Multidisciplinary approaches to the evolutionary history of felids: phylogeny disparity and biomechanics in living and fossil cats

作为有颌脊椎动物进化过程中的一项关键创新，咬食使得捕食策略多样化，并因此成功地扩展到生态位。不同的解剖适应支撑着不同的摄食功能。因此，我们对形式和功能之间关系的理解是梳理与头骨和颌骨生物力学功能相关的进化变化的关键。然而，功能的定量分析及其与结构修饰和解剖约束的关联需要新的方法和对当前范式的重新定义。为了研究功能，我们将使用几个指标，特别是咬合力。这提供了一个很好的定量代理，因为它由一个单一的数值来表示，该数值概括了多个功能参数，包括颅面和颌的结构，肌肉位置和肌肉大小。然而，咬合力与体型密切相关（动物越大，咬合力就越强）。由于这个原因，一个标准化的和尺寸无关的相对咬合力将被用来比较咬合力的变化，这是专门与功能。作为个案研究，我们将以猫科动物（Felidae）为研究对象，分析其进食功能及其与头骨形状、体型和进化史的关系。猫是生物力学分析的优秀研究对象。与普遍的看法相反，它们看似保守的头骨形态实际上能够广泛地发挥与进食有关的功能，尤其是在小型猫科动物中。猫的祖先相对较近（现存猫的辐射大约在1000万年左右，而最基本的猫科动物的祖先则在2000 - 2500万年左右），种类繁多（41种），体型各异（5到300多公斤），分布广泛。所有这些因素都表明，这一群体经历了快速的辐射，最终形成了过去和现在生态系统中的一些顶级掠食者（例如狮子和剑齿猫）。该项目由两个主要且完整的部分组成。首先，我们将测试功能性状的进化模式。这将通过使用一套数值分析协议（在系统发育比较方法的保护伞下松散地组装）来完成。这些方法使我们能够将系统发育信息整合到功能性状数据集中，并沿着系统发育树的分支测试系统发育信号或性状变化的任何模式。在功能数据中测试系统发育的影响是很重要的，因为我们在数据中观察到的一些可变性实际上可能是因为被比较物种的亲缘关系。例如，在标准化的咬合力数据中，较小的猫显示出很大的差异，而较大的猫（大型猫）显示出的差异要小得多。这背后可能真的有一些生物学上的原因；也许较小的猫占据了不同的功能龛。然而，小型猫科动物在分类上是多样化的，属于多个谱系，而大型猫科动物都是近亲；体型较小的猫的功能值变异性较大，而体型较大的猫的功能值变异性较小，这可以很好地归因于系统发育距离。因此，必须测试系统发育对数据的影响，以便了解功能性状的进化限制或过程。其次，我们将比较功能性状的变化与形态变化的关系。这将通过使用几何形态计量学来完成。这种方法使用标记来捕捉猫的头骨和颌骨的显著特征，并允许识别形状随体型增加或系统发育（在分类组内或跨分类组）变化的模式。颅骨形状变化的模式可以与功能特征的变化进行比较，以确定可能与功能密切相关的关键形态特征。

As a key innovation in the evolution of jawed vertebrates, biting permits a diverse array of feeding strategies and, as a consequence, also successful expansion into niches. Various anatomical adaptations underpin different feeding functions. Our understanding of the relationship between form and function is thus key to teasing out evolutionary changes related to skull and jaw biomechanics function. However, quantitative analyses of function and their associations to structural modifications and anatomical constraints require novel approaches and redefinition of current paradigms. To study function, we will use several metrics, in particular bite force. This offers a good quantitative proxy, as it is represented by a single numerical value that summarises multiple functional parameters, including craniofacial and jaw configuration, muscle position, and muscle size. However, bite force is strongly correlated with body size (the bigger the animal the more powerful its bite). For this reason, a standardised and size-independent relative bite force will be used to compare the variation in bite force that is exclusively linked to function. As a case-study, we will use cats (Family Felidae) to analyze feeding function, and its relation to skull shape, body size, and evolutionary history. Cats are an excellent study group for biomechanical analyses. Contrary to popular belief, their seemingly conservative skull morphology is in fact capable of a wide range of feeding-related functions, particularly among smaller cats. Cats share a relatively recent ancestry (around 10 million years for the extant cats' radiation and 20-25 million years for the most basal stem members of the family), are extremely diverse (41 living species), exhibit a spectrum of body sizes (5 to 300+ kg), and are widely distributed. All these elements suggest that the group underwent a rapid radiation, culminating in some of the top predators of past and present ecosystems (e.g. lions and sabre-tooth cats). The project consists of two major and integrated parts. First, we will test for patterns of evolution in functional traits. This will be accomplished by using a suite of numerical analytical protocols (loosely assembled under the umbrella of phylogenetic comparative methods). These methods allow us to incorporate phylogenetic information into the dataset of functional traits and test for phylogenetic signals or any patterns in trait changes along the branches of the phylogenetic tree. Testing for the effects of phylogeny in functional data is important because some of the variability we observe in the data may actually be because of the relatedness of the species under comparison. For instance, smaller cats show a great deal of variation in standardised bite force data but the larger cats (great cats) show far less variance. There may truly be some biological reason behind this; perhaps smaller cats occupy diverse functional niches. However, the smaller cats are taxonomically diverse and belong to multiple lineages, whereas the great cats are all closely related; the greater variability of functional values in smaller cats and the lower variance in larger cats could very well be attributed to phylogenetic distances. Thus, the effects of phylogeny on the data must be tested in order to understand the evolutionary constraints or processes of functional traits. Second, we will compare changes in functional traits in relation to changes in morphology. This will be accomplished by using geometric morphometrics. This method uses landmarks to capture salient features of interest in the skulls and jaws of cats and allows for identifications of patterns in shape change with increasing body size or with phylogeny (within or across taxonomic groups). Patterns of shape change in skulls can be compared with changes in functional traits to identify key morphological features that may be associated closely with function.