Meeting Abstract
Animals must frequently negotiate variable landscapes while searching for food, pursuing mates, and evading predators. Though a multitude of studies have tested how various aspects of substrate, such as incline, compliance, and rugosity, impact locomotion, fewer have examined how animals respond to changes or transitions in substrate during a single locomotor event. Substrate perturbations represent a pertinent challenge to performance, particularly for animals moving at high speeds. Field observations of escapes between flat rock and loose dune sand led us to test the effect of a substrate transition on posture, speed, and stability in the Namib Day Gecko ( Rhoptropus afer ), a diurnal, cursorial, rock-dwelling gecko. Ten individuals were captured and recorded from a lateral position at 500 fps while running down a trackway approximately 2 meters in length. Two conditions were examined: a sandpaper control and a treatment with approximately 10 centimeters of level dune sand at its center. We measured stride variables and used body markers to measure velocity, acceleration, and body pitch over the course of an entire trial and for each stride within a trial. Duty factor increased with reduced swing time in sand strides, regardless of speed. Interestingly, average stride velocity increased in sand strides relative to pre-sand strides in approximately half of the trials, suggesting that approach speed may dictate subsequent performance. Furthermore, peak stride acceleration was typically highest in strides following the sand transition, as was body pitch, reflecting a potential loss of stability as the lizards encountered a more compliant substrate. Successful negotiation of perturbations likely depends on the right combination of these properties.
