Facial asymmetry tracks genetic diversity amongGorillasubspecies

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Abstract
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 threeGorillasubspecies 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.
Funding Information
  • Leakey Foundation
  • 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
  • H2020 Marie Skłodowska-Curie Actions (798117)
  • Smithsonian Collections Care and Preservation Fund
  • Investments for the Future Programme IdEx Université de Bordeaux / GPR Human Past
  • National Geographic Society (8486-08)
  • SYNTHESYS Project http://www.synthesys.info/ which is financed by European Community Research Infrastructure Action under the FP7 "Capacities" Program
  • The Ohio State University President's Postdoctoral Scholar's Program
  • Division of Behavioral and Cognitive Sciences (0964944, 1520221, 1753651, BCS 0852866)

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