Directional differences in head stabilisation in Acanthodactylus pardalis lizards.

Running inevitably causes the animal trunk to undulate. The consequential head rotations have to be stabilised in space for a steady gaze and an accurate sense of self-motion for balance. The ecology and anatomy of the species determine the necessity to stabilise the head in yaw, pitch, and roll direction. Terrestrial lizards, running with a sprawled body posture, are especially prone to undulations in the horizontal (yaw) plane. Measurements on an experimental oscillation platform show that Acanthodactylus pardalis lizards stabilise their head less in pitch direction (54% stabilisation) than in yaw and roll direction (66% and 64% stabilisation, respectively). Because we performed these experiments in darkness, the lizards based their head stabilisation on vestibular information. Hence, we hypothesised that their vestibular system is less sensitive in pitch direction than in yaw and roll direction. Yet, this was not confirmed by a detailed Fluid-Structure Interaction model of the membranous labyrinth, which showed that not pitch sensitivity (88% of yaw sensitivity), but roll sensitivity (73% of yaw sensitivity) is the lowest. So why is the head stabilisation in darkness almost as good in roll direction as in yaw direction? While this may be due to neurological nonlinearities, it seems worth noticing that the moment of inertia is lowest in roll direction due to the elongated head shape. Hence, less torque is needed to stabilise a head rotation in roll direction than in the other two directions.

Why the semicircular canals are not stimulated by linear accelerations.

Head accelerations are sensed by the vestibular system in the inner ear. Linear accelerations stimulate the otolith organs, while the semicircular canals (SCC) sense angular accelerations. Fluid-structure interaction (FSI) models of the cupula sensor (simulated with finite element method (FEM)) and the endolymph fluid (simulated with computational fluid dynamics (CFD)) in the semicircular canal offer the possibility to investigate why the SCC are not stimulated by linear accelerations. Two hypotheses exist in the literature. The first hypothesis focusses on the density of the cupula sensor in the SCC, while the second is based on the continuous loop of fluid in the semicircular canal. However, neither increasing the cupula density, nor disrupting the continuous fluid circulation substantially increase the cupula deformation under linear head acceleration, thereby rejecting both existing hypotheses. We propose an alternative hypothesis, based on the circular geometry of the semicircular canal. During angular head acceleration, the cupula intersects the body of endolymph and 'pushes' it forward because the cupula seals the semicircular canal like a diaphragm. This results in cupula deflection and neural stimulation. During linear head acceleration, on the other hand, a large part of the canal wall also 'pushes' the endolymph forward, which leads to hardly any cupula deflection.

Asymmetric cupula displacement due to endolymph vortex in the human semicircular canal.

The vestibular system in the inner ear senses angular head manoeuvres by endolymph fluid which deforms a gelatinous sensory structure (the cupula). We constructed computer models that include both the endolymph flow (using CFD modelling), the cupula deformation (using FEM modelling), and the interaction between both (using fluid-structure interaction modelling). In the wide utricle, we observe an endolymph vortex. In the initial time steps, both the displacement of the cupula and its restorative forces are still small. As a result, the endolymph vortex causes the cupula to deform asymmetrically in an S-shape. The asymmetric deflection increases the cupula strain near the crista and, as a result, enhances the sensitivity of the vestibular system. Throughout the head manoeuvre, the maximal cupula strain is located at the centre of the crista. The hair cells at the crista centre supply irregularly spiking afferents, which are more sensitive than the afferents from the periphery. Hence, the location of the maximal strain at the crista may also increase the sensitivity of the semicircular canal, but this remains to be tested. The cupula overshoots its relaxed position in a simulation of the Dix-Hallpike head manoeuvre (3 s in total). A much faster head manoeuvre of 0.222 s showed to be too short to cause substantial cupula overshoot, because the cupula time scale of both models (estimated to be 3.3 s) is an order of magnitude larger than the duration of this manoeuvre.

Mechanoreceptor distribution in stag beetle jaws corresponds to the material stress in fights.

Male stag beetles (Lucanidae) use their extremely elongated jaws to pinch their rivals forcefully in male-male battles. The morphology of these jaws has to be a compromise between robustness (to withstand the bite forces), length and weight. Cyclommatus metallifer stag beetles circumvent this trade-off by reducing their bite force when biting with their slender jaw tips. Here we describe the functional mechanism behind the force modulation behaviour. Scanning Electron Microscopy and micro CT imaging show large numbers of small sensors in the jaw cuticle. We find a strong correlation between the distribution of these sensors and that of the material stress in the same jaw region during biting. The jaw sensors are mechanoreceptors with a small protrusion that barely protrudes above the undulating jaw surface. The sensors stimulate dendrites that extend from the neuronal cell body through the entire thickness of the jaw exoskeleton towards the sensors at the external surface. They form a sensory field that functions in a feedback mechanism to control the bite muscle force. This negative feedback mechanism enabled the stag beetles to evolve massive bite muscles without risking overloading their valuable jaws.

Finite-element modelling reveals force modulation of jaw adductors in stag beetles.

Male stag beetles carry large and heavy mandibles that arose through sexual selection over mating rights. Although the mandibles of Cyclommatus metallifer males are used in pugnacious fights, they are surprisingly slender. Our bite force measurements show a muscle force reduction of 18% for tip biting when compared with bites with the teeth located halfway along the mandibles. This suggests a behavioural adaptation to prevent failure. We confirmed this by constructing finite-element (FE) models that mimic both natural bite situations as well as the hypothetical situation of tip biting without muscle force modulation. These models, based on micro-CT images, investigate the material stresses in the mandibles for different combinations of bite location and muscle force. Young's modulus of the cuticle was experimentally determined to be 5.1 GPa with the double indentation method, and the model was validated by digital image correlation on living beetles. FE analysis proves to be a valuable tool in the investigation of the trade-offs of (animal) weapon morphology and usage. Furthermore, the demonstrated bite force modulation in male stag beetles suggests the presence of mechanosensors inside the armature.

Density thresholds for Mopeia virus invasion and persistence in its host Mastomys natalensis.

Well-established theoretical models predict host density thresholds for invasion and persistence of parasites with a density-dependent transmission. Studying such thresholds in reality, however, is not obvious because it requires long-term data for several fluctuating populations of different size. We developed a spatially explicit and individual-based SEIR model of Mopeia virus in multimammate mice Mastomys natalensis. This is an interesting model system for studying abundance thresholds because the host is the most common African rodent, populations fluctuate considerably and the virus is closely related to Lassa virus but non-pathogenic to humans so can be studied safely in the field. The simulations show that, while host density clearly is important, sharp thresholds are only to be expected for persistence (and not for invasion), since at short time-spans (as during invasion), stochasticity is determining. Besides host density, also the spatial extent of the host population is important. We observe the repeated local occurrence of herd immunity, leading to a decrease in transmission of the virus, while even a limited amount of dispersal can have a strong influence in spreading and re-igniting the transmission. The model is most sensitive to the duration of the infectious stage, the size of the home range and the transmission coefficient, so these are important factors to determine experimentally in the future.

Effects of cohabitation with females on aggressive behavior between male mice.

Individual male mice of several groups were observed during daily encounters with a male intruder. The groups differed with regard to social and sexual experience of the resident animal and of the intruder. Aggressive behavior was most intense in residents actually living with a female and least intense in sexually naïve residents living alone. Residents that had only once cohabited with a female for a short time obtained intermediate scores. Experienced intruders were attacked less than naïve ones.