Three-dimensional hindlimb kinematics of water running in the plumed basilisk lizard (Basiliscus plumifrons)

Much of what is known about tetrapod locomotion is based upon movement over solid surfaces. Yet in the wild, animals are forced to move over substrates with widely varying properties. Basilisk lizards are unique in their ability to run across water from the time they hatch to adulthood. Previous studies have developed mechanical models or presented theoretical analyses of running across water, but no detailed kinematic descriptions of limb motion are currently available. The present study reports the first three-dimensional kinematic descriptions of plumed basilisk lizards (Basiliscus plumifrons) running across water, from hatchling (2.8 g) to adult (78 g) size range. Basilisks ran on a 4.6 m-long water track and were filmed with two synchronized high-speed cameras at 250 frames s(-1) and 1/1250 s shutter speed. All coordinates were transformed into three dimensions using direct linear transformation. Seventy-six kinematic variables and six morphological variables were measured or calculated to describe the motion of the hindlimb, but only 32 variables most relevant to kinematic motion are presented here. Kinematic variation among individuals was primarily related to size differences rather than sprint speed. Although basilisk lizards applied some of the same strategies to increase running velocity across water as other tetrapods do on land, their overall kinematics differ dramatically. The feet exhibit much greater medio-lateral excursions while running through water than do those of other lizards while running on land. Also, whereas the hindlimb kinematics of other lizards on land are typically symmetrical (i.e. limb excursions anterior to the hip are of similar magnitude to the limb excursions aft of the hip), basilisks running through water exhibit much greater excursions aft than they do anterior to the hip. Finally, ankle and knee flexion in early stance is a defining feature of a tetrapod step during terrestrial locomotion; yet this characteristic is missing in aquatic basilisk running. This may indicate that the basilisk limb acts primarily as a force producer - as opposed to a spring element - when locomoting on a highly damping surface such as water.