Functional Myology of the Neck in Primates

Abstract

The neck is often used to study postural and locomotor behaviors across primates. Partially due to the complexity of this 3D system, there has not yet been a quantitative study of comparative nuchal muscle function. In this study, we applied innovative methods to visualize, quantify, and compare the epaxial muscles among primate taxa to characterize the relationship between trunk posture, extreme head turning behaviors, and forelimb-dominated below branch suspensory locomotion and muscle function. DiceCT imaging, 3D muscle fascicle-tracking, and 3D muscle visualization were all used to compare the attachment sites, orientations, and estimates of force for 19 muscles across five species that displayed contrasting postural and locomotor behaviors. To explore estimates of muscle force and 3D orientation of muscle bellies, ternary diagrams were used. Results show that most muscle forces scale with isometry, but musculature varies according to positional behavior and function. Semisuspensory species have a more dorsoventrally oriented cranial bellies of the trapezius muscle than closely related, but non-suspensory taxa. Pronograde primates have more powerful longissimus capitis muscles compared to orthograde primates. Carlito, noted for its pronounced head turning behaviors, has a more transversely oriented and more powerful sternocleidomastoid in addition to more powerful obliquus capitis superior muscle. The latter is also seen in Propithecus. These findings support the hypothesis that estimates of muscle force for certain key neck muscles are related to positional behaviors in primates. Therefore, the bony attachment sites of the muscles that were observed to vary among taxa could aid in understanding locomotor behaviors in extinct primates.