THE MECHANICAL AND ENERGETIC ADVANTAGES OF FOOTFALL TIMINGS IN QUADRUPEDAL WALKING

One of the simplest and most readily available measurements from video of quadrupedal locomotion is that of footfall pattern and timing, making it an ideal approach for field studies. Previous analyses of the factors influencing footfall timings and gait selection have focused separately on the mechanical and energetic cost implications for animals moving on relatively simple substrates. We combine and build upon these factors to present a novel and holistic model of the functional constraints acting on the footfall timings of quadrupeds during walking, one that allows us to predict the environmental contexts in which animals will select footfall timings that maximize energetic efficiency, minimize rolling and pitching moments, or balance the two. We hypothesise that quadrupeds will prioritise energy recovery on broader, flatter substrates, while resisting pitch and roll will play a larger role on narrower and more complex supports. The highly flexible gait patterning of the squirrel monkey (Saimiri sciureus) makes an ideal study model with which to test some of these functional constraints. Here we do so using three rigid substrates – a flat surface, straight pole and side-branched pole – designed, respectively, to provide simple roll-reducing, simple roll-inducing and
complex roll-reducing environments. Walking steps on the flat surface tended towards timings consistent with optimizing energy exchange, while both poles tended towards those that reduced rolling and pitching moments. These data expand our understanding of the influence of substrate type on footfall pattern timings and provide insight into the competing pressures that may influence footfall timing choices in quadrupedal mammals with energetic efficiency being a priority on the ground, and safety being a competing priority in the trees. By using data gained under controlled conditions, we anticipate that this simple measure of gait
properties will be useful to understanding wild animal locomotion, particularly on arboreal substrates.