Comparative cranial biomechanics in two lizard species: impact of variation in cranial design

Cranial morphology in lepidosaurs is highly disparate and characterised by the frequent loss or reduction of bony elements. In varanids and geckos, the loss of the postorbital bar is associated with changes in skull shape, but the mechanical principles underlying this variation remain poorly understood. Here, we sought to determine how the overall cranial architecture and the presence of the postorbital bar relate to the loading and deformation of the cranial bones during biting in lepidosaurs. Using computer-based simulation techniques, we compared cranial biomechanics in the varanid Varanus niloticus and the teiid Salvator merianae, two large, active foragers. The overall strain magnitude and distribution across the cranium were similar in the two species, despite lower strain gradients in V. niloticus. In S. merianae, the postorbital bar is important for resistance of the cranium to feeding loads. The postorbital ligament, which in varanids partially replaces the postorbital bar, does not affect bone strain. Our results suggest that the reduction of the postorbital bar impaired neither biting performance nor the structural resistance of the cranium to feeding loads in V. niloticus. Differences in bone strain between the two species might reflect demands imposed by feeding and non-feeding functions on cranial shape. Beyond variation in cranial bone strain related to species-specific morphological differences, our results reveal that similar mechanical behaviour is shared by lizards with distinct cranial shapes. Contrary to the situation in mammals, the morphology of the circumorbital region, calvaria and palate appears to be important for withstanding high feeding loads in these lizards

The biomechanical role of the chondrocranium and the material properties of cartilage

The chondrocranium is the cartilage component of the vertebrate braincase. Among jawed vertebrates it varies greatly in structure, mineralisation, and in the extent to which it is replaced by bone during development. In mammals, birds, and some bony fish, most of the chondrocranium is replaced by bone whereas in lizards, amphibians, and chondrichthyan fish it may remain a significant part of the braincase complex in adulthood. To what extent this variation relates to differences in skull biomechanics is poorly understood. However, there have been examinations of chondrocranium histology, in vivo strain, and impact on rostrum growth following partial removal of the chondrocranium. These studies have led to suggestions that the chondrocranium may provide structural support or serve to dampen external loads. Advances in computing-power have also facilitated an increase in the number of three-dimensional computer-based models. These models can be analysed (in silico) to test specific biomechanical hypotheses under specified loading conditions. However, representing the material properties of cartilage is still problematic because these properties differ according to the speed and direction of loading. The relationship between stress and strain is also non-linear. Nevertheless, analyses to date suggest that the chondrocranium does not provide a vertical support in lizards but it may serve to absorb some loads in humans. We anticipate that future models will include ever more detailed representations of the loading, anatomy, and material properties, in tandem with rigorous forms of model validation. However, comparison among a wider range of vertebrate subjects should also be pursued, in particular larvae, juveniles, and very small adult animals

Electron Loss from 1.4-MeV / u U\u3csup\u3e4,6,10+\u3c/sup\u3e Ions Colliding with Ne, N₂, and Ar Targets

Absolute, total, single- and multiple-electron-loss cross sections are measured for 1.4-MeV / u U4,6,10+ ions colliding with neon and argon atoms and nitrogen molecules. It is found that the cross sections all have the same dependence on the number of electrons lost and that multiplying the cross sections by the initial number of electrons in the 6s, 6p, and 5f shells yields good agreement between the different projectiles. By combining the present data with previous measurements made at the same velocity, it is shown that the scaled cross sections slowly decrease in magnitude for incoming charge states between 1 and 10, whereas the cross sections for higher-charge-state ions fall off much more rapidly

Switching stiction and adhesion of a liquid on a solid

When a gecko moves on a ceiling it makes use of adhesion and stiction. Stiction-static friction-is experienced on microscopic and macroscopic scales and is related to adhesion and sliding friction. Although important for most locomotive processes, the concepts of adhesion, stiction and sliding friction are often only empirically correlated. A more detailed understanding of these concepts will, for example, help to improve the design of increasingly smaller devices such as micro-and nanoelectromechanical switches. Here we show how stiction and adhesion are related for a liquid drop on a hexagonal boron nitride monolayer on rhodium, by measuring dynamic contact angles in two distinct states of the solid-liquid interface: a corrugated state in the absence of hydrogen intercalation and an intercalation-induced flat state. Stiction and adhesion can be reversibly switched by applying different electrochemical potentials to the sample, causing atomic hydrogen to be intercalated or not. We ascribe the change in adhesion to a change in lateral electric field of in-plane two-nanometre dipole rings, because it cannot be explained by the change in surface roughness known from the Wenzel model. Although the change in adhesion can be calculated for the system we study, it is not yet possible to determine the stiction at such a solid-liquid interface using ab initio methods. The inorganic hybrid of hexagonal boron nitride and rhodium is very stable and represents a new class of switchable surfaces with the potential for application in the study of adhesion, friction and lubrication. © 2016 Macmillan Publishers Limited