprocity * Correspondence and present address: K. M. Parsons, NMML, Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sandpoint Way NE, Seattle, WA 98115, U.S.A. E-mail address: kim.parsons@mail.com (K.M. Parsons). 1 K. C. Balcomb, III is at the Center for Whale Research, Box 1577, Friday Harbor, WA 98250, U.S.A. 2 J. K. B. Ford is at Fisheries & Oceans Canada, Pacific Biological Station, Nanaimo, BC V9T 6N7, Canada. 3 J. W. Durban is at the NMML, Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sandpoint Way NE, Seattle, WA 98115, U.S.A. Contents lists available at ScienceDirect Animal Behaviour journal homepage: www.elsevier.com/locate/yanbe ARTICLE IN PRESS 0003-3472/$38.00 Ó 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2009.01.018 Animal Behaviour xxx (2009) 1–9 Please cite this article in press as: Parsons, K. M., et al., The social dynamics of southern resident killer whales and conservation implications for this endangered population, Animal Behaviour (2009), doi:10.1016/j.anbehav.2009.01.018