Performance and three-dimensional kinematics of bipedal lizards during obstacle negotiation

Abstract

SUMMARYBipedal running is common among lizard species, but although the kinematics and performance of this gait have been well characterized, the advantages in biologically relevant situations are still unclear. Obstacle negotiation is a task that is ecologically relevant to many animals while moving at high speeds, such as during bipedal running, yet little is known about how obstacles impact locomotion and performance. We examined the effects of obstacle negotiation on the kinematics and performance of lizards during bipedal locomotion. We quantified three-dimensional kinematics from high-speed video (500Hz) of six-lined racerunners (Aspidoscelis sexlineata) running on a 3m racetrack both with and without an obstacle spanning the width of the track. The lizards did not alter their kinematics prior to contacting the obstacle. Although contact with the obstacle caused changes to the hindlimb kinematics, mean forward speed did not differ between treatments. The deviation of the vertical position of the body center of mass did not differ between treatments, suggesting that in the absence of a cost to overall performance, lizards forgo maintaining normal kinematics while negotiating obstacles in favor of a steady body center of mass height to avoid destabilizing locomotion.

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

“…Obstacle negotiation has been studied among a variety of species including guinea fowl (Daley and Biewener, 2011;Clark and Higham, 2011), humans (Sparrow et al, 1996;Perry et al, 2010), cats (McFadyen et al, 1999), cockroaches (Harley et al, 2009) and lizards (Kohlsdorf and Biewener, 2006;Kohlsdorf and Navas, 2007;Olberding et al, 2012;Tucker and McBrayer, 2012;Self, 2012). To negotiate obstacles, anticipatory adjustments are essential for locomotion (McFadyen et al, 1999), especially if postural adjustments are to be timed appropriately.…”

“…Bipedal locomotion may enhance environmental perception in the presence of physical barriers, and may enlarge the animal's visual field by elevating the trunk above the obstacle before crossing (Kohlsdorf and Biewener, 2006;Tucker and McBrayer, 2012). For quadrupeds crossing obstacles, bipedality allows the body's center of mass (COM) to move over the obstacle without extensive change in vertical height (Olberding et al, 2012). Maintaining a balance between body stability and energy expenditure on uneven terrain is important to cursorial avian bipeds, ranging from quail to the larger ostrich (Birn-Jeffery et al, 2014), as well as to smaller, non-avian facultative bipedal species.…”

“…However, even though bipedal running can be sustained on unobstructed trackways, it may not be maintained during obstacle negotiation by lizards. For example, the six-lined racerunner (Aspidoscelis sexlineatus) does not continue running with a bipedal gait after obstacle contact, despite its frequent use of bipedal posture (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).…”

“…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).…”