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

TLDR

Facial expressions are a critical mode of non‑vocal communication for many mammals, especially non‑human primates. The study aims to describe the chimpanzee facial musculature and compare it with other catarrhines. Using dissections of preserved faces and a novel approach, the authors mapped and compared the arrangement of 23 mimetic muscles with previous studies of chimpanzees, prosimians, cercopithecoids, and humans. They identified 23 mimetic muscles, including a thin sphincter colli, a bi‑layered zygomaticus major, and a distinct risorius, showing minimal differences from humans and supporting a highly graded facial communication system distinct from other primates.

Abstract

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.