Femoral mechanics in the lesser bushbaby (Galago senegalensis): structural adaptations to leaping in primates.

One method used to examine the relationship between behavioral strategies and anatomical adaptation is to study the results of mechanical stress associated with a given behavior and compare this with skeletal adaptations to other behaviors. This comparative approach is appropriate for highlighting combinations of features that are specializations to specific types of behavior. The purpose of this paper is to compare femoral mechanics in Galago senegalensis with previously collected data for macaques and humans as a basis for discussing structural adaptations in the primate hindlimb to leaping. The stiffness and load carrying capabilities of the femoral diaphyses of 27 G. senegalensis were analyzed using the SCADS computer program. The data suggest that the galago femur is well adapted to sustain large sagittal plane compressive loads rather than large bending loads. The straightness of the femoral shaft and large midshaft area moments of inertia prevent buckling from these large compressive loads. Calculations indicate that the ratio of critical buckling load to body weight in galago is 31 times that in macaques and 55 times that in humans. The femur of this saltatory primate is morphologically adapted to resist buckling when subjected to large compressive loads, while those of macaques and humans are better adapted to resist bending moments caused by ground reaction forces acting on the extended limb. The differences between galago on the one hand and macaques and humans on the other suggest that relatively smaller moments about the hip and relatively larger moments about the knee accompany more quadrupedal and bipedal walking, while habitual leaping is associated with relatively larger moments about the hip. These data reinforce the apparent similarity of the mechanical effects of quadrupedal and bipedal locomotion on the femur and dissimilarity with femoral mechanics in habitually saltatory primates.