Linking individual variation in facial musculature to facial behavior in rhesus macaques

Kimock, CM ORCID: 0000-0001-9560-7427, Ritchie, C, Whitehouse, J ORCID: 0000-0003-2607-5492, Witham, C ORCID: 0000-0003-1182-0074, Tierney, CM ORCID: 0000-0002-0079-9946, Jeffery, N ORCID: 0000-0001-5166-2029, Waller, BM ORCID: 0000-0001-6303-7458 and Burrows, AM ORCID: 0000-0002-9952-8125 (2025) Linking individual variation in facial musculature to facial behavior in rhesus macaques Anatomical Record, 308 (12). pp. 3105-3121. ISSN 1932-8486, 1932-8494

Access the full-text of this item by clicking on the Open Access link.

Abstract

Facial expression is a key component of primate communication, and primates (including humans) have a complex system of facial musculature underpinning this behavior. Human facial musculature is highly variable across individuals, but to date, whether other primate species exhibit a similar level of inter-individual variation is unknown. Whether individual-level variation in facial musculature covaries with significant differences in facial movement within the same individual is also unknown. Here, we use facial dissection data from 31 adult rhesus macaques, the largest sample to date, to quantify inter-individual variation in facial muscle presence. We used a subsample of eight individuals to measure covariation between facial muscle presence and the presence of external facial movements (action units in the Facial Action Coding System, or FACS). We found, in contrast to humans, limited inter-individual variation in muscle presence, but the zygomatic region exhibited more gross anatomical variation in muscle presence and morphology than any other region of the macaque face. We also found a good correspondence between facial muscle presence and the presence of the associated action units. Our results indicate that the observed variation in rhesus macaque facial expressivity is not likely driven primarily by variation in facial muscle presence but may instead be due to other factors such as learned behavior and/or physiological differences. These findings provide insight into the anatomical basis of inter-individual variation in facial behavior in primates and suggest potential differences in variation between humans and other primate species.

Item Type:	Article
Uncontrolled Keywords:	face, facial expression, facial musculature, rhesus macaque
Divisions:	Faculty of Health & Life Sciences Faculty of Health & Life Sciences > Inst. Life Courses & Medical Sciences Faculty of Health & Life Sciences > Education Directorate
Depositing User:	Symplectic Admin
Date Deposited:	24 Apr 2025 08:37
Last Modified:	28 Feb 2026 16:54
DOI:	10.1002/ar.25650
Open Access URL:	https://doi.org/10.1002/ar.25650
Related Websites:	Author Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3191566
Disclaimer:	The University of Liverpool is not responsible for content contained on other websites from links within repository metadata. Please contact us if you notice anything that appears incorrect or inappropriate.