Received: 4 September 2017 | Revised: 17 January 2018 | Accepted: 1 April 2018 DOI: 10.1002/ajp.22761 RESEARCH ARTICLE Evidence for paternal kin bias in the social affiliation of adult female blue monkeys Marina Cords 1,2 | Taylor Minich 1 | Su-Jen Roberts 1,2,3 | Clio Sleator 4 1 Department of Ecology, Evolution and Environmental Biology, Columbia University, New York, New York 2 New York Consortium in Evolutionary Primatology, New York, New York 3 Department of Education, Wildlife Conservation Society, Bronx, New York 4 Department of Physics, University of California, Berkeley, California Correspondence Marina Cords, Department of Ecology, Evolution and Environmental Biology, Columbia University, 10th Floor Schermerhorn Extension, 1200 Amsterdam Avenue, New York, NY 10027. Email: mc51@columbia.edu Funding information American Association for the Advancement of Science; University of California Research Expeditions Program; Columbia University; Ford Foundation; Harry Frank Guggenheim Foundation; Leakey Foundation; National Science Foundation, Grant numbers: SBE 95- 23623, BCS 98-08273, BCS 05-54747, BCS 10-28471, DGE 03-3415, DGE 09-66166, 2 GRFs; Wenner-Gren Foundation If animals increase inclusive fitness by cooperating with relatives, nepotism should involve maternal and paternal kin equally, all else being equal. Evidence of a behavioral bias toward paternal half-siblings in primates is both limited and mixed, with most positive reports from papionins. To expand knowledge of paternal kin recognition, particularly in cercopithecine monkeys, we examined evidence for paternal kin bias in wild blue monkeys (Cercopithecus mitis), a species living mostly in one-male groups. Seasonal breeding and the amount of male reproductive skew in blue monkeys suggests that opportunities to distinguish paternal kin are plentiful, and their social system would make such discrimination beneficial. We compared spatial association and social contact (grooming and contact-sitting) of 20 adult females with at least one paternal half-sibling and at least one non-relative that were present at the same time. We used two data sets, one in which social partners were other parous females, the other in which they were juveniles. Data came from a 7-year period. When interacting with other adult females, subjects groomed and sat in contact with paternal half- siblings significantly more than with known non-kin, and there was a similar trend for spatial association. We detected no paternal kin bias in interactions with juvenile partners. Kin-biased affiliative contact with adult female partners did not appear to be based on age proximity, measured by birth cohort. The study species’ social system suggests phenotype matching as the most likely alternative mechanism, though we could not test it directly. Across both behaviors, there was no significant relationship between the number of matrilineal kin a subject had and the degree to which she preferred paternal half-siblings over non-kin as affiliative partners. These findings contribute to a comparative understanding of paternal kin recognition in primates. KEYWORDS kin discrimination, kin selection, kinship, nepotism, social behavior 1 | INTRODUCTION In many animals including primates, cooperative acts, which benefit others, are biased toward kin. If actors increase their inclusive fitness by cooperating with relatives, such nepotism should involve maternal and paternal kin equally, all else being equal. All else may not be equal, however, if maternal versus paternal kin are not equally available or distinguishable, or if their presence or behavior as social partners benefits the cooperative actor differentially (Berman, 2011; Chapais & Belisle, 2004). The importance of partner availability is implicit in the common observation that sex-biased natal dispersal correlates with sex-biased nepotism (Langergraber, 2012). When females are philopatric, as in most cercopithecine monkeys, groups comprise one or more matrilines, with many opportunities for cooperative Am J Primatol. 2018;e22761. wileyonlinelibrary.com/journal/ajp © 2018 Wiley Periodicals, Inc. | 1 of 8 https://doi.org/10.1002/ajp.22761