Data from: 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 smaller than 1 kg throughout the middle Cenozoic. We find no consistent support for either Cope's Rule or Bergmann's Rule in plantigrade mammals, the largest locomotor guild (n = 1186, 59% of species in the database). Some cold-specialist plantigrade mammals, such as beavers and marmots, showed dramatic increases in body mass following the Miocene Climate Optimum which may, however, be partially explained by Bergmann's rule. This study reemphasizes the necessity of considering the evolutionary history and resultant form and function of mammalian morphotypes when attempting to understand contemporary mammalian body size distributions.

尽管体型在进化生物学中居于核心地位，但学界对哺乳动物体型演化的解释始终难以令人满意。本研究提出一项模型，认为新北界（Nearctic）哺乳动物的体型受到新生代（Cenozoic）气候与植被变化的塑造。早始新世气候适宜期（Early Eocene Climate Optimum）结束后，森林逐渐退缩，被开阔林地与稀树草原景观取代，后续进一步演化为草原生态系统。在此类新生景观中辐射演化的诸多植食性动物，从啃食乔木灌木叶片的习性转向取食草本植物的放牧习性，伴随而来的是蹄行性（unguligrade）奔跑能力与体型的提升，其中体型增长的驱动因素为纤维素消化（发酵）的能量需求与相关生理约束。食肉动物的体型亦同步增大，同时趾行性（digitigrade）奔跑能力得到提升，以此占据能够捕获最宽泛体型范围猎物的体型分布区间。随着体型更大、移动更快的食肉动物出现，跖行性（plantigrade）哺乳动物难以演化出大型体型，其中绝大多数在新生代中期始终保持在1千克以下。本研究发现，作为数据库中占比最高的运动功能群（locomotor guild，样本量n=1186，占物种总数的59%），跖行性哺乳动物既未得到科普法则（Cope's Rule）的一致支持，也不符合伯格曼法则（Bergmann's Rule）。部分耐寒特化的跖行性哺乳动物，如河狸与旱獭，在中新世气候适宜期（Miocene Climate Optimum）后体型出现显著增长，但这一现象可部分通过伯格曼法则得到解释。本研究再次强调，若要理解当代哺乳动物的体型分布格局，必须充分考虑哺乳动物形态类型的演化历史及其衍生的形态与功能特征。