Loris locomotor behavior in relation to skeletal morphology: disjunction between assumed mobility and utilized range of motion

Abstract

An underlying assumption of many studies of primate functional anatomy is that skeletal morphology directly reflects an animal’s movement patterns. For example, expansion of humeral head is assumed to relate to greater shoulder mobility, but Chan (2008) found that passive shoulder mobility of lorises with a relative expanded humeral head is the same or less than that of lemurs. This project explores this disjunct between anatomy and range of motion by quantifying active 3D shoulder motion during slow climbing and bridging in lorises. These data are used to test the hypothesis that active, rather than passive, shoulder mobility influences shoulder morphology of lorises. We videorecorded two subjects each of Loris tardigradus (LT) (0.175-0.205kg) and Nycticebus pygmaeus (NP) (0.420-0.515kg), bridging across substrate gaps of several widths and orientations. Three-dimensional joint angles and limb positions were digitized using Innovision Systems, Inc© software to calculate excursion (flexion and abduction) of the humerus with respect to the trunk. These data were plotted on a polar coordinate system and compared to the passive range of motion reported by Chan (2008). The results showed that the excursion of the arm in both species is comparable to those used during arboreal quadrupedalism (LT mean51008; NP mean51238) which is less than the range of motion predicted lorisid skeletal morphology (LT mean52288; NP mean51718). These results suggest that previous methods of extrapolating mobility from various shoulder features, including glenohumeral skeletal morphology, are problematic and argue for cautious functional interpretation of primate fossil skeletal material.