Locomotor Ecology and Biomechanics lab

Recent Research

Walking the walk: the evolution of human bipedalism

Thorpe et al (2007) Science, 316:1328-1331.

A fundamental question in human evolution is how modern humans and their ancestors (hominins) became terrestrial bipeds. Human bipedalism is traditionally thought to have evolved from a quadrupedal terrestrial precursor, much like the common chimpanzee. But some recent paleontological and genetic evidence suggests that adaptations for bipedalism must have arisen in an arboreal context. However, the adaptive benefit of arboreal bipedalism has been unknown. In this paper we hypothesised that bipedal locomotion might be advantageous for arboreal apes moving on the thin terminal branches at the periphery of trees because their long prehensile toes can grip multiple small branches and thus maximize stability, while freeing one or both hands for balance, feeding or weight transfer while crossing gaps between tree crowns. Our results showed that bipedalism does indeed allow the most arboreal great ape, the orangutan, to access supports too flexible to be negotiated otherwise. Orangutans react to branch flexibility like humans running on springy tracks, by straightening their knee and hip joints, whereas all other primates do the reverse. We concluded that human bipedalism is less an innovation than an exploitation of a locomotor behavior retained from the common great ape ancestor.

for technical comments on this paper and our response see Begun et al (2007) Science 318:1065 and Crompton and Thorpe (2007) Science 318:1065.

Orangutans use flexible branches to lower the energetic cost of locomotion

Thorpe et al. (2007) Biology Letters 3: 253-256

Within the canopy of tropical rainforest, the shortest gaps between tree crowns lie between slender terminal branches. While the flexibility (compliance) of these supports has previously been shown to increase the energetic cost of gap crossing in arboreal animals, field observations suggest that some primates may be able to use support compliance to increase the energetic efficiency of locomotion. In this paper we calculated the energetic cost of alternative methods of gap crossing in orangutans (Pongo abelii). Tree sway (see movie) was found to be less than half as costly as jumping, and an order of magnitude less costly than descending the tree, walking to the vine and climbing it. Observations of wild orangutans suggest that they actually use support compliance in many aspects of their locomotor behaviour. This study seems to be the first to show that elastic compliance in arboreal supports can be used to reduce the energetic cost of gap crossing.

Neuromaturation of walking in children

O’Conner et al. (2007) Gait and Posture 25:469-474.

The vertical velocity of the foot centre of a man walking, with triangular markers indicating the timing of heel strike (HS) and toe off (TO) as determined by the foot velocity algorithm, and a simultaneous recording from a force plate which provides the ‘‘true’’ times for HS and TO for one step, which are marked with dashed lines.

This study uses electromyography to establish the development of normal activation patterns in the major muscles of the human leg during ontogeny. The project, in collaboration with the University of Cape Town and University College Dublin, will aid understanding of gait-related developmental problems, such as cerebral palsy.

We are in the process of writing up this work, but have completed the first step (!). The timing of heel strike (HS) and toe off (TO) are the events that mark the transitions between stance and swing phase of gait and are important measures when analysing gait. The timing of these events are normally identified from kinetic (force) data, but these approaches restrict the number of steps that can be analyzed. We developed an algorithm to automatically determine HS and TO from kinematic (movement) data recorded by a motion capture system. The foot velocity algorithm uses data from the heel and toe markers and identifies features in the vertical velocity of the foot which correspond to the gait events. Our foot velocity algorithm offers accurate results and is easy to implement, and should be applicable in a variety of gait analysis settings.

Evolution of ape locomotor diversity

The apes (including humans) share a suite of morphological adaptations in the trunk and upper limb that have been interpreted by most as characteristics linked to brachiation: orthograde (vertical trunk) forelimb-suspensory under-branch locomotion. The Asian apes, more than any other, remain restricted to an arboreal habitat and they are consequently an important model in the interpretation of the morphological commonalities of the apes.

In this study we carried out a detailed analysis of orangutan positional behavior (locomotion and posture). We show that orangutan positional behavior is highly complex, representing a diverse spectrum of positional modes. Overall, our results suggest that orangutans use orthograde suspensory behaviour substantially less, but orthograde compressive locomotor modes (such as hand-assisted bipedalism) far more than has been previously suggested. In general orangutan positional behavior is similar to that of the African apes, and in particular, lowland gorillas. In our conclusion we argue that it is orthogrady in general, rather than forelimb suspension specifically, that is the common inheritance from the last common-ape ancestor over 20 million years ago.

Locomotor Ecology of Sumatran orangutans (Pongo abelii)

The large body mass and exclusively arboreal lifestyle of Sumatran orangutans identify them as a key species in understanding the dynamic between primates and their environment. In this study we employ a multivariate modeling approach to identify key associations between orangutan locomotion, canopy level, support use and contextual behavior. Log-linear modelling is particularly appropriate because it is designed for categorical data (such as is collected during observational field studies), provides a systematic method for testing alternative hypotheses regarding interactions between variables, and allows interactions to be ranked numerically in terms of relative importance. The diameter, type and number of supports used were found to have the strongest associations with locomotor repertoire, suggesting that these variables, rather than height, contextual behaviour, or the age-sex category of the orangutan, have been the most important influences on the evolution of orangutan locomotor profiles.