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 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.
Highlights
Facial symmetry is widely regarded as a reliable indicator of attractiveness and reproductive success in humans, while asymmetry is often used as a measure of early life stress [1,2]
We investigate three alternative hypotheses to test whether genetic stress, environmental stress, or chewing side preference better correspond with facial asymmetry among gorilla subspecies
Mountain gorillas have relatively broader, more prognathic faces and more rectangular orbits framed by a flat facial asymmetry score (a)
Summary
Facial symmetry is widely regarded as a reliable indicator of attractiveness and reproductive success in humans, while asymmetry is often used as a measure of early life stress [1,2]. As both sides of bilaterally symmetric faces share the same genotype, it is expected that they will exhibit the same phenotype, except when individuals experience instability during development [2]. Studies measuring FA in non-human primate faces have focused on the link between FA and adult fitness or health outcomes [10], and not necessarily the environmental conditions under which individuals developed. Surprisingly little is known about the evolutionary significance of facial asymmetry, including the magnitude of facial FA in extinct hominins and our closest living relatives, the non-human apes [11,12]
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