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占据核心地位，但学界对哺乳动物体型演化的解释始终出人意料地难以达成共识。在此，我们提出一项模型，认为新北界哺乳动物（Nearctic mammals）的体型受新生代（Cenozoic）气候与植被变化塑造。始新世气候适宜期（Early Eocene Climate Optimum）结束后，森林逐渐退缩，取而代之的是开阔林地与稀树草原景观，后续进一步演化为草原。在这些新生生态景观中辐射演化的众多植食性类群，从取食木本植被转向取食草本植被，这一转变伴随蹄行性（unguligrade）奔跑能力与体型的提升，而体型增大则由纤维素消化（发酵）的能量需求与生理约束所驱动。食肉动物类群的体型与趾行性（digitigrade）奔跑能力亦同步提升，以占据能够捕捉体型跨度最广的猎物的生态位。随着体型更大、移动更快的食肉动物出现，跖行性（plantigrade）哺乳动物的体型演化受到约束，难以走向大体型，且在新生代中期的绝大多数时段内，其体型均小于1千克。作为数据库中规模最大的运动功能群（locomotor guild，n=1186，占数据库总物种数的59%），我们未发现跖行性哺乳动物存在符合柯普法则（Cope's Rule）或伯格曼法则（Bergmann's Rule）的一致证据。部分耐寒特化的跖行性哺乳动物，例如河狸与旱獭，在中新世气候适宜期（Miocene Climate Optimum）后体重出现显著增长，但这一现象亦可部分通过伯格曼法则解释。本研究再次强调，若要理解当代哺乳动物的体型分布格局，必须考量哺乳动物形态类群的演化历史及其衍生的形态与功能特征。