A simple model for calculating residential 60-Hz magnetic fields

A model is presented that permits the calculation of densities of 60-Hz magnetic fields throughout a residence from only a few measurements. We assume that residential magnetic fields are produced by sources external to the house and by the residential grounding circuit. The field from external sources is measured with a single probe. The field produced by the grounding circuit is calculated from the current flowing in the circuit and its geometry. The two fields are combined to give a prediction of the total field at any point in the house. A data-acquisition system was built to record the magnitude and phase of the grounding current and the field from external sources. The model's predictions were compared with measurements of the total magnetic field at a single location in 23 houses; a correlation coefficient of .87 was obtained, indicating that the model has good predictive capability. A more detailed study that was carried out in one house permitted comparisons of measurements with the model's predictions at locations throughout the house. Again, quite reasonable agreement was found. We also investigated the temporal variability of field readings in this house. Daily magnetic field averages were found to be considerably more stable than hourly averages. Finally, we demonstrate the use of the model in creating a profile of the magnetic fields in a home.     

On birds: scale effects in the neognath hindlimb and differences in the gross morphology of wings and hindlimbs

Scale effects on whole limb morphology (i.e. bones together with in situ overlying muscles) are well understood for the neognath forelimb. However, scale effects on neognath gross hindlimb morphology remain largely unexplored. To broaden our understanding of avian whole limb morphology, I investigated the scaling of hindlimb inertial properties in neognath birds, testing empirical scaling relationships against the model of geometric similarity. Inertial property data – mass, moment of inertia, centre of mass distance, and radius of gyration – were collected from 22 neognath species representing a wide range of locomotor specializations. When scaled against body mass, hindlimb inertial properties scale with positive allometry. Thus, in terms of morphology, larger bodied neognaths possess hindlimbs requiring disproportionately more energy to accelerate and decelerate relative to body mass than smaller bodied birds. When scaled against limb length, hindlimb inertial properties scale according to isometry. In the subclade Land Birds (sensu Hackett et al.), hindlimb inertial properties largely scale according to positive allometry. The contrasting results of positive allometry vs. isometry in neognaths are due to how hindlimb length scales against body mass. Negative allometry of hindlimb inertial properties, which would reduce terrestrial locomotion costs, would probably make the hindlimb susceptible to mechanical failure or too diminutive for its many ecological functions. Comparing the scaling relationships of wings and hindlimbs highlights how locomotor costs influence the scaling of limb inertial properties. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 14–31.     

Re-Examination of Globally Flat Space-Time

In the following, we offer a novel approach to modeling the observed effects currently attributed to the theoretical concepts of “dark energy,” “dark matter,” and “dark flow.” Instead of assuming the existence of these theoretical concepts, we take an alternative route and choose to redefine what we consider to be inertial motion as well as what constitutes an inertial frame of reference in flat space-time. We adopt none of the features of our current cosmological models except for the requirement that special and general relativity be local approximations within our revised definition of inertial systems. Implicit in our ideas is the assumption that at “large enough” scales one can treat objects within these inertial systems as point-particles having an insignificant effect on the curvature of space-time. We then proceed under the assumption that time and space are fundamentally intertwined such that time- and spatial-translational invariance are not inherent symmetries of flat space-time (i.e., observable clock rates depend upon both relative velocity and spatial position within these inertial systems) and take the geodesics of this theory in the radial Rindler chart as the proper characterization of inertial motion. With this commitment, we are able to model solely with inertial motion the observed effects expected to be the result of “dark energy,” “dark matter,” and “dark flow.” In addition, we examine the potential observable implications of our theory in a gravitational system located within a confined region of an inertial reference frame, subsequently interpreting the Pioneer anomaly as support for our redefinition of inertial motion. As well, we extend our analysis into quantum mechanics by quantizing for a real scalar field and find a possible explanation for the asymmetry between matter and antimatter within the framework of these redefined inertial systems.     

Turning in a Bipedal Robot

We report the development of turning behavior on a child-size bipedal robot that addresses two common scenarios: turning in place and simultaneous walking and turning. About turning in place, three strategies are investigated and compared, including body-first, leg-first, and body/leg-simultaneous. These three strategies are used for three actions, respectively: when walking follows turning immediately, when space behind the robot is very tight, and when a large turning angle is desired. Concerning simultaneous walking and turning, the linear inverted pendulum is used as the motion model in the single-leg support phase, and the polynomial-based trajectory is used as the motion model in the double-leg support phase and for smooth motion connectivity to motions in a priori and a posteriori single-leg support phases. Compared to the trajectory generation of ordinary walking, that of simultaneous walking and turning introduces only two extra parameters: one for determining new heading direction and the other for smoothing the Center of Mass (COM) trajectory. The trajectory design methodology is validated in both simulation and experimental environments, and successful robot behavior confirms the effectiveness of the strategy.     

Why do male snakes have longer tails than females?

In most snake species, males have longer tails than females of the same body length. The adaptive significance of this widespread dimorphism has attracted much speculation, but few tests. We took advantage of huge mating aggregations of red-sided gartersnakes (Thamnophis sirtalis parietalis) in southern Manitoba to test two (non-exclusive) hypotheses about the selective forces responsible for this dimorphism. Our data support both hypotheses. First, relative tail length affects the size of the male copulatory organs (hemipenes). Males with longer tails relative to body length have longer hemipenes, presumably because of the additional space available (the hemipenes are housed inside the tail base). Second, relative tail length affects male mating success. Males with partial tail loss (due to predation or misadventure) experienced a threefold reduction in mating success. Among males with intact tails, we detected strong stabilizing selection on relative tail length in one of the two years of our study. Thus, our data support the notion that sex divergence in tail length relative to body length in snakes reflects the action of sexual selection for male mating success.     

A dynamic model of thoracic differentiation for the control of turning in the stick insect

Leg movements of stick insects (Carausius morosus) making turns towards visual targets are examined in detail, and a dynamic model of this behaviour is proposed. Initial results suggest that front legs shape most of the body trajectory, while the middle and hind legs just follow external forces (Rosano H, Webb B, in The control of turning in real and simulated stick insects, vol. 4095, pp 145–156, 2006). However, some limitations of this explanation and dissimilarities in the turning behaviour of the insect and the model were found. A second set of behavioural experiments was made by blocking front tarsi to further investigate the active role of the other legs for the control of turning. The results indicate that it is necessary to have different roles for each pair of legs to replicate insect behaviour. We demonstrate that the rear legs actively rotate the body while the middle legs move sideways tangentially to the hind inner leg. Furthermore, we show that on average the middle inner and hind outer leg contribute to turning while the middle outer leg and hind inner leg oppose body rotation. These behavioural results are incorporated into a 3D dynamic robot simulation. We show that the simulation can now replicate more precisely the turns made by the stick insect. This work was supported by CONACYT México and the European Commission under project FP6-2003-IST2-004690 SPARK.     

Balancing requirements for stability and maneuverability in cetaceans

The morphological designs of animals represent a balance betweenstability for efficient locomotion and instability associatedwith maneuverability. Morphologies that deviate from designsassociated with stability are highly maneuverable. Major featuresaffecting maneuverability are positions of control surfacesand flexibility of the body. Within odontocete cetaceans (i.e.,toothed whales), variation in body design affects stabilityand turning performance. Position of control surfaces (i.e.,flippers, fin, flukes, peduncle) provides a generally stabledesign with respect to an arrow model. Destabilizing forcesgenerated during swimming are balanced by dynamic stabilizationdue to the phase relationships of various body components. Cetaceanswith flexible bodies and mobile flippers are able to turn tightlyat low turning rates, whereas fast-swimming cetaceans with lessflexibility and relatively immobile flippers sacrifice smallturn radii for higher turning rates. In cetaceans, body andcontrol surface mobility and placement appear to be associatedwith prey type and habitat. Flexibility and slow, precise maneuveringare found in cetaceans that inhabit more complex habitats, whereashigh-speed maneuvers are used by cetaceans in the pelagic environment.     

Multi-scale simulations of the T cell receptor reveal its lipid interactions,dynamics and the arrangement of its cytoplasmic region

The T cell receptor (TCR-CD3) initiates T cell activation by binding to peptides of Major Histocompatibility Complexes (pMHC). The TCR-CD3 topology is well understood but the arrangement and dynamics of its cytoplasmic tails remains unknown, limiting our grasp of the signalling mechanism. Here, we use molecular dynamics simulations and modelling to investigate the entire TCR-CD3 embedded in a model membrane. Our study demonstrates conformational changes in the extracellular and transmembrane domains, and the arrangement of the TCR-CD3 cytoplasmic tails. The cytoplasmic tails formed highly interlaced structures while some tyrosines within the immunoreceptor tyrosine-based activation motifs (ITAMs) penetrated the hydrophobic core of the membrane. Interactions between the cytoplasmic tails and phosphatidylinositol phosphate lipids in the inner membrane leaflet led to the formation of a distinct anionic lipid fingerprint around the TCR-CD3. These results increase our understanding of the TCR-CD3 dynamics and the importance of membrane lipids in regulating T cell activation.     

Offset of rotation centers creates a bias in isokinetics: a virtual model including stiffness or friction

The present paper deals with a virtual model devoted to isokinetics and isometrics assessment of a human muscular group in the common joints, knee, ankle, hip, shoulder, cervical spine, etc. This virtual model with an analytical analysis followed by a numerical simulation is able to predict measurement errors of the joint torque due to offset of rotation centers between the body segment and the ergometer arm. As soon as offset is present, errors increase due to the influence of inertial effects, gravity effects, stiffness due to the limb strapping on the ergometer arm or Coulomb friction between limb and ergometer. The analytical model is written in terms of Lagrange formalism and the numerical model uses ADAMS software adapted to multi-body dynamics simulations. Results of models show a maximal relative error of 11%, for a 10% relative offset between the rotation centers. Inertial contributions are found to be negligible but gravity effects must be discussed in regard to the measured torque. Stiffness or friction effects may also increase the torque error; in particular when offset occurs, it is shown that errors due to friction have to be considered for all torque level while only stiffness effects have to be considered for torque less than 25Nm. This study also emphasizes the influence of the angular range of motion at a given angular position.     

Testing the functional significance of tail streamers

Studies of the evolution of elaborate ornaments have concentrated on their role in increasing attractiveness to mates. The classic examples of such sexually selected structures are the elongated tails of some bird species. Elongated tails can be divided into three categories: graduated tails, pin tails and streamers. There seems to be little debate about whether graduated and pin tails are ornaments; i.e. costly signals used in mate choice. However, in the case of streamers there is considerable discussion about their function. It has been suggested that tail streamers could be (i) entirely naturally selected, (ii) entirely sexually selected, (iii) partly naturally and partly sexually selected. The prime example of a species with tail streamers is the swallow (Hirundo rustica) in which both sexes have tail streamers. In this paper we discuss the aerodynamic consequences of different types of manipulation of the streamer and/or outer tail feather. We make qualitative predictions about the aerodynamic performance of swallows with manipulated tail streamers; these predictions differ depending on whether streamers have a naturally or sexually selected function. We demonstrate that these hypotheses can only be separated if tail streamers are shortened and changes in aerodynamic performance measured during turning flight.     

A 3D interactive method for estimating body segmental parameters in animals: application to the turning and running performance of Tyrannosaurus rex

We developed a method based on interactive B-spline solids for estimating and visualizing biomechanically important parameters for animal body segments. Although the method is most useful for assessing the importance of unknowns in extinct animals, such as body contours, muscle bulk, or inertial parameters, it is also useful for non-invasive measurement of segmental dimensions in extant animals. Points measured directly from bodies or skeletons are digitized and visualized on a computer, and then a B-spline solid is fitted to enclose these points, allowing quantification of segment dimensions. The method is computationally fast enough so that software implementations can interactively deform the shape of body segments (by warping the solid) or adjust the shape quantitatively (e.g., expanding the solid boundary by some percentage or a specific distance beyond measured skeletal coordinates). As the shape changes, the resulting changes in segment mass, center of mass (CM), and moments of inertia can be recomputed immediately. Volumes of reduced or increased density can be embedded to represent lungs, bones, or other structures within the body. The method was validated by reconstructing an ostrich body from a fleshed and defleshed carcass and comparing the estimated dimensions to empirically measured values from the original carcass. We then used the method to calculate the segmental masses, centers of mass, and moments of inertia for an adult Tyrannosaurus rex, with measurements taken directly from a complete skeleton. We compare these results to other estimates, using the model to compute the sensitivities of unknown parameter values based upon 30 different combinations of trunk, lung and air sac, and hindlimb dimensions. The conclusion that T. rex was not an exceptionally fast runner remains strongly supported by our models-the main area of ambiguity for estimating running ability seems to be estimating fascicle lengths, not body dimensions. Additionally, the craniad position of the CM in all of our models reinforces the notion that T. rex did not stand or move with extremely columnar, elephantine limbs. It required some flexion in the limbs to stand still, but how much flexion depends directly on where its CM is assumed to lie. Finally we used our model to test an unsolved problem in dinosaur biomechanics: how fast a huge biped like T. rex could turn. Depending on the assumptions, our whole body model integrated with a musculoskeletal model estimates that turning 45 degrees on one leg could be achieved slowly, in about 1-2s.     

The gut microbiota and Bergmann’s rule in wild house mice

The extent to which the gut microbiota may play a role in latitudinal clines of body mass variation (i.e., Bergmann's rule) remains largely unexplored. Here, we collected wild house mice from three latitudinal transects across North and South America and investigated the relationship between variation in the gut microbiota and host body mass by combining field observations and common garden experiments. First, we found that mice in the Americas follow Bergmann's rule, with increasing body mass at higher latitudes. Second, we found that overall differences in the gut microbiota were associated with variation in body mass controlling for the effects of latitude. Then, we identified specific microbial measurements that show repeated associations with body mass in both wild‐caught and laboratory‐reared mice. Finally, we found that mice from colder environments tend to produce greater amounts of bacteria‐driven energy sources (i.e., short‐chain fatty acids) without an increase in food consumption. Our findings provide motivation for future faecal transplant experiments directly testing the intriguing possibility that the gut microbiota may contribute to Bergmann's rule, a fundamental pattern in ecology.     

The scaling of locomotor performance in predator-prey encounters: from fish to killer whales.

During predator-prey encounters, a high locomotor performance in unsteady manoeuvres (i.e. acceleration, turning) is desirable for both predators and prey. While speed increases with size in fish and other aquatic vertebrates in continuous swimming, the speed achieved within a given time, a relevant parameter in predator-prey encounters, is size independent. In addition, most parameters indicating high performance in unsteady swimming decrease with size. Both theoretical considerations and data on acceleration suggest a decrease with body size. Small turning radii and high turning rates are indices of maneuverability in space and in time, respectively. Maneuverability decreases with body length, as minimum turning radii and maximum turning rates increase and decrease with body length, respectively. In addition, the scaling of linear performance in fish locomotion may be modulated by turning behaviour, which is an essential component of the escape response. In angelfish, for example, the speed of large fish is inversely related to their turning angle, i.e. fish escaping at large turning angles show lower speed than fish escaping at small turning angles. The scaling of unsteady locomotor performance makes it difficult for large aquatic vertebrates to capture elusive prey by using whole-body attacks, since the overall maneuverability and acceleration of small prey is likely to be superior to that of large predators. Feeding strategies in vertebrate predators can be related to the predator-prey length ratios. At prey-predator ratios higher than approximately 10(-2), vertebrate predators are particulate feeders, while at smaller ratios, they tend to be filter feeders. At intermediate ratios, large aquatic predators may use a variety of feeding methods that aid, or do not involve, whole body attacks. Among these are bubble curtains used by humpback whales to trap fish schools, and tail-slapping of fish by delphinids. Tail slapping by killer whales is discussed as an example of these strategies. The speed and acceleration achieved by the flukes of killer whales during tail slaps are higher and comparable, respectively, to those that can be expected in their prey, making tail-slapping an effective predator behaviour.     

Size-structured demographic models of coral populations

The demographic processes of growth, mortality, and the recruitment of young individuals, are the major organizing forces regulating communities in open systems. Here we present a size-structured (rather than age-structured) population model to examine the role of these different processes in space-limited open systems, taking coral reefs as an example. In this flux-diffusion model the growth rate of corals depends both on the available free-space (i.e. density-dependence) and on the particular size of the coral. In our analysis we progressively study several different forms of growth rate functions to disentangle the effects of free space and size-dependence on the model's stability. Unlike Roughgarden et al. [1985. Demographic theory for an open marine population space-limited recruitment. Ecology 66(1), 54-67], whose principal result is that the growth of settled organisms is destabilizing, we find that size-dependent growth rate often has the potential to endow stability. This is particularly true, if the growth rate is dependent on available free space (i.e. density dependent), but examples are given for growth rates that even lack this property. Further insights into reef system fragility are found through studying the sensitivity of the model steady state to changes in recruitment.     

Upstroke wing flexion and the inertial cost of bat flight

Flying vertebrates change the shapes of their wings during the upstroke, thereby decreasing wing surface area and bringing the wings closer to the body than during downstroke. These, and other wing deformations, might reduce the inertial cost of the upstroke compared with what it would be if the wings remained fully extended. However, wing deformations themselves entail energetic costs that could exceed any inertial energy savings. Using a model that incorporates detailed three-dimensional wing kinematics, we estimated the inertial cost of flapping flight for six bat species spanning a 40-fold range of body masses. We estimate that folding and unfolding comprises roughly 44 per cent of the inertial cost, but that the total inertial cost is only approximately 65 per cent of what it would be if the wing remained extended and rigid throughout the wingbeat cycle. Folding and unfolding occurred mostly during the upstroke; hence, our model suggests inertial cost of the upstroke is not less than that of downstroke. The cost of accelerating the metacarpals and phalanges accounted for around 44 per cent of inertial costs, although those elements constitute only 12 per cent of wing weight. This highlights the energetic benefit afforded to bats by the decreased mineralization of the distal wing bones.     

A comparative study of aerodynamic function and flexural stiffness of outer tail feathers in birds

To transmit aerodynamic forces to the body, tail feathers should be stiff to resist lift forces with minimum deformation. Because aerodynamic theory predicts that such feathers do not produce lift forces beyond the point of the maximum continuum width of the tail, species with deeply forked tails should not require stiff outer rectrices distal to that point. I tested this prediction by comparing the relative thickness of the outer rectrix rachis between species with deeply forked tails to those with triangular or shallowly forked tails. Eleven pairs of closely related species belonging to families Fregatidae, Phalacrocoracidae, Accipitridae, Sternidae, Caprimulgidae, Trochilidae, Coraciidae, Tyrannidae, Cotingidae, and Hirundinidae were compared. All but one of the phylogenetically independent comparisons showed that the species with triangular or shallowly forked tails have higher relative rachis thickness than their deeply forked relatives. In addition, nine out of eleven of the species with deeply forked tails showed a proportionately greater increase in relative rachis thickness from distal to proximal parts of the feather. In contrast, triangular and shallowly forked tails showed an approximately linear relation between relative rachis thickness and relative rachis length. These results considered together are consistent with the idea that the distal part of outer rectrix rachis in species with deeply forked tails has not been selected to resist lift forces and may be adaptively reduced to attenuate the costs of a hypertrophied ornament.     

Effects of habitat destruction and resource supplementation in a predator-prey metapopulation model

We developed a mean field, metapopulation model to study the consequences of habitat destruction on a predator-prey interaction. The model complements and extends earlier work published by Bascompte and Solé (1998, J. theor. Biol.195, 383-393) in that it also permits use of alternative prey (i.e., resource supplementation) by predators. The current model is stable whenever coexistence occurs, whereas the earlier model is not stable over the entire domain of coexistence. More importantly, the current model permits an assessment of the effect of a generalist predator on the trophic interaction. Habitat destruction negatively affects the equilibrium fraction of patches occupied by predators, but the effect is most pronounced for specialists. The effect of habitat destruction on prey coexisting with predators is dependent on the ratio of extinction risk due to predation and prey colonization rate. When this ratio is less than unity, equilibrial prey occupancy of patches declines as habitat destruction increases. When the ratio exceeds one, equilibrial prey occupancy increases even as habitat destruction increases; i.e., prey "escape" from predation is facilitated by habitat loss. Resource supplementation reduces the threshold colonization rate of predators necessary for their regional persistence, and the benefit derived from resource supplementation increases in a nonlinear fashion as habitat destruction increases. We also compared the analytical results to those from a stochastic, spatially explicit simulation model. The simulation model was a discrete time analog of our analytical model, with one exception. Colonization was restricted locally in the simulation, whereas colonization was a global process in the analytical model. After correcting for differences between nominal and effective colonization rates, most of the main conclusions of the two types of models were similar. Some important differences did emerge, however, and we discuss these in relation to the need to develop fully spatially explicit analytical models. Finally, we comment on the implications of our results for community structure and for the conservation of prey species interacting with generalist predators.     

A model for seasonal phytoplankton blooms

We analyse a generic bottom-up nutrient phytoplankton model to help understand the dynamics of seasonally recurring algae blooms. The deterministic model displays a wide spectrum of dynamical behaviours, from simple cyclical blooms which trigger annually, to irregular chaotic blooms in which both the time between outbreaks and their magnitudes are erratic. Unusually, despite the persistent seasonal forcing, it is extremely difficult to generate blooms that are both annually recurring and also chaotic or irregular (i.e. in amplitude) even though this characterizes many real time-series. Instead the model has a tendency to 'skip' with outbreaks often being suppressed from 1 year to the next. This behaviour is studied in detail and we develop analytical expressions to describe the model's flow in phase space, yielding insights into the mechanism of the bloom recurrence. We also discuss how modifications to the equations through the inclusion of appropriate functional forms can generate more realistic dynamics.     

Frequency of tail loss reflects variation in predation levels,predator efficiency,and the behaviour of three populations of brown anoles

We investigated two predictions regarding the incidence of tail regeneration in lizards for three populations of brown anoles exposed to varying predation levels from the same predator (cats). Firstly although inefficient predators are likely to increase the incidence of regenerated tails (i.e. lizards can escape through tail autotomy), highly efficient predators will kill and eat the lizard and thus leave no evidence of autotomy. At the site with no cats, only 4% of anoles demonstrated signs of tail regeneration. This value was not significantly different from the site where feral cats (i.e. ‘efficient’ predators that would capture prey to eat, as supported by behavioural observation) were present (7%). By contrast, 25% of anoles present at the site with pet cats (well‐fed domesticated cats that caught and played with anoles, i.e. were ‘inefficient’ predators) exhibited regenerated tails. Secondly, more obvious lizards are more susceptible to predation attempts. Supporting this hypothesis, our data indicate a higher incidence of regenerated tails (28%) was recorded amongst adult males (which are territorial, occupying exposed positions) compared to females and subadult males (17%) or juveniles (1%). In conclusion, the behaviour of both the predator and the lizard influences the frequency of regenerated tails in brown anoles. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103 , 648–656.     

Fork tails evolved differently in swallows and swifts

Whether sexual or viability selection drives the evolution of ornamental traits is often unclear because current function does not clarify evolutionary history, particularly when the ornamentation is a modified version of the functional traits. Here, using a phylogenetic comparative approach, we studied how deeply forked tails—a classic example of sexually selected traits that might also be a mechanical device for enhancing aerodynamic ability—evolved in two groups of aerial foragers, swallows (family: Hirundinidae) and swifts (family: Apodidae). Although apparent fork depth, the target of sexual selection, increases with increasing outermost tail feather length, fork depth can also increase with decreasing central tail feather length, which impairs the lift generated by the tail. Thus, we predicted that sexual selection, but not viability selection, should favour the evolution of short central tail feathers in species with deeply forked tails, particularly in swifts, which are less reliant on the lift generated by their tail than in swallows. We found support for these predictions because central tail feather length decreased with increasing tail fork depth, particularly in swifts. Instead, the increase in outermost tail feather length per unit tail fork depth was higher in swallows than in swifts, indicating that a similar sexual ornamentation (i.e. forked tails) differently evolved in these two aerial insectivores perhaps due to the differential cost of ornamentation. We also found support for an optical illusion that changes the relative importance of central and outermost tail feather length in sexual selection.