S Tonia Hsieh | PubMed Health

PubMed2004

Running on water: Three-dimensional force generation by basilisk lizards.

Water provides a unique challenge for legged locomotion because it readily yields to any applied force. Previous studies have shown that static stability during locomotion is possible only when the center of mass remains within a theoretical region of stability. Running across a highly yielding surface could move the center of mass beyond the edges of the region of stability, potentially leading to tripping or falling. Yet basilisk lizards are proficient water runners, regularly dashing across bodies of water to evade predators. We present here direct measurements of time-averaged force produced by juvenile plumed basilisk lizards (Basiliscus plumifrons) while running across water. By using digital particle image velocimetry to visualize fluid flow induced by foot movement, we show that sufficient support force is generated for a lizard to run across water and that novel strategies are also required to run across a highly yielding surface. Juvenile basilisk lizards produce greatest support and propulsive forces during the first half of the step, when the foot moves primarily vertically downwards into the water; they also produce large transverse reaction forces that change from medial (79% body weight) to lateral (37% body weight) throughout the step. These forces may act to dynamically stabilize the lizards during water running. Our results give insight into the mechanics of how basilisk lizards run across water and, on a broader scale, provide a conceptual basis for how locomotor surface properties can challenge established rules for the mechanics of legged locomotion.

Animals↗

PubMed2003

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.

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