Facial musculature in the rhesus macaque (Macaca mulatta): evolutionary and functional contexts with comparisons to chimpanzees and humans.
Level 5 - mechanism / opinion, no new human data
non-human anatomical dissection study (bench/animal research)
PubMed 19563473 · doi:10.1111/j.1469-7580.2009.01113.x
What was done
Formalin-fixed faces from six adult male rhesus macaques (Macaca mulatta) were dissected using a novel dissection technique. Researchers examined muscle morphology, attachments, three-dimensional relationships, and variability, comparing these anatomical structures to known musculature in chimpanzees (Pan troglodytes) and humans.
What was found
The authors identified 24 distinct facial muscles in rhesus macaques, revealing greater muscular complexity than previously documented. This included variably present muscles not previously reported in this species: zygomaticus minor, levator labii superioris alaeque nasi, depressor septi, anterior auricularis, inferior auricularis, and depressor supercilii. Musculature closely resembled chimpanzee and human anatomy, though Macaca mulatta lacked a risorius muscle.
Why it matters
The findings demonstrate that complex facial musculature is not restricted to hominoids, aligning anatomical structure with the intricate facial communication repertoire observed in macaques. This informs evolutionary models of primate communication and biomedical studies using macaques to model human facial movement.
Limits
The study relied on a very small sample size of six formalin-fixed specimens, restricted entirely to adult males, precluding assessment of sex-based variation. Living functional movement, innervation dynamics, and tissue biomechanics were not directly tested.
Cited by
- contradicts Great apes have facial musculature and control similar to humans, whereas monkeys and dogs lack the musculature to produce those types of facial expressions.