Diversity of the masticatory apparatus among extant rodents: 3D analysis and modeling of form and function

啮齿动物是哺乳动物中最大的目（啮齿目），包括2000多个物种。然而，尽管它们的多样性，所有的啮齿类动物都可以根据颌骨肌肉的排列分为三组：松鼠样啮齿类动物（sciuromorphs），豚鼠样啮齿类动物（hystricomorphs）和大鼠样啮齿类动物（myomorphs）。有两个族群，松鼠形和豪猪形，也有不同的进食模式-前者主要是啃食者，而后者主要是咀嚼者。Myomorphs是多面手，并不专注于他们的进食模式。这提出了一个非常有趣的问题--不同的进食行为和相关的肌肉排列是否驱动了啮齿动物头骨中的骨骼形式？令人惊讶的是，没有人解决这个根本性的重要问题，尽管啮齿动物是地球上生物研究，生物多样性和环境健康方面最重要的群体之一。我们的目标是利用我们在成像，计算机建模和进化生物学方面的集体专业知识，通过三个核心假设来解决啮齿动物的形式和功能问题：首先，假设啮齿动物中发现的两种极端进食模式（咀嚼和啃咬）与头骨上明显不同的应力和应变模式有关。据预测，适应这两种喂养方式的大鼠将表现出一种中间的应激和紧张模式。其次，假设不同的应力/应变模式将解释三组之间头骨形状和肌肉附着点的差异。另一种情况是，其他因素，如大脑大小，身体大小或进化历史，对头骨形状的影响与进食相同或更大。最后，据预测，大鼠要么从hystricomorph头骨上的sciuromorph肌肉发育而来，要么从sciuromorph头骨上的hystricomorph肌肉发育而来。这预示着，通过修改豚鼠和松鼠模型，我们将能够创建一个非常类似于大鼠的虚拟模型。为了检验上述假设，将从计算机断层扫描（CT）和磁共振成像（MRI）扫描重建三维数字模型。这些模型将使用有限元分析（FEA）进行分析-这是一种预测数字结构中的变形，应变和应力的技术。将使用这些技术，因为它们是非侵入性的，从而消除了对活体动物实验的需要，它们提供了空间共配准的数据，并且还因为可以操纵数字模型来测试特定的假设或产生自然界中通常不存在的假设形态。该项目的结果将导致更好地了解啮齿动物头骨上的进食效果和下颌肌肉施加的限制。这将有助于解释啮齿目动物在哺乳动物中的特殊进化成功，特别是啮齿类动物中大鼠和小鼠的成功。我们预计，这项研究将有利于对生物多样性和哺乳动物物种形成模式感兴趣的环境和进化生物学家。

Rodents form the largest order (Rodentia) of mammals comprising well over two thousand species. However, despite their diversity, all rodents can be assigned to one of just three groups based on the arrangement of the jaw muscles: the squirrel-like rodents (sciuromorphs), the guinea pig-like rodents (hystricomorphs), and the rat-like rodents (myomorphs). Two groups, the sciuromorphs and hystricomorphs, also have distinct modes of feeding - the former are predominantly gnawers whereas the latter are mostly chewers. The myomorphs are generalists and do not specialise in their mode of feeding. This raises a very interesting question - are the differing feeding behaviours and related muscle arrangements driving skeletal form in the rodent skull? Surprisingly, no one has addressed this fundamentally important question, even though rodents are one of the most important groups on the planet in terms of biological research, biodiversity, and environmental health. Our aim is to draw on our collective expertise in imaging, computer modelling and evolutionary biology to address the rodent form and function question by way of three core hypotheses: Firstly, it is hypothesised that the two extreme modes of feeding found among rodents (chewing and gnawing) are associated with notably different patterns of stress and strain across the skull. It is also predicted that rats, which are adapted to both modes of feeding, will show an intermediate pattern of stress and strain. Secondly, it is hypothesised that the different stress/strain patterns will explain differences in the shape of the skull and the attachment points of the muscles between the three groups. The alternative scenario is that other factors such as brain size, body size or evolutionary history are having an equal or greater effect on the skull shape than feeding. Lastly, it is predicted that rats either develop from sciuromorph muscles on a hystricomorph skull or hystricomorph muscles on a sciuromorph skull. This predicts that by modifying guinea pig and squirrel models, we will be able to create a virtual model that closely resembles a rat. To test the above hypotheses, three-dimensional digital models will be reconstructed from computed tomography (CT) and magnetic resonance imaging (MRI) scans. The models will be analysed using finite element analysis (FEA) - a technique that predicts deformation, strain and stress in a digital structure. These techniques will be used because they are non-invasive, thereby removing the need for live animal experimentation, they provide spatially co-registered data, and also because the digital models can be manipulated to test particular hypotheses or to produce hypothetical morphologies not normally found in nature. The results of this project will lead to a greater understanding of the effect of feeding on the rodent skull and the constraints imposed by the jaw muscles. This will help explain the exceptional evolutionary success of the order Rodentia amongst the mammals, and, in particular, the success of the rats and mice within the rodents. We anticipate that the research will benefit environmental and evolutionary biologists interested in modes of biodiversity and mammalian speciation.