The evolution of mammal body sizes: responses to Cenozoic climate change in North American mammals

Explanations for the evolution of body size in mammals have remained surprisingly elusive despite the central importance of body size in evolutionary biology. Here, we present a model which argues that the body sizes of Nearctic mammals were moulded by Cenozoic climate and vegetation changes. Following the early Eocene Climate Optimum, forests retreated and gave way to open woodland and savannah landscapes, followed later by grasslands. Many herbivores that radiated in these new landscapes underwent a switch from browsing to grazing associated with increased unguligrade cursoriality and body size, the latter driven by the energetics and constraints of cellulose digestion (fermentation). Carnivores also increased in size and digitigrade, cursorial capacity to occupy a size distribution allowing the capture of prey of the widest range of body sizes. With the emergence of larger, faster carnivores, plantigrade mammals were constrained from evolving to large body sizes and most remained sma...尽管体型在进化生物学中具有核心地位，但学界对哺乳动物体型演化的解释始终令人意外地难以捉摸。本研究提出一则演化模型，主张新北界（Nearctic）哺乳动物的体型演化受新生代气候与植被变化塑造。早始新世气候最适事件发生后，森林逐渐退缩，被开阔林地与稀树草原景观取代，后续更演化为草原。在此类新兴景观中辐射演化的诸多植食性动物，从取食嫩枝嫩叶转向啃食草本植物，伴随而来的是蹄行性奔跑能力与体型的提升；其中体型增大的驱动因素为纤维素消化（发酵）的能量代谢需求与生理约束。食肉动物的体型与趾行性奔跑能力亦同步提升，以此占据能够捕获最广泛体型跨度猎物的体型分布区间。随着体型更大、移动速度更快的食肉动物出现，跖行性哺乳动物的体型演化受到限制，绝大多数物种始终保持着较小的体型。