This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyrightAuthor's personal copyMale chimpanzees form enduring and equitable social bondsJohn C. Mitani*Department of Anthropology, University of Michiganarticle infoArticle history:Received 29 September 2008Initial acceptance 28 October 2008Final acceptance 18 November 2008Published online 20 January 2009MS. number: A08-00627RKeywords:chimpanzeePan troglodytessocial behavioursocial relationshipControversy exists regarding the nature of primate social relationships. While individual primates arefrequently hypothesized to form enduring social bonds with consp ecifics, recent studies sug gest thatrelationships are labile, with animals interacting only over short periods to satisfy their immediate needs.Here I use data collected over 10 years on a community of chimpanzees, Pan troglodytes, at Ngogo, KibaleNational Park, Uganda, to investigate whether male chimpanzees establish long-term social relationshipsand to determine the factors that affect variation in relationship quality and the stability of social bonds.Kinship and domina nce rank influenced the quality of relationships. Maternal brothers and males of thesame dominance rank class groomed each other more equitably than did unrelated males and males thatwere dissimilar in rank. In addi tion, males that formed strong social bonds groomed more equitably thandid males that displayed weaker bonds. Social bonds were stable over time, with relationships in oneyear predicting those in subsequent years. Kinship and the quality of social relationships affected bondstability. Maternal half siblings and males that groomed each other equitably maintained longer-lastingbonds than did nonkin and males that groomed each other unevenly. Virtually all of the males estab-lished at least one enduring relationship with another individual. The most enduring bonds formedbetween a few pairs of maternal brothers and dyads that maintained balanced grooming interactions.These results indicate that male chimpanzees maintain long-lasting and equitable social bonds whoseformation is affected by maternal kinship and the quality of social relationships.Ó 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.Primates are unusually gregarious mammals, which typicallyinteract repeatedly with individually recognizable conspecifics inrelatively stable social groups (Smuts et al. 1987). As a result, theirsocial relationships have been subject of intensive study for overa generation (Hinde 1983; Cheney et al. 1986; Silk 2007). Adoptingan influential framework developed by Hinde (1979), early field-work described primate social relationships with regard to whatindividuals do with each other, how they do it, and how ofteninteractions occur. These studies provided data on the content,quality, relative frequency and patterning of interactions betweenindividuals (e.g. Seyfarth 1976; Harcourt 1979; Dunbar 1983).Additional research suggested that kin and unrelated individualscultivate relationships to obtain adaptive benefits (Kummer 1978).For example, maternally related females in many Old Worldmonkey species form long-term alliances to acquire and maintaintheir dominance rank (Kapsalis 2004). High rank in turn correlatespositively with several indirect measures of fitness (Harcourt 1987).Recent observations now indicate that female baboons that formstrong social bonds with others reproduce more than do femalesthat develop weaker bonds (Silk et al. 2003). Similarly, femalebaboons derive fitness benefits including protection againstinfanticide and conspecific aggression by establishing long-termfriendships with unrelated males (Smuts 1985; Palombit et al.1997). As a result of this long history of research, primate socialrelationships are frequently depicted as differentiated andenduring, and as having important reproductive consequences(Hinde 1983; Cheney et al. 1986; Silk 2007).Recently, this canonical view of primate social relationships hasbeen challenged. In a series of thought-provoking papers, Barrettand colleagues (Henzi & Barrett 1999, 2007; Barrett & Henzi 2002;Barrett et al. 2007) have argued that primates typically do not formlong-term relationships with conspecifics. Instead, they suggestthat primate social relationships ‘. need not, and probably do not,take the long-term, temporally consistent form that has beenattributed to them .’ and that ‘. short-term contingent responseto current need, may provide a more satisfactory evolutionaryaccount of . coexistence .’(Henzi & Barrett 2007, page 73). Tosupport this claim, they furnish observations of female chacmababoons that displayed unstable grooming and proximity rela-tionships over a 4-year period (Barrett & Henzi 2002). Citing this asevidence, they conclude that primates are short-term ‘. ‘‘businesspartners’’, not friends’ (page 274).Resolving these different views is difficult because surprisinglyfew data exist regarding the stability of primate social relationships*Correspondence: J. C. Mitani, 1085 South University Avenue, Department ofAnthropology, University of Michigan, Ann Arbor, MI 48109-1107, U.S.A.E-mail address: [email protected] lists available at ScienceDirectAnimal Behaviourjournal homepage: www.elsevier.com/locate/yanbe0003-3472/$38.00 Ó 2008 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.anbehav.2008.11.021Animal Behaviour 77 (2009) 633–640Author's personal copy(but see Silk et al. 2006). Chimpanzees, Pan troglodytes, providea model system to investigate whether primates form enduringsocial bonds. Male chimpanzees in particular are extremelygregarious and engage in a variety of affiliative and cooperativebehaviours, including association, grooming, proximity,