Cursorial ground birds are paragons of bipedal running that span a 500-fold mass range from quail to ostrich. Here we investigate the task-level control priorities of cursorial birds by analysing how they negotiate single-step obstacles that create a conflict between body stability (attenuating deviations in body motion) and consistent leg force–length dynamics (for economy and leg safety). We also test the hypothesis that control priorities shift between body stability and leg safety with increasing body size, reflecting use of active control to overcome size-related challenges. Weight-support demands lead to a shift towards straighter legs and stiffer steady gait with increasing body size, but it remains unknown whether non-steady locomotor priorities diverge with size. We found that all measured species used a consistent obstacle negotiation strategy, involving unsteady body dynamics to minimise fluctuations in leg posture and loading across multiple steps, not directly prioritising body stability. Peak leg forces remained remarkably consistent across obstacle terrain, within 0.35 body weights of level running for obstacle heights from 0.1 to 0.5 times leg length. All species used similar stance leg actuation patterns, involving asymmetric force–length trajectories and posture-dependent actuation to add or remove energy depending on landing conditions. We present a simple stance leg model that explains key features of avian bipedal locomotion, and suggests economy as a key priority on both level and uneven terrain. We suggest that running ground birds target the closely coupled priorities of economy and leg safety as the direct imperatives of control, with adequate stability achieved through appropriately tuned intrinsic dynamics.
↵* Present address: Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK
↵‡ These authors contributed equally to this work
All authors discussed and commented on the manuscript. M.A.D. designed and supervised experiments. J.W.H. planned and co-supervised simulations. A.V.B.-J. and Y.B. collected and analysed experimental data. C.M.H. ran simulations and optimisations. D.R. co-supervised simulations. A.V.B.-J., C.M.H. and M.A.D. wrote the paper.
The authors declare no competing financial interests.
The Biotechnology and Biological Sciences Research Council, UK funded the comparative experiments (BB/H005838/1 to M.A.D.). The Human Frontier Science Program funded simulations (RGY0062/2010 to M.A.D. and J.W.H.). Deposited in PMC for immediate release.
Supplementary material available online at http://jeb.biologists.org/lookup/suppl/doi:10.1242/jeb.102640/-/DC1
- © 2014. Published by The Company of Biologists Ltd
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