Faster fowl fall frequently: speed and force regulation during turning maneuvers by guinea fowl on high and low friction terrains.

Safely navigating variable terrains requires animals to balance competing demands of speed, stability, maneuverability and injury avoidance. Straight-line locomotion has been extensively studied, but less is known about how animals coordinate turning maneuvers. The physics of turning creates a coupling between speed, turn sharpness and ground reaction force (GRF) demands, resulting in a trade-off between speed and maneuverability. Here, we investigated locomotor strategies as guinea fowl navigated turns in high and low friction substrates. We measured center of mass trajectories and GRF in four conditions: control straight, control turns, slippery straight and slippery turns. We hypothesized that guinea fowl would slow down in turns to maintain peak GRF similar to that in steady, straight conditions, and that slippery terrain would lead to a shift towards slower speeds and shallower turn angles for slip avoidance. We found that guinea fowl slowed down by 14% in high friction turns and 27% in slippery turns compared with straight running and maintained GRF peaks within the 95% prediction interval for straight runs. Contrary to predictions, guinea fowl used similar turn strategies in low and high friction terrain, executing gradual turns with ∼7 deg change in heading per step, shifting from aerial to grounded running and leaning into the turn. Substantial individual variation in preferred speeds persisted across terrains, and preferred speed correlated with slip and fall rates (faster birds fell more frequently), suggesting individual variation in risk tolerance. Our findings support the hypothesis that animals modulate speed and ground reaction forces to balance competing mechanical demands in unsteady maneuvers, although the underlying control mechanisms remain to be determined.