Previous analyses of factors influencing footfall timings and gait selection in quadrupeds have focused on the implications for energetic cost or gait mechanics separately. Here we present a model for symmetrical walking gaits in quadrupedal mammals that combines both factors, and aims to predict the substrate contexts in which animals will select certain ranges of footfall timings that (1) minimize energetic cost, (2) minimize rolling and pitching moments, or (3) balance the two. We hypothesize that energy recovery will be a priority on all surfaces, and will be the dominant factor determining footfall timings on flat, ground-like surfaces. The ability to resist pitch and roll, however, will play a larger role in determining footfall choice on narrower and more complex branch-like substrates. As a preliminary test of the expectations of the model, we collected sample data on footfall timings in a primate with relatively high flexibility in footfall timings – the squirrel monkey (Saimiri sciureus) – walking on a flat surface, straight pole, and a pole with laterally-projecting branches to simulate simplified ground and branch substrates. We compare limb phase values on these supports to the expectations of the model. As predicted, walking steps on the flat surface tended towards limb phase values that promote energy exchange. Both pole substrates induced limb phase values predicted to favor reduced pitching and rolling moments. These data provide novel insight into the ways in which animals may choose to adjust their behavior in response to movement on flat versus complex substrates and the competing selective factors that influence footfall timing in mammals. These data further suggest a pathway for future investigations using this perspective.

中文翻译：

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步行时肢体阶段的灵活性：松鼠猴（Saimiri sciureus）的测试案例

先前对影响四足动物脚步时间和步态选择的因素的分析分别集中于能量消耗或步态力学的影响。在这里，我们提出了一个四足哺乳动物对称行走步态的模型，该模型结合了这两个因素，旨在预测动物将选择特定范围的脚步时间的基质环境，以（1）最小化能量消耗，（2）最小化滚动和俯仰力矩，或（3）平衡两者。我们假设能量回收将是所有表面上的优先事项，并将成为决定平坦、地面状表面上的脚步时间的主导因素。然而，抵抗俯仰和滚动的能力将在决定更窄和更复杂的树枝状基底上的脚步选择方面发挥更大的作用。作为对模型期望的初步测试，我们收集了一种脚步时间灵活性相对较高的灵长类动物——松鼠猴（Saimiri sciureus）的脚步时间样本数据——在平坦的表面、直杆和带有杆子的杆子上行走。横向投影分支来模拟简化的地面和分支基板。我们将这些支撑上的肢体阶段值与模型的期望进行比较。正如预测的那样，在平坦表面上行走的脚步倾向于促进能量交换的肢体相位值。两个极基板诱导的肢体相位值预计有利于减少俯仰和滚动力矩。这些数据为动物如何选择调整其行为以响应平坦和复杂基质上的运动以及影响哺乳动物脚步时间的竞争性选择因素提供了新的见解。这些数据进一步为未来利用这一视角进行调查提供了一条途径。