Craniofacial fluctuating asymmetry in gorillas, chimpanzees, and macaques

Abstract

Objectives: Craniofacial fluctuating asymmetry (FA) refers to the random deviations from symmetry exhibited across the craniofacial complex and can be used as a measure of developmental instability for organisms with bilateral symmetry. This article addresses the lack of data on craniofacial FA in nonhuman primates by analyzing FA magnitude and variation in chimpanzees, gorillas, and macaques. We offer a preliminary investigation into how FA, as a proxy for developmental instability, varies within and among nonhuman primates. Materials and Methods:We generated 3D surface models of 121 crania from Pan troglodytes troglodytes, Gorilla gorilla gorilla, and Macaca fascicularis fascicularis. Using geometric morphometric techniques, the magnitude of observed FA was calculated and compared for each individual, sex, and taxon, along with the variation of FA across cranial regions and for each bilateral landmark.Results: Gorillas and macaques exhibited higher and more similar magnitudes of FA to each other than either taxon did to chimpanzees; variation in magnitude of FA followed this same trend. No significant differences were detected between sexes using pooled data across species, but sex did influence FA magnitude within taxa in gorillas. Further, variation in FA variance across cranial regions and by landmark was not distributed in any particular pattern. Conclusion:Possible environmentally induced causes for these patterns of FA magnitude include differences in growth rate and physiological stress experienced during life. Developmental stability may be greatest in chimpanzees in this sample. Additionally, these results point to appropriate landmarks for future FA analyses and may help suggest more urgent candidate taxa for conservation efforts.

Skeletal age during hurricane impacts fluctuating asymmetry in Cayo Santiago rhesus macaques

“…Distribution of the data can be observed in Figure 3, where the frequency of FA scores is shown in a histogram (A) and the distribution of FA scores is illustrated by sex (B) and skeletal maturity at death (C). While mean FA is not comparable to other studies (because each Procrustes superimposition is unique), the variance and standard error here are slightly lower than those reported for Macaca fascicularis in Romero et al (2022). This could be because the sample size in our study is much larger and thus provides a more accurate reflection of species‐level variation.…”

“…These results support earlier findings that sex has little influence on FA in the Cayo Santiago macaque population (Hallgrímsson, 1999), which aligns with many studies on FA across animal clades (Caccavo et al, 2021;Castilheiro et al, 2022;Hallgrímsson, 1993;Hopton et al, 2009;Landi et al, 2021;Van Dongen, 2015;Wauters et al, 1996) but does not align with a handful of studies in humans (Schlager & Rüdell, 2015), gorillas (Romero et al, 2022), olive baboons (Romero et al, unpublished data), lizards (Simbula et al, 2021), and shrews (Badyaev et al, 2000). These studies used a variety of data collection methods (e.g., caliper measurements, 2D photographs, 3D landmark patches) and measured different body components (e.g., mandibles, crania, femoral pores), making consistency impossible and comparisons relatively difficult.…”

“…In addition to age, other demographic variables-most notably sex-have been hypothesized to impact skeletal FA, with varying degrees of support. Measurements of cranial FA in humans and nonhuman primates have largely yielded no sex-specific patterns (e.g., Hallgrímsson, 1993Hallgrímsson, , 1999Van Dongen, 2015), with two notable exceptions: an increase in osseous nasal FA in male humans from multiple populations as compared to females (Schlager & Rüdell, 2015) and a similar increase in overall cranial FA in male gorillas relative to females (Romero et al, 2022). Beyond primates, sex was not observed to drive differences in FA within either hurricane-affected or control-group deer mice (Hopton et al, 2009), nor in red squirrels occupying either disturbed or undisturbed woodland habitats (Wauters et al, 1996).…”

Temporomandibular joint shape in anthropoid primates varies widely and is patterned by size and phylogeny

“…This variability is most likely a result of variation in food bolus size, shape, and material properties throughout the chewing cycle, and is also likely related to inter‐individual variation in the morphology of the muscles, TMJ, and teeth (Ross and Iriarte‐Diaz, 2014). One additional source of this variation could also be fluctuating asymmetry across the cranium and/or mandible (e.g., Romero et al, 2022).…”

Facial asymmetry tracks genetic diversity amongGorillasubspecies

“…As such, the possible stressors behind facial FA beyond the classical ‘environmental or genetic' dichotomy remain poorly understood. Moreover, 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 ].…”