Comparative anatomy of the levator scapulae, serratus anterior and rhomboid muscles in primates: Morphological adaptations related to the primate locomotion evolution

The scapula connects the forelimb to the axial skeleton, and its morphology varies among primates, showing similarities across species with comparable locomotor behaviours, particularly forelimb-suspensory locomotion. The serratus anterior (SA), levator scapulae (LS) and rhomboid (Rh), collectively referred to as the LSR, connect the scapula to the trunk. The morphology of the LSR shows interspecific variation among primates, but its diversity and determinants have not been fully elucidated. This study explored the morphological adaptations in primate locomotor evolution by analysing the morphology and innervation of the LSR. We examined 27 limbs of 11 primate species across Catarrhini, Platyrrhini and Strepsirrhini. Based on the LS origin, the LSR were classified as: Type I (Continuous type), where LS is attached to all the cervical vertebrae and continuous with the SA; Type II (Separated type), where LS is attached to the first to fourth cervical vertebrae and separated from the SA; and Type III (Intermediate type), where the demonstrated features are between Types I and II. In all specimens, the LSR were innervated by C3-8, with C4-7 consistently present. In Type II, C5 innervated both the lower LS and the upper SA, whereas in Type I and Type III, C5 innervated the lower LS. Type I was found across all groups, suggesting its representation as the ancestral form. Both Type II and Type III appear to be derived from the Type I. In Type II, separation of LS and SA reflects division of the C5-innervated region rather than loss of the lower LS. Type II occurred only in Hominoidea, suggesting increased functional demands on the upper SA. Type III is considered a subtype of Type I, retaining C5 innervation in the LS despite morphological separation. The occurrence of Type III in small-bodied species of the family Cebidae suggests that it represents a subtype associated with variation in body size. These findings suggest that the morphology of the LSR in primates is shaped not only by phylogenetic background but also by species-specific functional demands.