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Mountain gorillas are particularly inbred compared to other gorillas and even the most inbred human populations. As mountain gorilla skeletal material accumulated during the 1970s, researchers noted their pronounced facial asymmetry and hypothesized that it reflects a population-wide chewing side preference. However, asymmetry has also been linked to environmental and genetic stress in experimental models. Here, we examine facial asymmetry in 114 crania from three

Mountain gorillas are particularly inbred compared to other gorillas and even the most inbred human populations. As mountain gorilla skeletal material accumulated during the 1970s, researchers noted their pronounced facial asymmetry and hypothesized that it reflects a population-wide chewing side preference. However, asymmetry has also been linked to environmental and genetic stress in experimental models. Here, we examine facial asymmetry in 114 crania from three Gorilla subspecies using 3D geometric morphometrics. We measure fluctuating asymmetry (FA), defined as random deviations from perfect symmetry, and population-specific patterns of directional asymmetry (DA). Mountain gorillas, with a current population size of about 1000 individuals, have the highest degree of facial FA (explaining 17% of total facial shape variation), followed by Grauer gorillas (9%) and western lowland gorillas (6%), despite the latter experiencing the greatest ecological and dietary variability. DA, while significant in all three taxa, explains relatively less shape variation than FA does. Facial asymmetry correlates neither with tooth wear asymmetry nor increases with age in a mountain gorilla subsample, undermining the hypothesis that facial asymmetry is driven by chewing side preference. An examination of temporal trends shows that stress-induced developmental instability has increased over the last 100 years in these endangered apes. European Union [798117]; Ohio State University President's Postdoctoral Scholars Program, National Geographic Society [8486-08]; National Science Foundation [BCS 0852866, 0964944, 1520221, 1753651]; Leakey Foundation; Smithsonian Collections Care and Preservation Fund; 2010 Max Planck Research Award; French Government; SYNTHESYS Project - European Community Research Infrastructure Action under the FP7 'Capacities' Program Published version This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 798117, The Ohio State University President's Postdoctoral Scholars Program, National Geographic Society (8486-08), National Science Foundation (BCS 0852866, 0964944, 1520221, 1753651), The Leakey Foundation, the Smithsonian Collections Care and Preservation Fund, and the 2010 Max Planck Research Award to Timothy G. Bromage endowed by the German Federal Ministry of Education and Research to the Max Planck Society and the Alexander von Humboldt Foundation. This study received support from the French Government in the framework of the Investments for the Future Programme IdEx Universite de Bordeaux/GPR Human Past, and from the SYNTHESYS Project http://www.synthesys.info, which is financed by European Community Research Infrastructure Action under the FP7 'Capacities' Program.