COMPLIANT INTERACTIONS AND LIMB MECHANICS DURING ARBOREAL LOCOMOTION IN TROPICAL FOREST ENVIRONMENTS

苏门答腊猩猩是最大的哺乳动物，完全生活在树上，但它们习惯于在树木细长的外围树枝--末端树枝--导航，那里是大多数美味水果和树冠之间最窄的间隙所在的地方。从理论上讲，他们应该通过悬挂在下面，或者在上面用四条腿走路，高度弯曲的关节来接近这些细长的树枝，以减少树枝的振动。但与这些预测相反，我们发现猩猩实际上是两足行走(也就是说，像人类一样)在非常小的灵活的树枝上行走，使用它们长长的可缠绕的脚趾抓住多个支撑物并增加稳定性，同时腾出一只或两只手去接触水果或其他树枝进行缺口杂交。有趣的是，与迄今测试过的所有其他猴子和类人猿(包括黑猩猩和大猩猩)不同，猩猩在树上两足行走时保持非常笔直的腿。这样做的好处还不清楚，但由于三个关键原因，它很重要。首先，猩猩是树栖动物运动生态以及大体质量与末端分枝生态位关系的重要模型。其次，树栖两足动物被越来越多地认为是所有类人猿共同祖先运动能力的基本组成部分，阐明其生态和机制将有助于解释中新世化石记录和现存类人猿运动多样性的进化。最后，越来越多的证据表明，人类陆地两足动物的起源在于在树栖动物而不是陆地环境中的运动，研究树栖两足动物的运动生态学和力学可能有助于揭示我们自己两足动物进化的形成阶段。两足动物是在树上进化的，自旧大陆猴子分离以来在某种程度上一直存在，这是区分人类和黑猩猩化石记录的关键，因为传统上人们把适应两足动物的行为定义为人类祖先与其他非洲类人猿的祖先。在这项建议中，我们结合了对野生猩猩、动物园猩猩和人类的研究，以量化树栖两足动物的机制，以及至关重要的是，这些机制如何随着树枝机械结构的变化而变化，因为树枝逐渐变细，朝着末端变得更灵活。就像人类运动员在有弹性的跑道上跑步时恢复能量一样，伸直腿的姿势可能会使能量在关节之间转移，或者从灵活的树枝转移到猩猩身上。为了研究这些，我们将使用步态分析中的一种相对较新的技术--超声波，来记录人类和猩猩在运动过程中肌肉和肌腱的行为。当与肌肉活动和力学的数据合并时，这将使我们能够获得树栖两足动物内部和外部能量储存和转移的完整图像。我们还将用计算机建模来补充这些数据，以模拟在不可能或不实用于实验测试的情况下从分支和关节之间进行的能量转移。

Sumatran orangutans are the largest mammal to live an exclusively arboreal lifestyle and yet they habitually navigate the slender, peripheral branches of trees - the terminal branch niche - where the majority of tasty fruits and the narrowest gaps between tree crowns are situated. Theoretically they should access these slender branches either by suspending underneath them or walking on all fours on top with highly flexed joints to reduce branch vibrations. But contrary to these predictions we have shown that orangutans actually walk bipedally (that is, like humans) on the very smallest flexible branches, using their long prehensile toes to grip multiple supports and increase stability, while freeing one or both hands to reach fruits or other branches for gap crossing. Interestingly and unlike all other monkeys and apes tested to date (including chimpanzees and gorillas), orangutans maintain very straight legs when they walk bipedally in the trees. The benefits of this are unclear, but are important for 3 key reasons. Firstly, orangutans are an important model for the locomotor ecology of arboreal animals and the relationship between large body mass and the terminal branch niche. Secondly, arboreal bipedalism is increasingly thought to have been a fundamental component of the locomotor repertoire of the common ancestor of all apes and elucidating its ecology and mechanics will aid interpretation of the Miocene fossil record and the evolution of locomotor diversity in the living apes. Finally, increasing evidence suggests that the origins of human terrestrial bipedality lie in locomotion in an arboreal rather than terrestrial setting and studying the locomotor ecology and mechanics of arboreal bipedalism may shed light on the formative stages of the evolution of our own bipedality. The suggestion that bipedality evolved in the trees and has been present to some extent since the split from the old world monkeys is key to the distinction between the human and chimpanzee fossil record, since adaptations for bipedality have traditionally been taken to define human ancestors from those of the other African apes. In this proposal we combine studies of wild orangutans with those of zoo orangutans and humans to quantify the mechanics of arboreal bipedality, and crucially, how these change in accordance with changes in the mechanics of the branches, since branches taper and become more flexible towards the ends. It is possible that the straight legged postures might enable either transfer of energy between joints; or the transfer of energy from the flexible branches to the orangutan, in the same way as human athletes recover energy when running on springy running tracks. To study these we will use a relatively new technique in gait analysis, ultrasound, to record the behaviour of the muscles and tendons during locomotion in humans and orangutans. When merged with data on muscle activity and mechanics this will allow us to obtain a complete picture of internal and external energy storage and transfer during arboreal bipedality. We will also complement these data with computer modeling to mimic energy transfer from branches and between joints in situations that are not possible or practical to test experimentally.