In search of the pitching momentum that enables some lizards to sustain bipedal running at constant speeds

Abstract

The forelimbs of lizards are often lifted from the ground when they start sprinting. Previous research pointed out that this is a consequence of the propulsive forces from the hindlimbs. However, despite forward acceleration being hypothesized as necessary to lift the head, trunk and forelimbs, some species of agamids, teiids and basilisks sustain running in a bipedal posture at a constant speed for a relatively long time. Biomechanical modelling of steady bipedal running in the agamid Ctenophorus cristatus now shows that a combination of three mechanisms must be present to generate the angular impulse needed to cancel or oppose the effect of gravity. First, the trunk must be lifted significantly to displace the centre of mass more towards the hip joint. Second, the nose-up pitching moment resulting from aerodynamic forces exerted at the lizard's surface must be taken into account. Third, the vertical ground-reaction forces at the hindlimb must show a certain degree of temporal asymmetry with higher forces closer to the instant of initial foot contact. Such asymmetrical vertical ground-reaction force profiles, which differ from the classical spring-mass model of bipedal running, seem inherent to the windmilling, splayed-legged running style of lizards.

The effects of multiple obstacles on the locomotor behavior and performance of a terrestrial lizard

“…Two studies show that a bipedal posture is associated with holding the tail horizontally, and suggest that this may increase the stability of the body COM (Irschick and Jayne, 1999b;Self, 2012). However, other studies suggest elevating the tail above the COM is needed during sustained bipedal running and that both tail elevation and bipedal running are related to rapid initial acceleration (Van Wassenbergh and Aerts, 2013;Aerts et al, 2003;Clemente, 2014). When approaching an obstacle, lizards had an average bipedal body angle of 17.7±1.6 deg (Table 2), similar to the body angle of 15 deg observed in Van Wassenbergh and Aerts (2013) and of 25 deg observed in bipedal running cockroaches (Alexander, 2004).…”

“…Or, a horizontal tail may counterbalance the weight of the increased body angle during bipedal running, as it would position the COM of the tail as far as possible from the center of the hip (Irschick and Jayne, 1999b), thus providing a potential benefit for lizards using a bipedal running posture. In contrast, elevating the tail ∼12-15 deg above a horizontal plane allows lizards to sustain bipedal running at lower accelerations (Van Wassenbergh and Aerts, 2013;Aerts et al, 2003;Clemente, 2014). However, studies have yet to show that bipedal running can be sustained when lizards negotiate obstacles (Olberding et al, 2012).…”

“…If lizards cannot sprint at (or near) maximum capacity, then their ability to elude predators would be affected (McMillan et al, 2011;De Barros et al, 2010;Okafor, 2010), particularly on uneven substrates containing obstacles. Whereas some studies have examined bipedal locomotion in lizards from a biomechanical standpoint (Hsieh and Lauder, 2004;Kohlsdorf and Biewener, 2006;Olberding et al, 2012;Van Wassenbergh and Aerts, 2013), the present study will examine a mechanistic link between bipedalism and ecologically relevant features of the habitat (obstacles).…”

“…Alternatively, a bipedal posture may be a mechanical consequence of rapid initial acceleration. Here, the torque generated by the hip would cause a pitching rotation that lifts the head and trunk (Aerts et al, 2003;Van Wassenbergh and Aerts, 2013). Yet for several lizard species, acceleration is not a prerequisite for sustained bipedal running (Van Wassenbergh and Aerts, 2013).…”

“…Here, the torque generated by the hip would cause a pitching rotation that lifts the head and trunk (Aerts et al, 2003;Van Wassenbergh and Aerts, 2013). Yet for several lizard species, acceleration is not a prerequisite for sustained bipedal running (Van Wassenbergh and Aerts, 2013). Active tail lifting during acceleration results in the upward rotation of the trunk through increased angular momentum of the tail, making sustained bipedal running possible at lower acceleration (Aerts et al, 2003;Clemente, 2014).…”