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

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.