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，所有的啮齿动物都可以根据颌部肌肉的排列被归类为三组中的一组：类似松鼠的啮齿动物(坐骨神经形态)、类似豚鼠的啮齿动物(半形动物)和类似老鼠的啮齿动物(肌形动物)。坐骨神经形体和半形体这两类动物也有不同的取食方式--前者主要是啃食动物，而后者主要是咀嚼动物。肌形体是多面手，并不专门研究它们的进食方式。这提出了一个非常有趣的问题--不同的摄食行为和相关的肌肉排列是否推动了啮齿动物头骨的骨骼形态？令人惊讶的是，尽管啮齿动物在生物学研究、生物多样性和环境健康方面是地球上最重要的群体之一，但还没有人解决这个根本上的重要问题。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态和功能问题：首先，假设在啮齿动物中发现的两种极端的进食方式(咀嚼和啃咬)与颅骨上明显不同的压力和紧张模式有关。还预测，适应这两种喂养模式的大鼠将显示出一种中间的应力和应变模式。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，大脑大小、身体大小或进化史等其他因素对头骨形状的影响与进食相同或更大。最后，据预测，大鼠要么是从超形态头骨上的坐骨神经形态肌肉发育出来的，要么是从坐骨神经形态头骨上的超形态肌肉发育而来的。这预示着，通过修改豚鼠和松鼠模型，我们将能够创建一个与老鼠非常相似的虚拟模型。为了验证上述假设，将根据计算机断层扫描(CT)和磁共振成像(MRI)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。这些技术将被使用，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，它们提供了空间上共同登记的数据，还因为数字模型可以被操纵来测试特定的假设或产生自然界中通常找不到的假设形态。这个项目的结果将使人们更好地了解摄食对啮齿类动物头骨的影响以及颌部肌肉施加的约束。这将有助于解释啮齿目在哺乳动物中的特殊进化成功，特别是啮齿动物中的大鼠和小鼠的成功。我们预计，这项研究将使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受益。

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.