MARINE MAMMAL SCIENCE, 24(4): 929–940 (October 2008) C  2008 by the Society for Marine Mammalogy DOI: 10.1111/j.1748-7692.2008.00233.x Preliminary analysis of the social structure of killer whales, Orcinus orca, at subantarctic Marion Island CHERYL A. TOSH P. J. NICO DE BRUYN MARTH ´ AN N. BESTER Mammal Research Institute, Department of Zoology & Entomology, University of Pretoria, Pretoria 0002, South Africa E-mail: catosh@zoology.up.ac.za ABSTRACT Studies of social differentiation between populations of killer whales (Orcinus orca) are important due to the cosmopolitan nature of the species, both in terms of distribution and feeding habits. The following research provides preliminary findings describing the social structure of the killer whale, Orcinus orca, population at subantarctic Marion Island. We provide evidence for consistent, observable patterns of social interactions with animals associating and disassociating in nonrandom patterns. We show that the social structure of this population may follow a new pattern of association, displaying a blend of the traditional resident/transient model displayed in the Northern Hemisphere. However, we emphasize the critical need for further studies related to the sociality, biology, and life history of Southern Ocean killer whales. Key words: long-term identikit, Orcinus orca, killer whale, Southern Ocean, social- ity, shore-based observation. Social structure in cetaceans has been extensively described and analyzed (Matkin et al. 1999, Baird and Whitehead 2000, Mann et al. 2000, Gowans et al . 2001, Weinrich et al. 2006), however, it is acknowledged that intraspecific generalizations cannot be made in animals of advanced social structure (Di Fiore and Rendall 1994, Costa and Fitzgerald 1996). An ideal example is the differences in diet, behavior, and social organization of “transient” and “resident” killer whales in the North Pacific (Heimlich-Boran 1988, Bigg et al. 1990, Felleman et al. 1991, Baird et al. 1992, Ford et al. 1998, Baird and Whitehead 2000). It is thus particularly valuable to document the variability in such factors for different populations. The degree of social organization and the ability of animals to “learn” from individuals within the same population have implications for long-term survival, especially in marine environments (Boran and Heimlich 1999, Whitehead 2007a). Knowledge of the social differentiation between different populations of killer whales is vital for 929