Hsieh, S.T.: Ontogenetic 3-D kinematics of water running in green basilisk lizards (Basiliscus plumifrons)
Most research on tetrapod locomotion has focused on movement over solid substrates. The question of how animals locomote on fluid substrates is just beginning to be explored. Basilisk lizards are known for their ability to run across the surface of water to evade predators. Previous research indicates that this method of escape depends largely on how much force the animal generates when slapping the water, relative to its body mass. I ran eight green basilisk lizards (Basiliscus plumifrons)across a 3.7 m long water track. Animals ranged in size and age from two week-old hatchlings to year-old juveniles (2-60g). Runs were filmed with two high-speed Redlake video cameras operating at 250fps and 1/2500 shutter speed. Digitized coordinates from each camera were transformed into three dimensions using direct linear transformation. I divided each stride into four phases: slap, stroke, upwards recovery, and downwards recovery. In comparison to a stride on land, the slap and stroke phases are analogous to periods when the foot is in contact with the ground, while the recovery phases are analogous to when the foot is not in contact with the ground. Preliminary results suggest that there is extreme variation in the kinematics of running, both within and between individuals. Within one run, velocity may vary by as much as 18% (87.9 to 107.2 cm/s); and the slap may occur with the toe first contacting the water (toe strike), or the whole foot contacting the water (flat-foot strike). Equally large variations could be observed in 3D angular motions of the hip, knee, and ankle joints. However, while contact time (tc, slap and stroke phases combined) may last between 0.040s and 0.064s, duty factor (tc/stride period) appears to be relatively invariant (0.458�0.013, mean � se).
