Social insect colonies represent distinct units of selection. Most individuals evolve by kin selection and forgo individual reproduction. Instead, they display altruistic food sharing, nest ...maintenance and self-sacrificial colony defence. Recently, altruistic self-removal of diseased worker ants from their colony was described as another important kin-selected behaviour. Here, we report corroborating experimental evidence from honey bee foragers and theoretical analyses. We challenged honey bee foragers with prolonged CO₂ narcosis or by feeding with the cytostatic drug hydroxyurea. Both treatments resulted in increased mortality but also caused the surviving foragers to abandon their social function and remove themselves from their colony, resulting in altruistic suicide. A simple model suggests that altruistic self-removal by sick social insect workers to prevent disease transmission is expected under most biologically plausible conditions. The combined theoretical and empirical support for altruistic self-removal suggests that it may be another important kin-selected behaviour and a potentially widespread mechanism of social immunity.
Vegetation buffers local diurnal land surface temperatures, however, this effect has found limited applications for remote vegetation characterization. In this work, we parameterize diurnal ...temperature variations as the thermal decay rate derived by using satellite daytime and nighttime land surface temperatures and modeled using Newton's law of cooling. The relationship between the thermal decay rate and vegetation depends on many factors including vegetation type, size, water content, location, and local conditions. The theoretical relationships are elucidated, and empirical relationships are presented. Results show that the decay rate summarizes both vegetation structure and function and exhibits a high correlation with other established vegetation-related observations. As proof of concept, we interpret 15-year spatially explicit trends in the annual thermal decay rates over Africa and discuss results. Given recent increases in availability of finer spatial resolution satellite thermal measurements, the thermal decay rate may be a useful index for monitoring vegetation.
The kinetics of methanol oxidation on supported 47.5 wt.% Pt and 54 wt.% PtRu (with nominal Pt:Ru ratios of 2:3) catalysts are measured in 0.5 M H
2SO
4 and 0.1 NaOH at 295 and 333 K using thin-film ...rotating disk electrode (RDE) method. It was found that the activity of Pt and PtRu for methanol oxidation is a strong function of pH of solution and temperature. The kinetics are much higher in alkaline than in acid solution; at 333 K, a factor of 30 for Pt and a factor of 20 for Pt
2Ru
3 at 0.5 V. The pH effect is attributed to the pH competitive adsorption of oxygenated species with anions from supporting electrolytes. The activity of Pt and Pt
2Ru
3 catalysts at 333 K is higher (a factor of 5) than at 295 K. Irrespective of pH, only negligible differences in the kinetics are observed between Pt and on high Ru content Pt alloys, presumably owing to a slow rate of methanol dehydrogenation on the Ru-rich surface and insufficient number of Pt sites required for dissociative chemisorption of methanol.
In condensed matter systems, formation of long-range order (LRO) is often accompanied by new excitations. However, in many geometrically frustrated magnetic systems, conventional LRO is suppressed, ...while non-trivial spin correlations are still observed. A natural question to ask is then what is the nature of the excitations in this highly correlated state without broken symmetry? Frequently, applying a symmetry breaking field stabilizes excitations whose properties reflect certain aspects of the anomalous state without LRO. Here we report a THz spectroscopy study of novel excitations in quantum spin ice Yb2 Ti2 O7 under a directed magnetic field. At large positive fields, both right- and left-handed magnon and two-magnon-like excitations are observed. The g-factors of these excitations are dramatically enhanced in the low-field limit, showing a crossover of these states into features consistent with the quantum string-like excitations proposed to exist in quantum spin ice in small fields.
The breathing pyrochlore lattice material Ba3Yb2Zn5O11 exists in the nearly decoupled limit, in contrast to most other well-studied breathing pyrochlore compounds. As a result, it constitutes a ...useful platform to benchmark theoretical calculations of exchange interactions in insulating Yb3+ magnets. Here we study Ba3Yb2Zn5O11 at low temperatures in applied magnetic fields as a further probe of the physics of this model system. Experimentally, we consider the behavior of polycrystalline samples of Ba3Yb2Zn5O11 with a combination of inelastic neutron scattering and heat capacity measurements down to 75 mK and up to fields of 10 T. Consistent with previous work, inelastic neutron scattering finds a level crossing near 3 T, but no significant dispersion of the spin excitations is detected up to the highest applied fields. Refinement of the theoretical model previously determined at zero field can reproduce much of the inelastic neutron scattering spectra and specific heat data. A notable exception is a low temperature peak in the specific heat at ∼0.1 K. This may indicate the scale of interactions between tetrahedra or may reflect undetected disorder in Ba3Yb2Zn5O11.
We describe a comparative study of the oxygen reduction reaction on two carbon-supported Pt-based alloy catalysts in aqueous acidic electrolyte at low temperature. Both alloys have the bulk ...compositions of 50 and 75 at. % Pt, with the alloying elements being Ni and Co. Comparison is made to a pure Pt catalyst on the same carbon support, Vulcan XC-72, having the same metal loading (20 wt %) and nominally the same particle size (4 ± 2 nm). High-resolution electron microscopy was used to determine the size and shape of the particles as well as the particle size distribution on all catalysts. Electrochemical measurements were performed using the thin-film rotating ring−disk electrode method in 0.1 M HClO4 at 20−60 °C. Hydrogen adsorption pseudocapacitance was used to determine the number of Pt surface atoms and to estimate the surface composition of the alloy catalysts. Kinetic analysis in comparison to pure Pt revealed a small activity enhancement (per Pt surface atom) of ca. 1.5 for the 25 at. % Ni and Co catalysts, and a more significant enhancement of a factor of 2−3 for the 50 at. % Co. The 50 at. % Ni catalyst was less active than the Pt standard and unstable at oxygen electrode potentials. Ring-current collection measurements for peroxide indicated no significant differences between the Pt−Co catalysts or the 25 at. % Ni catalyst and pure Pt, while the 50 at. % Ni catalyst had a higher peroxide yield. Together with the observed Tafel slopes and activation energies, it was concluded that the kinetic enhancement is contained in the preexponential factor of the conventional transition state theory rate expression. It is, however, not clear why the alloying with Ni or Co produces this change in the preexponential factor.
The oxygen reduction reaction (orr) has been studied on carbon supported multimetallic catalysts in aqueous acidic electrolyte. The bimetallic catalysts had the nominal compositions of 50 and 75 at.% ...Pt with the alloying elements being Ni and Co. A ternary catalyst consisted of Pt, Rh and Fe in the ratio of 1:0.3:0.7. Comparison is made to carbon supported Pt. The particle size distribution of the metallic phases was obtained by high resolution transmission electron microscopy (HRTEM). An estimation of the surface composition was obtained from the hydrogen adsorption charge and the particle size. Electrochemical measurements were performed using the thin-film rotating ring-disk electrode (RRDE) method in 0.1 M HClO
4 at 20–60
°C. Kinetic analysis in comparison to pure Pt revealed an activity enhancement (per Pt surface atom) of a factor of 2 to 3 for the 50 at.% Co-catalyst. The 25 at.% Co(Ni) and the ternary catalyst revealed similar activity compared with pure Pt. The 50 at.% Ni catalyst was actually less active than the Pt standard and unstable at oxygen electrode potentials. Ring-current collection measurements for peroxide indicated no significant differences between the alloy catalysts and pure Pt with the exception of the 50 at.% Ni catalyst which had a higher peroxide yield. Comparison was also made to results obtained under equal experimental conditions on polycrystalline (pc) Pt
3Ni and Pt
3Co alloy electrodes. These bulk alloys were prepared in UHV to obtain a well defined surface composition of 75 at.% Pt and 25 at.% Co and Ni, respectively. A Pt(pc) electrode served for comparison. The bulk alloys also have an enhanced activity for the orr by about a factor of 2 versus the identically prepared Pt(pc) electrode.
The purpose of this review is to discuss progress in the understanding of electrocatalytic reactions through the study of model systems with surface spectroscopies. Pure metal single crystals and ...well-characterized bulk alloys have been used quite successfully as models for real (commercial) electrocatalysts. Given the sheer volume of all work in electrocatalysis that is on fuel cell reactions, we will focus on electrocatalysts for fuel cells. Since Pt is the model fuel cell electrocatalyst, we will focus entirely on studies of pure Pt and Pt bimetallic alloys. The electrode reactions discussed include hydrogen oxidation/evolution, oxygen reduction, and the electrooxidation of carbon monoxide, formic acid, and methanol. Surface spectroscopies emphasized are FTIR, STM/AFM and surface X-ray scattering (SXS). The discussion focuses on the relation between the energetics of adsorption of intermediates and the reaction pathway and kinetics, and how the energetics and kinetics relate to the extrinsic properties of the model system, e.g. surface structure and/or composition. Finally, we conclude by discussing the limitations that are reached by using pure metal single crystals and well-characterized bulk alloys as models for real catalysts, and suggest some directions for developing more realistic systems.
Summary Background Tubal ectopic pregnancy can be surgically treated by salpingectomy, in which the affected Fallopian tube is removed, or salpingotomy, in which the tube is preserved. Despite ...potentially increased risks of persistent trophoblast and repeat ectopic pregnancy, salpingotomy is often preferred over salpingectomy because the preservation of both tubes is assumed to offer favourable fertility prospects, although little evidence exists to support this assumption. We aimed to assess whether salpingotomy would improve rates of ongoing pregnancy by natural conception compared with salpingectomy. Methods In this open-label, multicentre, international, randomised controlled trial, women aged 18 years and older with a laparoscopically confirmed tubal pregnancy and a healthy contralateral tube were randomly assigned via a central internet-based randomisation program to receive salpingotomy or salpingectomy. The primary outcome was ongoing pregnancy by natural conception. Differences in cumulative ongoing pregnancy rates were expressed as a fecundity rate ratio with 95% CI, calculated by Cox proportional-hazards analysis with a time horizon of 36 months. Secondary outcomes were persistent trophoblast and repeat ectopic pregnancy (expressed as relative risks RRs with 95% CIs) and ongoing pregnancy after ovulation induction, intrauterine insemination, or IVF. The researchers who collected data for fertility outcomes were masked to the assigned intervention, but patients and the investigators who analysed the data were not. All endpoints were analysed by intention to treat. We also did a (non-prespecified) meta-analysis that included the findings from the present trial. This trial is registered, number ISRCTN37002267. Findings 446 women were randomly assigned between Sept 24, 2004, and Nov 29, 2011, with 215 allocated to salpingotomy and 231 to salpingectomy. Follow-up was discontinued on Feb 1, 2013. The cumulative ongoing pregnancy rate was 60·7% after salpingotomy and 56·2% after salpingectomy (fecundity rate ratio 1·06, 95% CI 0·81–1·38; log-rank p=0·678). Persistent trophoblast occurred more frequently in the salpingotomy group than in the salpingectomy group (14 7% vs 1 <1%; RR 15·0, 2·0–113·4). Repeat ectopic pregnancy occurred in 18 women (8%) in the salpingotomy group and 12 (5%) women in the salpingectomy group (RR 1·6, 0·8–3·3). The number of ongoing pregnancies after ovulation induction, intrauterine insemination, or IVF did not differ significantly between the groups. 43 (20%) women in the salpingotomy group were converted to salpingectomy during the initial surgery because of persistent tubal bleeding. Our meta-analysis, which included our own results and those of one other study, substantiated the results of the trial. Interpretation In women with a tubal pregnancy and a healthy contralateral tube, salpingotomy does not significantly improve fertility prospects compared with salpingectomy. Funding Netherlands Organisation for Health Research and Development (ZonMW), Region Västra Götaland Health & Medical Care Committee.
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