Neck Muscle Morphology and Function in Primates

Abstract

The cervical spine lies at intersection between the cranium and postcranium and plays a crucial role in orienting the head relative to the substrate during locomotion. Previous work demonstrates that bony features of the cranial base and cervical spine are related to positional behavior in primates. How soft tissues of the neck vary across primate taxa is relatively unknown yet is critical for understanding function, head carriage, and locomotion in this clade. The objective of this study was to describe the relationship between the extrinsic and intrinsic muscles of the neck and postural and locomotor behaviors in primates. We collected and dissected 26 primate cadavers from 17 different genera representing each major primate clade. We hypothesized that trunk and neck posture, as well as forelimb dominated below‐branch suspensory behaviors and neck rotational capabilities all influence the attachment sites and size of the dorsal neck musculature.Using gross anatomical dissection, we qualitatively described and compared the attachment sites of the dorsal neck, back, and shoulder muscles of taxa that display contrasting behaviors and quantified muscle mass. Using the dissection data combined with contrast enhanced microCT data, we constructed 3D models of the head and neck to visualize the in situ neck muscle attachment sites for Propithecus, Aotus, and Carlito.Gross anatomical dissections demonstrate that among nonhuman apes, hylobatids lack cranial attachments of the trapezius and rhomboideus. Additionally, strepsirrhines display anatomy that is more nonprimate mammal‐like in the distribution of the trapezius, rhomboideus, and semispinalis muscles. Results of phylogenetic regressions indicate that neck muscle masses are best predicted by body size, but also demonstrated significant results for both postural and locomotor behaviors as predictors of overall muscle mass (p < 0.05). This study represents the first comprehensive description of the functional anatomy of the neck muscles across primates. Future work will focus on quantifying biomechanically‐relevant variables such as physiological cross‐sectional area using novel fascicle‐tracking methods and estimating muscle moments to compare across taxa.Support or Funding InformationThis work was funded by the Leakey Foundation, National Science Foundation, American Association of Anatomists, Western University of Health Sciences, and University of Missouri.