2,114 research outputs found
Growth and maturity of salmon sharks (Lamna ditropis) in the eastern and western North Pacific, and comments on back-calculation methods
Age and growth estimates for salmon sharks (Lamna ditropis) in the eastern North Pacific were derived from 182 vertebral centra collected from sharks ranging in length from 62.2 to 213.4 cm pre-caudal length (PCL) and compared to previously published age and growth data for salmon sharks in the western North Pacific. Eastern North Pacific female and male salmon sharks were aged up to 20 and 17 years, respectively. Relative marginal increment (RMI) analysis showed that postnatal rings form annually between January and March. Von Bertalanffy growth parameters derived from vertebral length-at-age data are L∞ =207.4 cm PCL, k=0.17/yr, and t0=−2.3 years for females (n=166), and L∞ =182.8 cm PCL, k=0.23/yr , and t0=−1.9 years for males (n=16). Age at maturity was estimated to range from six to nine years for females (median pre-caudal length of 164.7 cm PCL) and from three to five years old for males (median precaudal length of 124.0 cm PCL). Weight-length relationships for females and males in the eastern North Pacific are W=8.2 × 10_05 × L2.759 –06 × L3.383
(r2 =0.99) and W=3.2 × 10 (r2 =0.99), respectively. Our results show that female and male salmon sharks in the eastern North Pacific possess a faster growth rate, reach sexual maturity earlier, and attain greater weight-at-length than their same-sex counterparts living in the western North Pacific
Floating phase in a dissipative Josephson junction array
We consider dissipative quantum phase transitions in Josephson junction
arrays and show that the disordered phase in this extended system can be viewed
as an unusual floating phase in which the states of local -dimensional
elements (single Josephson junctions) can slide past each other despite
arbitrary range spatial couplings among them. The unusual character of the
metal-superconductor quantum critical point can be tested by measurements of
the current voltage characteristic. This may be the simplest and most natural
example of a floating phase.Comment: 4 pages, RevTex4. The revised version contains higher order
renormalization group equations and the corresponding phase diagra
Valence Quark Distribution in A=3 Nuclei
We calculate the quark distribution function for 3He/3H in a relativistic
quark model of nuclear structure which adequately reproduces the nucleon
approximation, nuclear binding energies, and nuclear sizes for small nuclei.
The results show a clear distortion from the quark distribution function for
individual nucleons (EMC effect) arising dominantly from a combination of
recoil and quark tunneling effects. Antisymmetrization (Pauli) effects are
found to be small due to limited spatial overlaps. We compare our predictions
with a published parameterization of the nuclear valence quark distributions
and find significant agreement.Comment: 18pp., revtex4, 4 fig
How Costly Is Hospital Quality? A Revealed-Preference Approach
One of the most important and vexing issues in health care concerns the cost to improve quality. Unfortunately, quality is difficult to measure and potentially confounded with productivity. Rather than relying on clinical or process measures, we infer quality at hospitals in greater Los Angeles from the revealed preference of pneumonia patients. We then decompose the joint contribution of quality and unobserved productivity to hospital costs, relying on heterogeneous tastes among patients for plausibly exogenous quality variation. We find that more productive hospitals provide higher quality, demonstrating that the cost of quality improvement is substantially understated by methods that do not take into account productivity differences. After accounting for these differences, we find that a quality improvement from the 25th percentile to the 75th percentile would increase costs at the average hospital by nearly fifty percent. Improvements in traditional metrics of hospital quality such as risk-adjusted mortality are more modest, indicating that other factors such as amenities are an important driver of both hospital costs and patient choices.
Re-Evaluation of the Elastic Scattering of Supersymmetric Dark Matter
We examine the cross sections for the elastic scattering of neutralinos
on nucleons , as functions of in the constrained minimal
supersymmetric standard model. We find narrow bands of possible values of the
cross section, that are considerably lower than some previous estimates. The
constrained model is based on the minimal supergravity-inspired framework for
the MSSM, with universal scalar and gaugino masses , and
and the MSSM Higgs masses treated as dependent parameters. We explore
systematically the region of the plane where LEP and other
accelerator constraints are respected, and the relic neutralino density lies in
the range preferred by cosmology. We update
previous discussions of both the spin-independent and -dependent scattering
matrix elements on protons and neutrons, using recent analyses of low-energy
hadron experiments.Comment: 16 pages, latex, 14 eps figure
Tuning Curves, Neuronal Variability, and Sensory Coding
Tuning curves are widely used to characterize the responses of sensory neurons to external stimuli, but there is an ongoing debate as to their role in sensory processing. Commonly, it is assumed that a neuron's role is to encode the stimulus at the tuning curve peak, because high firing rates are the neuron's most distinct responses. In contrast, many theoretical and empirical studies have noted that nearby stimuli are most easily discriminated in high-slope regions of the tuning curve. Here, we demonstrate that both intuitions are correct, but that their relative importance depends on the experimental context and the level of variability in the neuronal response. Using three different information-based measures of encoding applied to experimentally measured sensory neurons, we show how the best-encoded stimulus can transition from high-slope to high-firing-rate regions of the tuning curve with increasing noise level. We further show that our results are consistent with recent experimental findings that correlate neuronal sensitivities with perception and behavior. This study illustrates the importance of the noise level in determining the encoding properties of sensory neurons and provides a unified framework for interpreting how the tuning curve and neuronal variability relate to the overall role of the neuron in sensory encoding
Comprehensive computational analysis of Hmd enzymes and paralogs in methanogenic Archaea
<p>Abstract</p> <p>Background</p> <p>Methanogenesis is the sole means of energy production in methanogenic Archaea. H<sub>2</sub>-forming methylenetetrahydromethanopterin dehydrogenase (Hmd) catalyzes a step in the hydrogenotrophic methanogenesis pathway in class I methanogens. At least one <it>hmd </it>paralog has been identified in nine of the eleven complete genome sequences of class I hydrogenotrophic methanogens. The products of these paralog genes have thus far eluded any detailed functional characterization.</p> <p>Results</p> <p>Here we present a thorough computational analysis of Hmd enzymes and paralogs that includes state of the art phylogenetic inference, structure prediction, and functional site prediction techniques. We determine that the Hmd enzymes are phylogenetically distinct from Hmd paralogs but share a common overall structure. We predict that the active site of the Hmd enzyme is conserved as a functional site in Hmd paralogs and use this observation to propose possible molecular functions of the paralog that are consistent with previous experimental evidence. We also identify an uncharacterized site in the N-terminal domains of both proteins that is predicted by our methods to directly impart function.</p> <p>Conclusion</p> <p>This study contributes to our understanding of the evolutionary history, structural conservation, and functional roles, of the Hmd enzymes and paralogs. The results of our phylogenetic and structural analysis constitute datasets that will aid in the future study of the Hmd protein family. Our functional site predictions generate several testable hypotheses that will guide further experimental characterization of the Hmd paralog. This work also represents a novel approach to protein function prediction in which multiple computational methods are integrated to achieve a detailed characterization of proteins that are not well understood.</p
The evolution and functional repertoire of translation proteins following the origin of life
<p>Abstract</p> <p>Background</p> <p>The RNA world hypothesis posits that the earliest genetic system consisted of informational RNA molecules that directed the synthesis of modestly functional RNA molecules. Further evidence suggests that it was within this RNA-based genetic system that life developed the ability to synthesize proteins by translating genetic code. Here we investigate the early development of the translation system through an evolutionary survey of protein architectures associated with modern translation.</p> <p>Results</p> <p>Our analysis reveals a structural expansion of translation proteins immediately following the RNA world and well before the establishment of the DNA genome. Subsequent functional annotation shows that representatives of the ten most ancestral protein architectures are responsible for all of the core protein functions found in modern translation.</p> <p>Conclusions</p> <p>We propose that this early robust translation system evolved by virtue of a positive feedback cycle in which the system was able to create increasingly complex proteins to further enhance its own function.</p> <p>Reviewers</p> <p>This article was reviewed by Janet Siefert, George Fox, and Antonio Lazcano (nominated by Laura Landweber)</p
Effects of resonant tunneling in electromagnetic wave propagation through a polariton gap
We consider tunneling of electromagnetic waves through a polariton band gap
of a 1-D chain of atoms. We analytically show that a defect embedded in the
structure gives rise to the resonance transmission at the frequency of a local
polariton state associated with the defect. Numerical Monte-Carlo simulations
are used to examine properties of the electromagnetic band arising inside the
polariton gap due to finite concentration of defects.Comment: 12 pages, 6 figures, RevTe
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