Posture and mobility of the primate head and neck in relation to cervical vertebral morphology (ASUF - 30006670)

Posture and mobility of the primate head and neck in relation to cervical vertebral morphology (ASUF - 30006670) Posture and mobility of the primate head and neck in relation to cervical vertebral morphology Introduction Hominoids are highly variable and exceedingly specialized in their locomotor modes. Gibbon brachiation, orangutan suspensory behaviors, gorilla and chimpanzee knuckle walking, and human bipedality involve diverse suites of skeletal adaptations that evolved in equally diverse environmental conditions. Much research has been conducted on the adaptive morphology of pelvis, limbs, and lumbar spine; however, the adaptive morphology of the head in neck as it relates to those distinct forms of locomotion and substrate use among primates. The purpose of this study is to evaluate the functional morphology of the head and neck as a means for understanding their evolution within the hominoid clade in the context of these varied locomotor modes. The neck functions in two critical ways: it allows for a mobile visual field and maintains a stable head and field of vision during locomotion. The head and neck can be modeled as a bow and string with the neck muscles supporting the position through tension (1) (Fig. 1, App. 1). Following this, taxa with more pronograde necks, i.e. those whose necks are habitually oriented perpendicular to the force of gravity, require greater support than those with more orthograde necks, or positioned parallel to the gravity vector. This change in gravitational loading affects how the neck maintains the position of the head. To maintain postural function, either the muscular effort must be larger or the muscle forces must be oriented in more mechanically advantageous configurations. Therefore, differences in posture should affect the form of the cervical vertebral column. Strait and Ross (2) quantified neck posture through filming habitual primate locomotion. Neck inclination was measured along the flattest surface of the neck during midstance or midswing. However, this dermal inclination of the neck is poorly correlated with the inclination of the vertebrae in humans (3). It is likely that the large neck musculature found in other primates, like gorillas, severely influence this measure. A skeletal measure of posture will better estimate the biomechanical loads placed on the spine during habitual locomotion and, therefore, elucidate the relationship between cervical form and postural function. Head posture during arboreal locomotion has been shown to correlate with the inclination of primates substrate (2, 4, 5). Therefore, arboreal primates, who locomote on substrates with more variable inclinations should possess larger ranges of neck motion than terrestrial primates with less substrate variability. These differences in ranges of motion should be reflected in cervical form. In humans, intervertebral differences in ranges of motion, due to morphological variations, are known (6, 7). For example, the spinous process (Fig. 2, App. 1) physically inhibits extension (Fig. 3, App. 1) (6) and becomes increasingly longer from the first cervical vertebra (C1) to the seventh (C7). Therefore, one would expect a corresponding decrease in range of extension from upper to lower cervical vertebrae across taxa. Graf and colleagues (8) quantified head and neck mobility in humans and three species of monkey without considering cervical form. Unexpectedly, the results of Graf et al. show no inter- or intraspecific patterns in range of motion (8). This study suffers from small sample size as well as limited taxonomic breadth. A wider range of primate taxa should elucidate functional patterns. Hypotheses The goal of this research is to quantify the way in which the neck functions in posture and range of motion to test the hypotheses that cervical vertebral form is adapted to maintaining 1) posture during locomotion and 2) head and neck mobility. Multiple functional predictions exist for each aspect of the vertebral form. 1. Predictions related to habitual posture during locomotion: a. Spinous processes will be shorter and less cranially inclined in orthograde species in comparison to species with more pronograde necks. Longer spinous processes allow for larger nuchal musculature and increase the moment arm, thereby increasing the resulting moment (9). b. Laminae will have greater cross-sectional areas in taxa with larger nuchal musculature in order to resist the greater stress and strain placed on them (10). c. Articular facets will be oriented more coronally in species with more orthograde postures in order to better resist potential shear forces of gravity. d. Vertebral bodies will be relatively larger in more orthograde taxa due to the increased amount of weight bearing that coincides with more vertical neck postures. 2. Predictions related to mobility: a. Spinous processes will be short in species or vertebral levels with greater ranges of extension because they are known to physically inhibit extension(6). b. Transverse processes will be short and more cranially oriented in species and vertebral levels with greater ranges of lateral flexion. Long transverse processes physically inhibit lateral flexion while a cranial orientation improves the leverage of the lateral flexors(11). c. Articular facets will be oriented more coronally in species or vertebral levels with larger ranges of rotation. The effects of articular facet orientation on ranges of motion are known within the human vertebral column. A coronally flat facet, like that found in the joint between C1 and C2, exhibits larger rotational ranges of motion while a facet that is oriented parallel with the dorsal surface have larger ranges of lateral flexion. d. Uncinate processes are known to inhibit lateral flexion(6). Therefore, species and vertebral levels with larger uncinate processes will exhibit smaller ranges of lateral flexion. Pilot Project Aims This pilot project is confined to two species, the lar gibbon (Hylobates lar) and humans (Homo sapiens). Due to the morphological variation within the cervical column, it is possible to test the range of motion predictions within a single species. Relationships established will be used to retrodict the head and neck mobility and vertebral functions of fossil hominoids, which is key to further elucidate the contexts of their evolution. The postural hypothesis, however, requires a taxonomically broader dataset and, thus, the data collected in this project will be incorporated into a larger dissertation data set in order to address the hypotheses above. Methods Postural data will be collected via 3D markers, which will be placed on palpable craniocervical skeletal landmarks of the participants including: glabella, inion, cervical spinous processes, and the first through fourth thoracic spinous processes. The gibbon is trained to brachiate along a rigid substrate in front of 3D motion analysis system, Frame DIAS IV (DKH, Japan). Human participants will be asked to walk normally (5-6mph) on a treadmill in front of a 3D motion analysis system, Vicon (Motion Systems Ltd., UK). I will measure neck posture during locomotion using the angle between 1) a line through the spinous process markers of the 1st-7th cervical vertebrae and 2) the vertical line of gravity. Mobility data will be collected through radiography, in the gibbon, and computed tomography imaging, in humans. Ranges of motion will be calculated using these images taken in maximum active ranges of motion in flexion, extension, lateral flexion, and rotation while the thorax is stationary. Radiographs and CT scans will be used to compare the orientation of the dorsal aspect of the neck to the vertebral inclination within it. This comparison will be used to calculate the difference between the superficial and skeletal inclinations of the neck, allowing a skeletal measure of posture to be calculated. For the dissertation project, these methods will be repeated for a variety of primate species and compared to vertebral form. Dr. Hiroo Kumakura (University of Osaka) has the equipment and primate facility required to complete this research and we are in the process of submitting the protocols to the Japanese Animal Research Committee. When approved, these protocols will be submitted to the American Institutional Animal Care and Use Committee. Dr. David Raichlen (University of Arizona) has the equipment and experience to oversee the human kinematic study. The CT scanning will be conducted at the University of Arizona Medical Center. These protocols are under review by the Institutional Review Board. Morphological data will be collected using a 3D scanner (4DDynamics) at the Osaka Museum of Natural History and the Nubian Skeletal Collection at Arizona State University and analyzed using the program Amira (FEI Visualization) available at my home institution. Form-function relationships will be established for extant taxa and the behavior of fossil species will be reconstructed using their vertebral form. These methods will be repeated on a wider range of primate taxa for my dissertation research and the pilot data obtained will be used to support funding applications for such a project. Significance Locomotion within the hominoid clade is highly variable, and often unique, but very little is known about the evolution of the cervical spine within the hominoid clade. I expect to find positive relationships between cervical form and cervical function. These unique forms of locomotion create unprecedented selection pressures on postcranial anatomy. This pilot study the will further our understanding of the function of the cervical spine within primates and how the evolution of fundamentally affected the posture of the head and neck.