Comparison of the bipedal locomotion of
gibbons, bonobos and humans

Background of the study
The development of habitual bipedal locomotion is a key factor in the hominisation process. Bipedalism, together with quadrupedalism, climbing (see also "climbing in bonobos")and brachiation, makes part of the locomotory behaviour of many primates. But, during human evolution, bipedalism has formed an increasing percentage of the locomotory repertoire and ultimately becomes the only type of locomotion in modern humans.
This gradual development of habitual bipedalism is coupled with the transition of an arboreal way of life, where the foot with an opposable hallux has mainly a grasping function, to a terrestrial way of life, where the foot functions as a rigid lever. These changes in locomotion and biotope are reflected in the structure of the foot, the shortening of the phalanges, the occurrence of a longitudinal foot arch and the gradual loss of the opposability of the hallux.

Why study gibbons and bonobos ?
The question can be asked why only hominids became habitual bipeds. Indeed, all recent genera of the Hominoidea (apes) show some bipedalism in their locomotor repertoire. It may thus seem surprising that the closest relatives of man, namely the common chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus), make only limited use of bipedal locomotion, despite the large morphological and morphometrical similarity with humans and the hominid ancestors. Gibbons (Family Hylobatidae), the furthest relatives of man within the Hominoidea, are, in contrast, the most bipedal of all non-human primates and this despite their specific adaptations to an other type of locomotion, namely brachiation.

The Hylobatidae split off the line leading (eventually) to the Hominidae approximately 20 million year ago. It is thus not unlikely that the bipedal locomotion of gibbons developped independently in gibbons and the much more recent Pan and Homo. To support (or to reject) this hypothesis, we first have to answer the following question: what are the functional morphological and kinesiological differences and similarities between the bipedalism of Hylobatidae and Hominidae?

What do we want to know ?

• The kinesiology of the terrestrial bipedal locomotion of the Hylobatidae (especially the white-handed and the white-cheeked gibbon) and,
• the kinesiology of the terrestrial bipedal and quadrupedal locomotion of the bonobo, Pan paniscus.

• The foot morphology of the Hylobatidae (especially the white-handed and the white-cheeked gibbon) and,
• the foot morphology of the bonobo, Pan paniscus.

How do we study the functional morphology ?
We use computerised tomography (CT-scan), whereby serial radiographic sections of the cadaver (or a part of it) are made every 0,7 mm. By using contrast liquids, the weak tissues can also be visualised. Starting from these digital sections, the three-dimensional morphology can be reconstructed and virtual sections can be obtained in each arbitrary plane.

This approach has the advantage that the movement possibilities of the different joints of the foot-ankle-complex can be studied in situ. This part of the research is in co-operation with Dr. S. Van Sint Jan from the Laboratory for Human Anatomy of the Université Libre de Bruxelles (see the Vakhum project). For verification, mainly for what concerns the musculatory and ligamentous elements of the foot-ankle-complex, detailed dissections and morphometrical analyses will be carried out. For comparison with the foot and ankle morphology of humans, we collaborate with the Department for Movement and Sport Science of the University of Ghent (Belgium); Prof. Dr. D. De Clercq).

How do we study the kinesiology ?
For the kinesiological aspect of the study, there will be taken video, force and plantar pressure recordings of gibbons from the colonies of the animal park of Planckendael (Mechelen). Herefore the set-up, at the moment applied for the ongoing bonobo project, will be used. This set-up consists of a catwalk with built-in force plates and pressure mats, a reference wall and three video cameras (see also bipedal walking in bonobos). In this manner, video images are completed with 3D-groundreaction forces as well as pressure profiles from foot contact at a very high spatial and temporal resolution. By collecting those two data simultaneously, exact quantification is possible. In combination with the kinematica these exact data are necessary to calculate the intern action of the forces near the joints within the ankle-foot-complex.