Tibia and fibula strength and locomotor behaviour in hominoids

The fibula has rarely been taken into consideration in comparative morphological studies, probably due to its relatively minor role in carrying mechanical load. However, some differences in morphology (and inferred function) of the fibula between humans and apes, and within apes have been previously noted, and related to differences in positional behavior. Therefore, the study of tibio-fibular relations may be useful in characterizing such differences. This study examines cross-sectional geometric (CSG) properties (cortical area and polar section modulus, Zp) of the tibia and fibula at mid-diaphysis across a sample (N=87) of humans, chimpanzees, gorillas, orangutans, and gibbons. The fibula is compared against the tibia in the different groups. From the analysis it appears that the robusticity of the fibula relative to that of the tibia can be explained in terms of differences in positional behavior. In particular, more arboreal hominoids (gibbons, orangutans, and chimpanzees) show a relatively more robust fibula than more terrestrial hominoids (gorillas and bipedal humans). The difference may be due to the more mobile fibula necessary in an arboreal environment. Apart from providing interesting CSG data on a bone not investigated so far using this technique, these results may also be helpful in the interpretation of fossil hominoids of uncertain locomotor assignment.