Meeting Abstract
The more quickly an animal runs, the less time it has to adjust to and recover from sudden perturbations. A combination of neural feedback, feed-forward control, preflexes, and distributed mechanical feedback, are known to be important recovery strategies for humans, guinea fowl, and cockroaches. Some investigators have postulated that increased leg number naturally leads to greater locomotor robustness against perturbations. While this seems to be largely supported for multi-legged, forward moving animals, it is unknown whether this also applies to sideways moving animals. The Atlantic ghost crab (Ocypode quadrata) is a high-speed, sideways runner. The goal of this study was to determine how these crabs respond to an unexpected slip perturbation. We hypothesized that these crabs would maintain constant locomotor kinematics and limb phasing, similar to that observed in cockroaches, in spite of an applied slip perturbation. Thirty running ghost crabs were filmed at 500 fps, capturing one dorsal and two lateral views. Crabs were randomly assigned to one of two treatments: (I) a control treatment, in which crabs ran unperturbed along a sand trackway; and (II) a slip treatment, in which crabs encountered a low-traction surface in the middle of the trackway. Analyses show that ghost crabs’ limbs slipped up to 80% carapace width on the low-traction surface. Surprisingly, in spite of such long-distance slips, ghost crabs exhibited few signs of instability. Limb kinematics (e.g., stride frequency and duty factor) and phasing parameters in perturbed trials were statistically similar to those observed during control trials (P>0.05, mixed-model ANCOVAs), suggesting feed-forward control. As a result, we conclude that sideways running, multi-legged systems likely use similar strategies as forward running systems to maintain locomotor stability when perturbed.
