Bipedal gait versatility in the Japanese macaque (Macaca fuscata) Naomichi Ogihara a, b, * , Eishi Hirasaki c , Emanuel Andrada d , Reinhard Blickhan e a Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi Kohoku-ku, Yokohama 223-8522, Japan b Department of Biological Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-0033, Japan c Primate Research Institute, Kyoto University, Inuyama, Aichi 484-8506, Japan d Institute of Systematic Zoology and Evolutionary Biology with Phyletic Museum, Jena, Germany e Science of Motion, Friedrich-Schiller-University, 07749 Jena, Germany article info Article history: Received 25 January 2018 Accepted 7 September 2018 Keywords: Macaque Grounded running Skipping Running abstract It was previously believed that, among primates, only humans run bipedally. However, there is now growing evidence that at least some non-human primates can not only run bipedally but can also generate a running gait with an aerial phase. Japanese macaques trained for bipedal performances have been known to exhibit remarkable bipedal locomotion capabilities, but no aerial-phase running has previously been reported. In the present study, we investigated whether Japanese macaques could run with an aerial phase by collecting bipedal gait sequences from three macaques on a level surface at self- selected speeds (n ¼ 188). During our experiments, body kinematics and ground reaction forces were recorded by a motion-capture system and two force plates installed within a wooden walkway. Our results demonstrated that macaques were able to utilize a variety of bipedal gaits including grounded running, skipping, and even running with an aerial phase. The self-selected bipedal locomotion speed of the macaques was fast, with Froude speed ranging from 0.4 to 1.3. However, based on congruity, no single trial that could be categorized as a pendulum-like walking gait was observed. The parameters describing the temporal, kinematic, and dynamic characteristics of macaque bipedal running gaits follow the patterns previously documented for other non-human primates and terrestrial birds that use running gaits, but are different from those of humans and from birds' walking gaits. The present study confirmed that when a Japanese macaque engages in bipedal locomotion, even without an aerial phase, it generally utilizes a spring-like running mechanism because the animals have a limited ability to stiffen their legs. That limitation is due to anatomical restrictions determined by the morphology and structure of the macaque musculoskeletal system. The general adoption of grounded running in macaques and other non-human primates, along with its absence in human bipedal locomotion, suggests that abandonment of compliant gait was a critical transition in the evolution of human obligatory bipedalism. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction With the aim of understanding the origin and evolution of hu- man bipedalism, the bipedal locomotion of non-human primates has been experimentally investigated in the field of physical an- thropology for more than half a century. To date, the bipedal locomotion of chimpanzees, bonobos, gibbons, baboons, Japanese macaques, spider monkeys, capuchin monkeys, and sifaka have been studied in terms of spatiotemporal gait parameters, kine- matics, kinetics, electromyography, and energetics, and the obtained results have been compared with those of humans (for a review, see: Schmitt, 2003; D'Août and Vereecke, 2011; for more recent literature not cited in these reviews, see: Thorpe et al., 2004; Kimura and Yaguramaki, 2009; Hirasaki et al., 2010; Ogihara et al., 2010, 2012; Demes, 2011; Demes and O'Neill, 2013; Pontzer et al., 2014; Demes et al., 2015; O'Neill et al., 2015; Druelle et al., 2016). Such experimental studies on bipedal locomotion in extant non- human primates have provided profound insights into possible morphofunctional prerequisites to bipedalism and are helping us to understand the process and consequences of the evolution of the human bipedal locomotion. Based on a spatiotemporal criterion, the presence or absence of an aerial phase (Hildebrand, 1965), bipedal gaits have been classi- fied into walking and running. For a long time, it was supposed that * Corresponding author. E-mail address: ogihara@mech.keio.ac.jp (N. Ogihara). Contents lists available at ScienceDirect Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol https://doi.org/10.1016/j.jhevol.2018.09.001 0047-2484/© 2018 Elsevier Ltd. All rights reserved. Journal of Human Evolution 125 (2018) 2e14