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Chapitre D'ouvrage Année : 2023

Convergence of Bipedal Locomotion: Why Walk or Run on Only Two Legs

Résumé

The adaptive reason(s) as to why some tetrapods walk or run on only two legs is far from straightforward. Compared to a quadruped, maintaining balance is obviously challenging for a biped, since the number of ground contacts per cycle is reduced. Consequently, cyclic limb loading can also be expected to be higher. In association with these mechanical constraints, some species show clear adaptations that enable them to continuously walk and/or run bipedally. Others, however, can only perform bipedally for short bouts after which they proceed further on all fours. Apparently, in the latter case, an optimal functional anatomical context (the so-called ‘evolutionary spandrel’) favours occasional bipedal performance. In this context, it is possible that the morphological features in the extant species that routinely practice bipedal walking and/or running (i.e., birds and humans) are convergent. Indeed, a similar (analogous) adaptive trait might have evolved independently in these ‘bipedal’ taxa as a result of similar selection pressures. Similarly, since occasional bipedalism should be a mandatory intermediate evolutionary stage leading to habitual, or obligate, bipedalism, the evolutionary pathways leading to bipedal adaptations can be explored in extant animals practicing different levels of bipedal performance. In the present chapter, we discuss different functional and evolutionary contexts that have led species with different Bauplans to use bipedal walking and/or running (for short or longer behavioural bouts). Firstly, we consider the involuntary but dynamically imposed bipedal running observed in extant lizards. Secondly, the voluntary but anatomically constrained bipedal walking behaviour of extant non-human primates is discussed. Thirdly, the use of the bipedal posture in birds and humans is compared (i.e. for the species for which this mode of progression is anatomically imposed and constrained). Finally, in an attempt to link the mechanical constraints and the potential evolutionary pathways related to occasional, habitual and obligate bipedalism, we argue that bipedal tetrapods should converge upon the same functional anatomical outcomes. Indeed, based on a virtual limb that would connect the body’s centre of mass to the foot, angular impulses about the body’s centre of mass over a (half-) stance time of the (vertical) ground reaction forces should cancel each other out. This might leave only two adaptive pathways open that could lead to a more habitual use of bipedalism: (1) make the bipedal functional/anatomical limb coincide with the virtual limb, or (2) make the virtual limb coincide with the anatomical (quadrupedal) limb.
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Dates et versions

hal-04133385 , version 1 (19-06-2023)

Licence

Paternité - Pas d'utilisation commerciale - Partage selon les Conditions Initiales

Identifiants

Citer

François Druelle, Anick Abourachid, Menelia Vasilopoulou-Kampitsi, Peter Aerts. Convergence of Bipedal Locomotion: Why Walk or Run on Only Two Legs. Convergent Evolution, Springer International Publishing, pp.431-476, 2023, Fascinating Life Sciences, 978-3-031-11440-3. ⟨10.1007/978-3-031-11441-0_14⟩. ⟨hal-04133385⟩
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