Search

Percentage reduction in maximum incisor bite force with each pair of masticatory muscles set to zero activation

Supplementary figures of the mandibles of extant Glires and a modified version of Fig. 4

Supplementary information contains: - Instruction sheet describing the protocol followed by the three investigators to sculpt muscle volumes; - Additional detail on MDA modelling method; - Discussion of investigator experience and expertise; - Supplementary figures; - Supplementary tables

Additional information on methods and results

R-code of the function predict.size()

(1)Hull York Medical School, University of York, United Kingdom; (2) Department of Earth Sciences, University of Oxford, United Kingdom; (3)Natural History Museum, London, United Kingdom; (4) Department of Musculoskeletal Ageing Science, Institute of Life Course Medical Sciences, University of Liverpool, United Kingdom; (5)Departament de Dinàmica de la Terra i de l’Oceà, Facultat de Ciències de la Terra, Universitat de Barcelona, Barcelona, Spain; (6)Departament de Biodiversitat i Conservació, Institut Mediterrani d’Estudis Avançats (CSIC-UIB), Esporles, Mallorca, Spain; (7)Department of Archaeology, University of York, United Kingdom Drastic morphological changes often occur within mammalian species during prolonged isolation on islands. The depauperate nature of insular ecosystems is often associated with phenomena known as insular dwarfism and gigantism, with such changes in size particularly well-known in the fossil taxa of Mediterranean islands. Insular gigantism has been noted in rodents, lagomorphs and insectivores. Within Rodentia, dormice appear to increase body size more frequently during isolation than other rodents. At least eight different Mediterranean islands have been occupied by giant dormice from the onset of the Miocene.

File containing supplementary information regarding methods and additional results

Contains reconstructions of each specimen showing incisor shape and position within the mandible

Convergence—the independent evolution of similar phenotypes in distantly related clades—is a widespread and much-studied phenomenon. An often-cited, but hitherto untested, case of morphological convergence is that between the aye-aye and squirrels. The aye-aye (Daubentonia madagascariensis) is a highly unusual lemuriform primate that has evolved a dentition similar to that of rodents: it possesses large, ever-growing incisors which it uses to strip the bark from trees in order to feed on wood-boring beetle larvae. Indeed, such is the similarity that some of the earliest classifications of the aye-aye placed it in the squirrel genus Sciurus. Here, we aimed to test the degree of convergence between the skulls and lower jaws of squirrels and the aye-aye. Three-dimensional landmarks were recorded from the crania and mandibles of 46 taxa representing the majority of families in the Euarchontoglires. Results were plotted as phylomorphospaces and convergence measures were calculated. The convergence between squirrels and the aye-aye was shown to be statistically significant for both the cranium and mandible, although the mandibles seem to converge more closely in shape. The convergence may indicate strong functional drivers of morphology in these taxa, i.e. the use of the incisors to produce high bite forces during feeding. Overall, we have shown that this classic case of convergence stands up to quantitative analysis.

Further details on selection of sample, GMM methods, description and illustration of cranial and mandibular landmarks, and presentation of warped surfaces.

Summary of main online repositories for 3D digital morphological data.

The African mole-rats (Bathyergidae) are a family of rodents highly adapted for life underground. Previous research has shown that chisel-tooth digging mole-rats (which use their incisors to dig burrows) are clearly distinguishable from scratch diggers (which only use the forelimbs to tunnel) on the basis of morphology of the skull, and that the differences are linked to the production of high bite forces and wide gapes. We hypothesized that the skull of a chisel-tooth digging mole-rat would perform better at wider gapes than that of a scratch digging mole-rat during incisor biting. To test this hypothesis, we created finite-element models of the cranium of the scratch digging Bathyergus suillus and the chisel-tooth digging Fukomys mechowii, and loaded them to simulate incisor bites at different gapes. Muscle loads were scaled such that the ratio of force to surface area was the same in both models. We measured three performance variables: overall stress across the cranium, mechanical efficiency of biting and degree of deformation across the skull. The Fukomys model had a more efficient incisor bite at all gapes, despite having greater average stress across the skull. In addition, the Fukomys model deformed less at wider gapes, whereas the Bathyergus model deformed less at narrower gapes. These properties of the cranial morphology of Fukomys and Bathyergus are congruent with their respective chisel-tooth and scratch digging behaviours and, all other factors being equal, would enable the more efficient production of bite force at wider gapes in Fukomys. However, in vivo measurements of muscle forces and activation patterns are needed to fully understand the complex biomechanics of tooth digging.

The African mole-rats (Bathyergidae) are a family of rodents highly adapted for life underground. Previous research has shown that chisel-tooth digging mole-rats (which use their incisors to dig burrows) are clearly distinguishable from scratch diggers (which only use the forelimbs to tunnel) on the basis of morphology of the skull, and that the differences are linked to the production of high bite forces and wide gapes. We hypothesized that the skull of a chisel-tooth digging mole-rat would perform better at wider gapes than that of a scratch digging mole-rat during incisor biting. To test this hypothesis, we created finite-element models of the cranium of the scratch digging Bathyergus suillus and the chisel-tooth digging Fukomys mechowii, and loaded them to simulate incisor bites at different gapes. Muscle loads were scaled such that the ratio of force to surface area was the same in both models. We measured three performance variables: overall stress across the cranium, mechanical efficiency of biting and degree of deformation across the skull. The Fukomys model had a more efficient incisor bite at all gapes, despite having greater average stress across the skull. In addition, the Fukomys model deformed less at wider gapes, whereas the Bathyergus model deformed less at narrower gapes. These properties of the cranial morphology of Fukomys and Bathyergus are congruent with their respective chisel-tooth and scratch digging behaviours and, all other factors being equal, would enable the more efficient production of bite force at wider gapes in Fukomys. However, in vivo measurements of muscle forces and activation patterns are needed to fully understand the complex biomechanics of tooth digging.