Hominoid Locomotion

The fibula has rarely been considered in anthropological studies. However differences in morphology - and inferred function - of the fibula between human and non-human apes have been noted in the past and related to differences in locomotor behavior. Recent studies have pointed out the correlation between diaphyseal rigidity of the fibula and tibia and locomotor behavior in living hominids, and its possible application for inferring early hominin locomotor behavior. The problem with the application of the method proposed in these studies is the extreme rarity of associated early hominin fibula and tibia. Additionally, previous studies investigating morphological traits of fibulotalar articular facets to infer the
degree of arboreality in fossil australopiths were often qualitative. In the present study, articular measurements of the distal fibula of living great apes and humans (Pongo, Gorilla, Pan and Homo) are quantified and compared to Australopithecus afarensis distal fibulae. Quantitative analysis is carried out for articular areas and breadths of the fibulotalar articular facets, for the angles formed by the fibulotalar articular facets and the longitudinal axis of the fibula, and for the angle between the proximal fibulotalar articular facet and the subcutaneous triangular area. Results show that the fibula of A. afarensis bears some traits consistent with modern terrestrial bipedalism, like a more laterally facing lateral malleolus, in association with more ape-like traits, like the smaller distal fibulotalar articular facet area and the more inferiorly oriented fibulotalar articular facets, consistent with A. afarensis being a terrestrial hominin adapted for some form of arboreality.

The fibula has rarely been considered in comparative morphological studies. The bones normally used to investigate early hominin locomotor behavior are the largest bones of the hindlimb, femur and tibia. The reason for this choice is probably due to the relatively minor role of the fibula in carrying mechanical loads. However differences in morphology (and inferred function) of the fibula between human and non-human great apes, and within non-human great apes, have been noted in the past and related to differences in positional behavior. Recent research (Marchi, 2007; Marchi and Shaw, 2011) have pointed out the correlation present between diaphyseal structural properties of the fibula and locomotor behavior in living hominids (i.e. Pongo, Gorilla, Pan and Homo), and its possible application to inferring early hominin locomotor behavior. The problem with the method proposed in these studies is the extreme rarity of complete fibular diaphyses in the early hominin fossil record. However, distal fibular fragments are present in the fossil record. In particular, five distal fibulae of Australopithecus afaresis (i.e. AL 288-1at, AL 333-1a, AL 333-1b, AL 333-85, AL 333w-37) from the Hadar region of Ethiopia are available. Several morphological traits in the distal part of the fibula have been used in the past to infer locomotory adaptations and in particular the degree of arboreality in fossil hominins (Stern and Susman, 1983). However, the descriptions of these traits are often qualitatite. In this preliminary research articular measurements of the distal fibula of living hominids (Pongo, Gorilla, Pan and Homo, N = 107) are quantified. Besides articular area and breadth of the fibulotalar articular surfaces (i.e. proximal and distal), quantitative analysis is provided here of: 1. the angles formed by the fibulotalar articular surfaces with the longitudinal axis of the fibula; 2. the angle between the proximal portion of the fibulotalar articular surface and the subcutaneous triangular area of the fibula, which was hypothesized to be related to the degree of arboreality in great apes (Stern and Susman, 1983). Results show that the morphological characteristics of the distal fibula bear signals correlated with different locomotor behaviors particularly useful to distinguish between bipedal and non-bipedal hominids. The application of this method to the five distal Au. afarensis fibulae available shows how these morphological traits can be added to the ones normally investigated by palaeoanthropologists to better understand early hominin locomotory behavior.

This is the first attempt to study cross-sectional geometry (using a moulding technique in combination with biplanar radiography) of metacarpals II-V, metatarsals I-V, and the tibio-fibular complex at 50% length across a sample of chimpanzees, gorillas, orang-utans, gibbons, and humans and to shed some light on their relationship with locomotor behaviour. The best results were obtained comparing tibia and fibula, and metacarpals and metatarsals of the same ray. On the whole, cross-sectional characteristics of the tibio-fibular complex, metacarpal and metatarsal rays are strongly related to the locomotor behaviour of hominoids, and may be useful in elucidating locomotor adaptations of fossil specimens.