Muscles of facial expression in the chimpanzee (Pan troglodytes): descriptive, comparative and phylogenetic contexts

Abstract

Facial expressions are a critical mode of non-vocal communication for many mammals, particularly non-human primates. Although chimpanzees ( Pan troglodytes ) have an elaborate repertoire of facial signals, little is known about the facial expression (i.e. mimetic) musculature underlying these movements, especially when compared with some other catarrhines. Here we present a detailed description of the facial muscles of the chimpanzee, framed in comparative and phylogenetic contexts, through the dissection of preserved faces using a novel approach. The arrangement and appearance of muscles were noted and compared with previous studies of chimpanzees and with prosimians, cercopithecoids and humans. The results showed 23 mimetic muscles in P. troglodytes , including a thin sphincter colli muscle, reported previously only in adult prosimians, a bi-layered zygomaticus major muscle and a distinct risorius muscle. The presence of these muscles in such definition supports previous studies that describe an elaborate and highly graded facial communication system in this species that remains qualitatively different from that reported for other non-human primate species. In addition, there are minimal anatomical differences between chimpanzees and humans, contrary to conclusions from previous studies. These results amplify the importance of understanding facial musculature in primate taxa, which may hold great taxonomic value.

A Cross-species Comparison of Facial Morphology and Movement in Humans and Chimpanzees Using the Facial Action Coding System (FACS)

“…However, initial attempts to produce a basic muscle plan of the chimpanzee face were hindered by the inconsistencies in reported dissections. Therefore, new dissections were conducted in order to clarify our understanding of cross-species differences in facial musculature, and these are described elsewhere (see Burrows et al, 2006). As there were no data available on facial muscle function in the chimpanzee, a comparative study of muscle action was also undertaken in order to ascertain whether independence of facial movements was a common feature, and also to document these actions in isolation .…”

“…ChimpFACS is based upon the comparative anatomy and facial morphology of chimpanzees. Although the literature revealed some disagreement regarding the muscle plan of the chimpanzee (Gibbs, Collard, & Wood, 2002;Huber, 1931;Pellatt, 1979;Seiler, 1971), many of these inconsistencies have been resolved using a more refined facial mask dissection method (see Burrows, Waller, Parr, & Bonar, 2006). Contrary to previous reports suggesting unique complexity and refinement of human facial musculature (Huber, 1931), there is in fact a striking similarity between humans and chimpanzees.…”

Intramuscular electrical stimulation of facial muscles in humans and chimpanzees: Duchenne revisited and extended.

“…Given small sample size, possible individual variation, and inconsistent reports (see Huber, 1931;Pellatt, 1979b), it is unclear whether the corrugator is always present in the chimpanzee (although recent dissections have found a distinct corrugator: Burrows, Waller, Parr & Bonar, 2006).…”

“…Duchenne did not demonstrate isolated contraction of the risorius, but showed the platysma stimulations (labeled the muscle of fright and terror), which may also involve risorius action. Although not often referred to directly in the chimpanzee, the dissection drawings of both Pellatt (1979b) and Huber (1931) show a bundle of fibers from the platysma crossing the triangularis and inserting onto the modiolus in the chimpanzee, and recent dissections have also located a risorius (Burrows et al, 2006).…”

“…Where the specific muscle is not shown, the general area is circled. Muscles not shown in the central table reported only in human, but see Burrows, Waller, Parr and Bonar (2006) (Zi = Zygomatic Minor, R = Risorius). Human diagram adapted from Hager (2000) and chimpanzee dissection diagram adapted from Pellatt (1979b).…”