The mammalian cranium (skull without lower jaw) is representative of mammalian diversity and is thus of particular interest to mammalian biologists across disciplines. One widely retrieved pattern accompanying mammalian cranial diversification is referred to as ‘craniofacial evolutionary allometry’ (CREA). This posits that adults of larger species, in a group of closely related mammals, tend to have relatively longer faces and smaller braincases. However, no process has been officially suggested to explain this pattern, there are many apparent exceptions, and its predictions potentially conflict with well-established biomechanical principles. Understanding the mechanisms behind CREA and causes for deviations from the pattern therefore has tremendous potential to explain allometry and diversification of the mammalian cranium. Here, we propose an amended framework to characterise the CREA pattern more clearly, in that ‘longer faces’ can arise through several kinds of evolutionary change, including elongation of the rostrum, retraction of the jaw muscles, or a more narrow or shallow skull, which all result in a generalised gracilisation of the facial skeleton with increased size. We define a standardised workflow to test for the presence of the pattern, using allometric shape predictions derived from geometric morphometrics analysis, and apply this to 22 mammalian families including marsupials, rabbits, rodents, bats, carnivores, antelopes, and whales. Our results show that increasing facial gracility with size is common, but not necessarily as ubiquitous as previously suggested. To address the mechanistic basis for this variation, we then review cranial adaptations for harder biting. These dictate that a more gracile cranium in larger species must represent a structural sacrifice in the ability to produce or withstand harder bites, relative to size. This leads us to propose that facial gracilisation in larger species is often a product of bite force allometry and phylogenetic niche conservatism, where more closely related species tend to exhibit more similar feeding ecology and biting behaviours and, therefore, absolute (size-independent) bite force requirements. Since larger species can produce the same absolute bite forces as smaller species with less effort, we propose that relaxed bite force demands can permit facial gracility in response to bone optimisation and alternative selection pressures. Thus, mammalian facial scaling represents an adaptive by-product of the shifting importance of selective pressures occurring with increased size. A reverse pattern of facial ‘shortening’ can accordingly also be found, and is retrieved in several cases here, where larger species incorporate novel feeding behaviours involving greater bite forces. We discuss multiple exceptions to a bite force-mediated influence on facial proportions across mammals which lead us to argue that ecomorphological specialisation of the cranium is likely to be the primary driver of facial scaling patterns, with some developmental constraints as possible secondary factors. A potential for larger species to have a wider range of cranial functions when less constrained by bite force demands might also explain why selection for larger sizes seems to be prevalent in some mammalian clades. The interplay between adaptation and constraint across size ranges thus presents an interesting consideration for a mechanistically grounded investigation of mammalian cranial allometry.

中文翻译：

---

面对事实：身体尺寸范围的适应性权衡决定了哺乳动物的颅面尺度

哺乳动物的颅骨（没有下颌的头骨）代表了哺乳动物的多样性，因此跨学科的哺乳动物生物学家特别感兴趣。一种广泛检索的伴随哺乳动物颅骨多样化的模式被称为“颅面进化异速生长”（CREA）。这表明，在一组密切相关的哺乳动物中，较大物种的成年个体往往具有相对较长的面孔和较小的脑壳。然而，官方还没有提出任何过程来解释这种模式，存在许多明显的例外，并且其预测可能与既定的生物力学原理相冲突。因此，了解 CREA 背后的机制以及偏离模式的原因对于解释哺乳动物颅骨的异速生长和多样化具有巨大的潜力。在这里，我们提出了一个修正的框架，以更清楚地描述 CREA 模式，因为“更长的脸”可以通过几种进化变化而出现，包括嘴部的延长、下颌肌肉的回缩，或者更窄或更浅的头骨，所有这些都会导致面部骨骼普遍细化，尺寸增大。我们定义了一个标准化工作流程来测试模式的存在，使用几何形态测量分析得出的异速形状预测，并将其应用于 22 个哺乳动物科，包括有袋动物、兔子、啮齿动物、蝙蝠、食肉动物、羚羊和鲸鱼。我们的研究结果表明，面部轮廓随着尺寸的增加而增加是常见的，但不一定像之前建议的那样普遍存在。为了解决这种变化的机制基础，我们随后回顾了更难咬合的颅骨适应。这些表明，相对于尺寸而言，较大物种的更纤细的头盖骨必须代表在产生或承受更硬咬伤的能力方面的结构性牺牲。这使我们提出，较大物种的面部细化通常是咬合力异速生长和系统发育生态位保守性的产物，其中关系更密切的物种往往表现出更相似的摄食生态和咬合行为，因此，绝对（与大小无关）咬合力的要求。由于较大的物种可以用更少的努力产生与较小的物种相同的绝对咬合力，因此我们建议放松的咬合力需求可以允许面部优雅，以响应骨骼优化和替代选择压力。因此，哺乳动物的面部缩放代表了随着体型增大而发生的选择性压力的重要性发生变化的适应性副产品。因此，也可以发现面部“缩短”的相反模式，并且在此处的几个案例中得到检索，其中较大的物种采用涉及更大咬合力的新颖摄食行为。我们讨论了咬合力介导的对哺乳动物面部比例影响的多个例外，这使我们认为颅骨的生态形态专业化可能是面部缩放模式的主要驱动因素，而一些发育限制可能是次要因素。当受咬合力需求的限制较小时，较大的物种可能具有更广泛的颅骨功能，这也可能解释了为什么在某些哺乳动物进化枝中对较大体型的选择似乎很普遍。因此，跨尺寸范围的适应和约束之间的相互作用为哺乳动物颅骨异速生长的机械基础研究提供了有趣的考虑。