Fine-branch models have long played a central role in primate evolutionary research. Nevertheless, recent studies of positional behavior in nonprimate arboreal mammals have challenged the idea that synapomorphic primate features, such as grasping extremities, uniquely facilitated access to the fine-branch zone. We test the alternative hypothesis that grasping extremities specifically improve locomotor performance in a fine-branch environment by examining how support diameter influences locomotor mechanics in one sciurid rodent (Sciurus carolinensis) and two platyrrhine primates (Callithrix jacchus and Saimiri boliviensis). These species were chosen to broadly model different stages in the evolution of primate grasping morphology. The results showed that transitioning from broad to narrower supports required the greatest kinematic adjustment in squirrels and the least adjustment in squirrel monkeys, with marmosets displaying an intermediate level of adjustment. Moreover, on any given support, squirrels' locomotor mechanics differed from marmosets' in a manner consistent with a greater need for stability, despite superficial ecomorphological similarities between sciurid rodents and callitrichine primates. Morphological analyses of autopodial size and proportions suggest that variation in locomotor performance more closely tracked variation in overall hand and foot size rather than digit length per se. Indeed, a broad comparative analysis revealed that for their body mass, primates have longer hands than similarly sized arboreal rodents and marsupials (although only the primate-rodent comparison was significant after incorporating phylogenetic relatedness). Inclusion of fossil stem primates (plesiadapiforms) and euprimates (adapiforms) in these analyses suggests that this primate-wide grade shift in relative autopodial size must have occurred early in the evolutionary history of the group. Overall, our findings show that basal primate morphological adaptations may have specifically facilitated improved locomotor performance in a fine-branch niche, rather than merely permitting access to the environment. As such, future adaptive hypotheses of primate origins should incorporate the import of primate-like morphology on locomotor performance as well.

长期以来，精细分支模型在灵长类动物进化研究中发挥了核心作用。尽管如此，最近在非灵长类动物栖类哺乳动物中的位置行为研究挑战了这样的观点，即突触型灵长类特征（例如抓住四肢）独特地促进了进入细分支区。我们通过检验支点直径如何影响一只短尾啮齿动物（Scuirus carolinensis）和两只白y灵长类动物（Callithrix jacchus和Saimiri boliviensis）的支配直径如何影响运动力学，来测试另一种假设，即在四肢环境中，抓住四肢可以特别改善运动性能。）。选择这些物种是为了广泛地模拟灵长类动物抓握形态演化的不同阶段。结果表明，从宽到窄的支撑过渡需要松鼠的运动学调整最大，而松鼠的猴子的调整最少，而，猴显示出中等的调整水平。而且，在任何给定的支持下，尽管松鼠啮齿类动物和卡利替利丁灵长类动物之间在表面生态学上有相似性，但松鼠的运动力学与mos猴的运动力学在一定程度上与对稳定性的需求一致。对自足体大小和比例的形态学分析表明，运动能力的变化更能追踪整体手脚大小的变化，而不是手指长度本身的变化。确实，广泛的比较分析表明，灵长类动物的体重要比同等大小的树栖啮齿动物和有袋动物更长（尽管只有灵长类与啮齿类动物的比较才具有系统发育相关性）。在这些分析中包括化石干的灵长类动物（plesiadapiforms）和大戟类动物（adapiforms）表明，相对于自足体大小的这种灵长类动物的等级变化一定是在该组进化史的早期发生的。总体而言，我们的发现表明，基础灵长类动物的形态适应可能特别促进了细枝小生境中运动能力的改善，而不是仅仅允许进入环境。因此，未来对灵长类起源的适应性假说也应纳入运动能力方面的类似灵长类形态的输入。