Meeting Abstract
Natural terrain varies enormously in surface properties and contour; yet animals are able to move rapidly over these surfaces with apparent ease. Much is known about compensatory strategies of bipedal parasagittal and crouched runners over perturbations. For example, guinea fowl have been shown to lengthen their legs over sudden drops in terrain, whereas humans tend to change the stiffness of their limbs to compensate for drops, obstacles and changes in surface compliance. In contrast, dogs and cockroaches do not change their limb stiffness. Little is known, however, about how bipedal sprawled runners contend with perturbations and why such a diversity of strategies might exist. We ran 4 basilisk lizards (Basiliscus vittatus) over flat terrain (control) as well as drops of 40% their leg length. Basilisks were able to accomplish this task without any detriment to their running speed (p = 0.962). In general, they shortened their stride (p = 0.0025), while keeping stride frequency and duty factor constant. Lizards landed in the drop with a more upright body angle, touchdown angle and tail pitch while using a wider step than when unperturbed (p <0.05). They also straightened their leg (p = 0.0227) but did not stiffen it. Lizards reached intermediate or control level values of tail pitch, body angle and touchdown angle by the next step. This response is very similar to the that used by guineafowl, for which their stability has been associated with the crouched limb posture. We propose that the sprawled, crouched posture found among many vertebrates and invertebrates can potentially convey greater locomotor stability for a similar reason—the ability to lengthen the limb can compensate for unexpected changes in surface contour.
