Physiological tipping points in the relationship between foraging success and lifetime fitness of a long-lived mammal

Although anthropogenic change is often gradual, the impacts on animal populations may be precipitous if physiological processes create tipping points between energy gain, reproduction or survival. We use 25 years of behavioural, diet and demographic data from elephant seals to characterise their relationships with lifetime fitness. Survival and reproduction increased with mass gain during long foraging trips preceding the pupping seasons, and there was a threshold where individuals that gained an additional 4.8% of their body mass (26 kg, from 206 to 232 kg) increased lifetime reproductive success three-fold (from 1.8 to 4.9 pups). This was due to a two-fold increase in pupping probability (30% to 76%) and a 7% increase in reproductive lifespan (6.0 to 6.4 years). The sharp threshold between mass gain and reproduction may explain reproductive failure observed in many species and demonstrates how small, gradual reductions in prey from anthropogenic disturbance could have profound implications for animal populations.     

High Q Long-Range Surface Plasmon Polariton Modes in Sub-wavelength Metallic Microdisk Cavity

Metal-capped microdisk cavity supporting surface plasmon polaritons (SPP)-guided whispering gallery mode (WGM) can achieve higher cavity factor Q than traditional microdisk cavity in sub-wavelength dimensions. We have numerically analyzed the limiting factors on Q using finite difference time domain method. The Q of SPP-guided WGM is primarily limited by the loss of metal. A thin metal-sandwiched microdisk cavity supporting long-range surface plasmon polariton mode was proposed to reduce the metal loss. The proposed cavities have been shown to increase cavity Q by more than 15-fold and reduce threshold gain by more than threefold as opposed to traditional microdisk cavities.     

Subcellular Distribution of Cadmium and Nickel in Chronically Exposed Wild Fish: Inferences Regarding Metal Detoxification Strategies and Implications for Setting Water Quality Guidelines for Dissolved Metals

The objective of this study was to investigate metal detoxification in chronically exposed juvenile yellow perch (YP: Perca flavescens) and to field test the commonly assumed threshold toxicity model. Fish were collected from lakes located along a cadmium (Cd) and nickel (Ni) concentration gradient. Ambient dissolved metal concentrations were measured to evaluate exposure and total hepatic metal concentrations were determined as a measure of metal bioaccumulation. Hepatic metal partitioning among potentially metal-sensitive fractions (heat-denatured proteins, organelles) and detoxified metal fractions (metallothionein) was determined after differential centrifugation of YP liver homogenates. Major proportions of hepatic Cd were found in the heat-stable cytosolic peptides and proteins fraction (HSP; including metallothioneins), whereas Ni was mainly found in the potentially metal-sensitive heat-denaturable proteins fraction (HDP). For these chronically exposed fish there was no threshold exposure concentration below which binding of Cd or Ni to the heat-denaturable protein fraction or the organelle fraction did not occur. Metal detoxification was clearly incomplete and P. flavescens was subject to some metal-related stress, as evidenced notably by endocrine perturbations. Similar subcellular partitioning results were obtained when juvenile yellow perch were transferred from a reference lake to a Cd-contaminated lake and Cd accumulation was followed over time; there was no accumulation threshold below which Cd binding to the putative metal-sensitive fractions (HDP and organelles) did not occur. The presence of Cd and Ni in these fractions, even for low exposure concentrations and low hepatic accumulation, contradicts the threshold toxicity model that underpins metal toxicology theory and that is implicitly used in setting water quality guidelines for metals. Chronically exposed YP appear to have settled for a tradeoff between the cost of turning on their detoxification apparatus at full capacity, to completely suppress metal binding to metal-sensitive sites, and the alternative cost of allowing some binding of inappropriate metals to metal-sensitive sites.     

Conditions for Loss Compensation of Surface Plasmon Polaritons Propagation on a Metal/Gain Medium Boundary

One way to compensate for the surface plasmon polariton (SPP) propagation losses is to use a gain medium. However, simply ensuring high enough gain is not sufficient because it may violate the bounded character of the wave. Therefore, a detailed theoretical analysis is needed for the determination of the conditions for lossless or amplified SPP propagation. Here presented is an exact theoretical analysis of the SPP propagation in the case of an infinite metal/gain medium boundary. It is shown that the conditions for lossless/amplified SPP propagation can be conveniently examined and presented as a simply connected region in the complex plane of the gain medium dielectric function. Effective and minimum gain parameters are introduced, which facilitates the simultaneous analyses of different gain media/metals combinations. The practical application of these results is illustrated for several gain media/metal (silver, gold and aluminium) systems.     

Threshold foraging behaviour of basking sharks on zooplankton: life on an energetic knife-edge?

The minimum threshold foraging response of basking sharks has not been determined despite the widely held view that has been perpetuated in the literature for the past 45 years that this species cannot use low prey densities for net energy gain and so lives on an energetic ''knife-edge''. An early theoretical estimate suggested basking sharks would expend more energy collecting zooplankton at concentrations less than 1.36 g m^-3 than could be obtained from it. This led to the claim that basking sharks will feed at an energetic loss for much of the annual cycle as zooplankton abundance outside summer months is too low for net energy gain to occur. Here I show from theoretical calculations and behavioural studies on individual and group-feeding sharks in the English Channel that basking sharks have a theoretical threshold prey density of between 0.55 and 0.74 g m^-3 and an observed foraging threshold of between 0.48 and 0.70 g m^-3 (mean = 0.62 g m^-3). The close agreement between theoretical and empirical threshold values suggests basking sharks can achieve net energy gain in much lower zooplankton densities than previously thought. The findings imply that this species may not be reliant upon the ''migration–hibernation'' energy conservation strategy it is purported to exhibit when seasonal zooplankton abundance decreases below 1.36 g m^-3.     

A study of the resistance of mass marine algae to heavy metals

The threshold and lethal concentrations of the heavy metals Hg, Cd, Pb, and Cu were evaluated for mass species of marine microalgae. Algae with high specific surface were found to be the most sensitive to the effects of toxic metals. Among the resilient species that survived a wide range of doses of toxic metal, ranging from low threshold to high lethal levels, were those possessing flexible structures and the ability to grow after toxic stress. Resistant species could survive at high threshold and high lethal metal concentrations. The ratio of sensitive and resistant microalgae can be used for monitoring the quality of marine water.     

A simple Markov model of sodium channels with a dynamic threshold

Characteristics of action potential generation are important to understanding brain functioning and, thus, must be understood and modeled. It is still an open question what model can describe concurrently the phenomena of sharp spike shape, the spike threshold variability, and the divisive effect of shunting on the gain of frequency-current dependence. We reproduced these three effects experimentally by patch-clamp recordings in cortical slices, but we failed to simulate them by any of 11 known neuron models, including one- and multi-compartment, with Hodgkin-Huxley and Markov equation-based sodium channel approximations, and those taking into account sodium channel subtype heterogeneity. Basing on our voltage-clamp data characterizing the dependence of sodium channel activation threshold on history of depolarization, we propose a 3-state Markov model with a closed-to-open state transition threshold dependent on slow inactivation. This model reproduces the all three phenomena. As a reduction of this model, a leaky integrate-and-fire model with a dynamic threshold also shows the effect of gain reduction by shunt. These results argue for the mechanism of gain reduction through threshold dynamics determined by the slow inactivation of sodium channels.     

Evaluation of muscle metaboreflex function through graded reduction in forearm blood flow during rhythmic handgrip exercise in humans

Hypoperfusion of active skeletal muscle elicits a reflex pressor response termed the muscle metaboreflex. Our aim was to determine the muscle metaboreflex threshold and gain in humans by creating an open-loop relationship between active muscle blood flow and hemodynamic responses during a rhythmic handgrip exercise. Eleven healthy subjects performed the exercise at 5 or 15% of maximal voluntary contraction (MVC) in random order. During the exercise, forearm blood flow (FBF), which was continuously measured using Doppler ultrasound, was reduced in five steps by manipulating the inner pressure of an occlusion cuff on the upper arm. The FBF at each level was maintained for 3 min. The initial reductions in FBF elicited no hemodynamic changes, but once FBF fell below a threshold, mean arterial blood pressure (MAP) and heart rate (HR) increased and total vascular conductance (TVC) decreased in a linear manner. The threshold FBF during the 15% MVC trial was significantly higher than during the 5% MVC trial. The gain was then estimated as the slope of the relationship between the hemodynamic responses and FBFs below the threshold. The gains for the MAP and TVC responses did not differ between workloads, but the gain for the HR response was greater in the 15% MVC trial. Our findings thus indicate that increasing the workload shifts the threshold for the muscle metaboreflex to higher blood flows without changing the gain of the reflex for the MAP and TVC responses, whereas it enhances the gain for the HR response.     

Rectification and spike synchronization in the Limulus lateral eye

The encoding of light stimuli into spike trains in the Limulus lateral eye is shown to be markedly nonlinear in our conditions of stimulation. Our experimental results support the conclusion that nonlinearities are enhanced by lateral inhibition but arise within the single ommatidium, and are due to the high gain in the transduction from generator potential to spike rate. This high gain is in turn related to the existence, in the steady relation between generator potential and spike rate, of a positive threshold on the generator potential.     

Spaser Based on Dark Quadrupolar Mode of a Single Metallic Nanodisk

A spaser based on dark quadrupolar mode of a single metallic nanodisk coated with a layer of gain media is studied theoretically. The absorption efficiency of the metallic structure, the gain efficiency of the gain media, and the scattering efficiency of the whole nanosystem are calculated separately. It is found that the ratio of the absorption and the scattering intensities (RAS) of the dark quadrupolar mode depends strongly on the gain coefficient, which increases from 0.8 to 4.39 with gain coefficient reaches the threshold, by contrast the bright dipolar mode keeps its RAS unchanged at 0.16. This is attributed to that the gain media mainly amplifies quadrupolar eigenmode composition of the dark quadrupolar mode, thus the scattering loss caused by dipolar eigenmode composition is under effective control, which changes the RAS and leads to super low-threshold spaser. Our works may benefit the achievement of spaser system with low scattering loss.

     

Theoretical aspects of sexual selection: A generalized model of mating behaviour

A model of mate selection is described in which females mate preferentially according to their probability of encounter with the males they prefer. In this model, different thresholds of response to the courtship of different male phenotypes determine the female mating preferences. Females with a lower threshold toward particular males require fewer encounters before mating with these males and more encounters before mating with any of the others. Such females mate preferentially if they encounter a male they prefer before they have been stimulated to the level of the higher threshold. At the higher threshold they mate at random. The number of the extra encounters required to raise the females' level of stimulation from the lower to the higher threshold is a parameter of the model. The frequency of the preferred males then determines the probability that a female encounters and mates with one of them before she has been sufficiently stimulated to mate at random. Sexual selection by differences in male courtship can also be described in terms of this model.The preferred characters may be determined either by dominant and recessive alleles or by each different genotype. When only one extra encounter is required before the females mate at random, the preferred males only gain a slight frequency-dependent advantage: Stable polymorphisms can only be maintained if the heterozygotes have the greater preference in their favor. When more than one extra encounter is required before random mating, the males gain a negative frequency-dependent advantage: Stable polymorphisms are generally maintained.The models are fitted to published data on the mating success of male Drosophila at varying frequencies and provide an explanation of the “rare male” effect in which less common males gain a mating advantage.     

Weber and noise adaptation in the retina of the toad Bufo marinus

Responses to flashes and steps of light were recorded intracellularly from rods and horizontal cells, and extracellularly from ganglion cells, in toad eyecups which were either dark adapted or exposed to various levels of background light. The average background intensities needed to depress the dark-adapted flash sensitivity by half in the three cell types, determined under identical conditions, were 0.9 Rh*s-1 (rods), 0.8 Rh*s-1 (horizontal cells), and 0.17 Rh*s-1 (ganglion cells), where Rh* denotes one isomerization per rod. Thus, there is a range (approximately 0.7 log units) of weak backgrounds where the sensitivity (response amplitude/Rh*) of rods is not significantly affected, but where that of ganglion cells (1/threshold) is substantially reduced, which implies that the gain of the transmission from rods to the ganglion cell output is decreased. In this range, the ganglion cell threshold rises approximately as the square root of background intensity (i.e. in proportion to the quantal noise from the background), while the maintained rate of discharge stays constant. The threshold response of the cell will then signal light deviations (from a mean level) of constant statistical significance. We propose that this type of ganglion cell desensitization under dim backgrounds is due to a post-receptoral gain control driven by quantal fluctuations, and term it noise adaptation in contrast to the Weber adaptation (desensitization proportional to the mean background intensity) of rods, horizontal cells, and ganglion cells at higher background intensities.     

Weak-Periodic Stochastic Resonance in a Parallel Array of Static Nonlinearities

This paper studies the output-input signal-to-noise ratio (SNR) gain of an uncoupled parallel array of static, yet arbitrary, nonlinear elements for transmitting a weak periodic signal in additive white noise. In the small-signal limit, an explicit expression for the SNR gain is derived. It serves to prove that the SNR gain is always a monotonically increasing function of the array size for any given nonlinearity and noisy environment. It also determines the SNR gain maximized by the locally optimal nonlinearity as the upper bound of the SNR gain achieved by an array of static nonlinear elements. With locally optimal nonlinearity, it is demonstrated that stochastic resonance cannot occur, i.e. adding internal noise into the array never improves the SNR gain. However, in an array of suboptimal but easily implemented threshold nonlinearities, we show the feasibility of situations where stochastic resonance occurs, and also the possibility of the SNR gain exceeding unity for a wide range of input noise distributions.     

The ins and outs of algal metal transport

Metal transporters are a central component in the interaction of algae with their environment. They represent the first line of defense to cellular perturbations in metal concentration, and by analyzing algal metal transporter repertoires, we gain insight into a fundamental aspect of algal biology. The ability of individual algae to thrive in environments with unique geochemistry, compared to non-algal species commonly used as reference organisms for metal homeostasis, provides an opportunity to broaden our understanding of biological metal requirements, preferences and trafficking. Chlamydomonas reinhardtii is the best developed reference organism for the study of algal biology, especially with respect to metal metabolism; however, the diversity of algal niches necessitates a comparative genomic analysis of all sequenced algal genomes. A comparison between known and putative proteins in animals, plants, fungi and algae using protein similarity networks has revealed the presence of novel metal metabolism components in Chlamydomonas including new iron and copper transporters. This analysis also supports the concept that, in terms of metal metabolism, algae from similar niches are more related to one another than to algae from the same phylogenetic clade. This article is part of a Special Issue entitled: Cell Biology of Metals.     

Acoustical and neural aspects of hearing in the Australian gleaning bats,Macroderma gigas andNyctophilus gouldi

1. The maximum acoustic gain of the external ear in Macroderma gigas was found to be 25-30 dB between 5-8 kHz and in Nyctophilus gouldi it reached 15-23 dB between 7-22 kHz. Pinna gain reached a peak of 16 dB near 4.5-6 kHz in M. gigas and 12-17 dB between 7-12 kHz in N. gouldi, with average gain of 6-10 dB up to 100 kHz. Pinna gain curves resemble that of a finite conical horn, including resonance. 2. The directional properties of the external ear in both species result from sound diffraction at the pinna face, as it approximates a circular aperture. The frequency dependent movement of the acoustic axis in azimuth and elevation is attributed to the asymmetrical structure of the pinnae. 3. Evoked potentials and neuronal responses were studied in the inferior colliculus. In M. gigas, the neural audiogram has sensitivity peaks at 10-20 kHz and 35-43 kHz, with extremely low thresholds (-18 dB SPL) in the low frequency region. In N. gouldi, the neural audiogram has sensitivity peaks at 8-14 kHz (lowest threshold 5 dB SPL) and 22-45 kHz. Removal of the contralateral pinna causes a frequency dependent loss in neural threshold sensitivity of up to 10-15 dB in both species. 4. The high frequency peak in the audiogram coincides with the sonar energy band in both species, whereas the low frequency region is used for social communication. Highly sensitive low frequency hearing is discussed in relation to hunting in bats by passive listening.     

Properties of human motoneurones and their synaptic noise deduced from motor unit recordings with the aid of computer modelling.

This paper reviews two new facets of the behaviour of human motoneurones; these were demonstrated by modelling combined with analysis of long periods of low-frequency tonic motor unit firing (sub-primary range). 1) A novel transformation of the interval histogram has shown that the effective part of the membrane's post-spike voltage trajectory is a segment of an exponential (rather than linear), with most spikes being triggered by synaptic noise before the mean potential reaches threshold. The curvature of the motoneurone's trajectory affects virtually all measures of its behaviour and response to stimulation. The 'trajectory' is measured from threshold, and so includes any changes in threshold during the interspike interval. 2) A novel rhythmic stimulus (amplitude-modulated pulsed vibration) has been used to show that the motoneurone produces appreciable phase-advance during sinusoidal excitation. At low frequencies, the advance increases with rising stimulus frequency but then, slightly below the motoneurones mean firing rate, it suddenly becomes smaller. The gain has a maximum for stimuli at the mean firing rate (the 'carrier'). Such behaviour is functionally important since it affects the motoneurone's response to any rhythmic input, whether generated peripherally by the receptors (as in tremor) or by the CNS (as with cortical oscillations). Low mean firing rates favour tremor, since the high gain and reduced phase advance at the 'carrier' reduce the stability of the stretch reflex.     

Theoretical Study of a Planar Structure Plasmonic Nanolaser in Visible Regime

The potential of a plasmonic nanolaser using semiconductor gain to compensate the metal loss was investigated theoretically in multilayer planar structure geometry. The propagation constant of surface plasmon (SP) mode, Purcell factor, and modal gain were calculated via transfer matrix method. Near SP resonance, the Purcell factor shows sensitive frequency dependence and exponential decay with distance. The huge Purcell factor leads to an impractical current density about several hundreds kiloampere/square centimeter (kA/cm²). When the spectra peak of optical gain in a semiconductor is shifted about 0.7 eV below SP resonance, the moderate Purcell factor shows a rather broadband enhancement. Net modal gain was achieved at an injected current density of 12.1 kA/cm², comparable to that of conventional photonic laser diodes. The structure is further optimized by inserting a (low permittivity) dielectric spacer between the semiconductor active region and metal.

     

Clonus is explained from increased reflex gain and enlarged tissue viscoelasticity

Upper motor neuron diseases (UMND), such as stroke and spinal cord injury (SCI), are assumed to produce alterations in muscle tissue in association with neural damage. Distinguishing between these two factors is of clinical importance in choosing appropriate therapy. We studied the effect of changes in the gain of the Ia reflex pathway and tissue viscoelasticity on the emergence, frequency, and persistence of ankle clonus: a clinically significant, involuntary oscillatory movement disorder. Monte Carlo simulations were performed to explain our experimental observations in patients with stroke (n = 3) and SCI (n = 4) using a nonlinear antagonistic muscle model of the human ankle joint. Ia reflex gain was varied by changing motor unit pool threshold and gain, and passive tissue viscosity and elasticity were varied by changing optimal muscle length. Tissue viscoelasticity appeared to have a strong effect on the emergence and persistence of clonus. Observed frequencies of ankle movement, prior to and after the experimental intervention of a sudden damper, was predicted by the model. The simulations revealed that reflex gains were largest in patients with the largest tissue viscoelasticity. We conclude that ankle clonus in stroke and SCI is the result of a combination of, and suggests a relation between, (i) a decrease in threshold and an increase in gain of the motor unit pool and (ii) a decrease in optimal muscle length.     

Carrying capacity in arid rangelands during droughts: the role of temporal and spatial thresholds

Assessing the carrying capacity is of primary importance in arid rangelands. This becomes even more important during droughts, when rangelands exhibit non-equilibrium dynamics, and the dynamics of livestock conditions and forage resource are decoupled. Carrying capacity is usually conceived as an equilibrium concept, that is, the consumer density that can co-exist in long-term equilibrium with the resource. As one of the first, here we address the concept of carrying capacity in systems, where there is no feedback between consumer and resource in a limited period of time. To this end, we developed an individual-based model describing the basic characteristics of a rangeland during a drought. The model represents a rangeland composed by a single water point and forage distributed all around, with livestock units moving from water to forage and vice versa, for eating and drinking. For each livestock unit we implemented an energy balance and we accounted for the gut-filling effect (i.e. only a limited amount of forage can be ingested per unit time). Our results showed that there is a temporal threshold above which livestock begin to experience energy deficit and burn fat reserves. We demonstrated that such a temporal threshold increases with the number of animals and decreases with the rangeland conditions (amount of forage). The temporal threshold corresponded to the time livestock take to consume all the forage within a certain distance from water, so that the livestock can return to water for drinking without spending more energy than they gain within a day. In this study, we highlight the importance of a time threshold in the assessment of carrying capacity in non-equilibrium conditions. Considering this time threshold could explain contrasting observations about the influence of livestock number on livestock conditions. In case of private rangelands, the herd size should be chosen so that the spatial threshold equals (or exceeds) the length of the drought.     

Location-dependent effects of inhibition on local spiking in pyramidal neuron dendrites

Cortical computations are critically dependent on interactions between pyramidal neurons (PNs) and a menagerie of inhibitory interneuron types. A key feature distinguishing interneuron types is the spatial distribution of their synaptic contacts onto PNs, but the location-dependent effects of inhibition are mostly unknown, especially under conditions involving active dendritic responses. We studied the effect of somatic vs. dendritic inhibition on local spike generation in basal dendrites of layer 5 PNs both in neocortical slices and in simple and detailed compartmental models, with equivalent results: somatic inhibition divisively suppressed the amplitude of dendritic spikes recorded at the soma while minimally affecting dendritic spike thresholds. In contrast, distal dendritic inhibition raised dendritic spike thresholds while minimally affecting their amplitudes. On-the-path dendritic inhibition modulated both the gain and threshold of dendritic spikes depending on its distance from the spike initiation zone. Our findings suggest that cortical circuits could assign different mixtures of gain vs. threshold inhibition to different neural pathways, and thus tailor their local computations, by managing their relative activation of soma- vs. dendrite-targeting interneurons.