Sexual dimorphism of body size in an African fossil ape, Nacholapithecus kerioi Yasuhiro Kikuchi a, * , Masato Nakatsukasa b , Hiroshi Tsujikawa c , Yoshihiko Nakano d , Yutaka Kunimatsu e , Naomichi Ogihara f , Daisuke Shimizu g , Tomo Takano h , Hideo Nakaya i , Yoshihiro Sawada j , Hidemi Ishida k a Division of Human Anatomy and Biological Anthropology, Department of Anatomy and Physiology, Faculty of Medicine, Saga University, Saga 849-8501, Japan b Laboratory of Physical Anthropology, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan c Department of Rehabilitation, Faculty of Medical Science and Welfare, Tohoku Bunka Gakuen University, Miyagi, 981-8551, Japan d Laboratory of Biological Anthropology, Graduate School of Human Sciences, Osaka University, Osaka 565-0871, Japan e Department of Business Administration, Faculty of Business Administration, Ryukoku University, Kyoto, 612-8577, Japan f Department of Biological Sciences, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan g Faculty of Nursing and Rehabilitation, Chubu Gakuin University, Seki, Gifu, 504-0837, Japan h Japan Monkey Centre, Aichi 484-0081, Japan i Department of Earth and Environment Science, Faculty of Science, Kagoshima University, Kagoshima, 890-0065, Japan j Shimane University, Shimane 690-8504, Japan k Kyoto University, Kyoto 606-8502, Japan article info Article history: Received 9 February 2018 Accepted 11 July 2018 Available online xxx Keywords: Fossil hominoid Miocene Sexual dimorphism Social structure Africa abstract Sexual size dimorphism in the African fossil ape Proconsul nyanzae (18 million years ago, 18 Ma) has been previously documented. However, additional evidence for sexual dimorphism in Miocene hominoids can provide great insight into the history of extant hominoid mating systems. The present study focused on body mass (BM) sexual dimorphism in Nacholapithecus kerioi from the Middle Miocene (16e15 Ma) in Africa. Bootstrap analysis revealed that P. nyanzae BM sexual dimorphism was lower than that in Pan troglodytes, which exhibits moderate sexual dimorphism, as reported previously. The same simulation revealed that BM sexual dimorphism of N. kerioi was comparable with that in Gorilla spp.; i.e., the males were approximately twice as large as the females. High sexual dimorphism in extant apes is usually indicative of a polygynous social structure (gorilla) or solitary/fission-fusion social system (orangutan). However, because of the high proportion of adult males in this fossil assemblage, the magnitude of dimorphism inferred here cannot be associated with a gorilla-like polygynous or oranguran-like solitary/ fission-fusion social structure, and may reflect either taphonomic bias, or some other social structure. Extant hominoids have a long evolutionary history owing to their deep branching, comprising only a few existing members of the original highly successful group. Therefore, it is not surprising that the mating systems of extant hominoids fail to provide fossil apes with a perfect “model”. The mating systems of extinct hominoids may have been more diverse than those of extant apes. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Among extant primate groups, hominoids are unusual in having highly diverse social structures despite a limited number of species. Hylobatids are monogamous (single-male single-female: Tenaza, 1975; Mitani, 1984, 1987; Palombit, 1996; Brockelman et al., 1998; Bartlett, 2008); chimpanzees and bonobos are polygynandrous (multi-male multi-female: Goodall, 1986; Fleagle, 1999; Nishida, 2011); orangutans have a (semi-) solitary social system (territory of one male covers that of several females) or fission-fusion society (Galdikas, 1985, 1988; van Schaik, 1999; Delgado and van Schaik, 2000; Setia et al., 2009; Singleton et al., 2009); gorillas are usually polygynous (single-male multi-female), but groups of mountain gorillas frequently include more than two silverbacks in Bwindi and Virungas, Congo (Yamagiwa, 1987; Yamagiwa et al., 1993, 2009; * Corresponding author. E-mail addresses: kikuchiy@cc.saga-u.ac.jp (Y. Kikuchi), nakatsuk@anthro.zool. kyoto-u.ac.jp (M. Nakatsukasa), ishida-h@grace.ocn.ne.jp (H. Ishida). Contents lists available at ScienceDirect Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol https://doi.org/10.1016/j.jhevol.2018.07.003 0047-2484/© 2018 Elsevier Ltd. All rights reserved. Journal of Human Evolution xxx (2018) 1e12 Please cite this article in press as: Kikuchi, Y., et al., Sexual dimorphism of body size in an African fossil ape, Nacholapithecus kerioi, Journal of Human Evolution (2018), https://doi.org/10.1016/j.jhevol.2018.07.003