Predicting Locomotion with the Radius: A study of Functional Morphology of the Radius in Apes and Monkeys

Comparison between suspensory, arboreal and terrestrial locomotion in primate locomotion has been extensively researched with heavy emphasis on the morphology of the forelimb. Most morphological studies of the forelimb focus attention around the elbow and wrist joints; however, many of these past studies have been done using the distal humerus and the proximal ulna. Although these studies are thorough and well conducted, research regarding the proximal end of the radius has been understudied in its functionality to the elbow joint. This study examines multiple osteological landmarks of the radius of anthropoid primates in order to better understand the possible connection between morphology and locomotor behavior. We also investigated body size scaling and phylogenetic differences among species. The goal of our study is to determine if the functional morphology of the radius correlates with locomotion over a larger and more diverse dataset of primates. We used extant species with known locomotion patterns and measured anatomical landmarks to ultimately determine which landmarks best predict locomotion. This information will be used to help predict locomotion patterns in fossil anthropoid species. We hypothesized that the joint morphology of the proximal radius is a useful measurement in predicting locomotion patterns, specifically in primates that use below branch vs. quadrupedal locomotion and arboreal vs. terrestrial locomotion. We created 3-D surface renderings by collecting data off of CT scans of the radius. Variables measured were humerus length, radial length, radial neck length, and mediolateral and anteroposterior diameter of the radial head. Most variables scaled isometrically with body size in both males and females. In addition, radial neck index was found to be significant between arboreal and terrestrial primates, with arboreal primates having a higher radial neck index. Brachial index, anterior posterior index and medial lateral index were all found to be not significant when comparing arboreal and terrestrial primates. Below branch individuals had higher brachial and radial neck index, while above branch individuals had higher anterior posterior and medial lateral indexes. In phylogenetic comparisons of cercopithecoids, hominoids, and platyrrhines, platyrrhines had the lowest brachial index and the highest radial neck index, anterior posterior and medial lateral index. The radial neck appears to be promising for using the radius to predict locomotion.