Sex ratio dynamics and fluctuating selection pressures in natural populations of the Trinidadian guppy,<Emphasis Type="Italic"> Poecilia reticulata</Emphasis>

In many species, population sex ratios have far-reaching consequences for a wide variety of population-level and behavioural processes and can directly influence sexual selection through differential effects on male and female mating behaviour. Although sex ratios are often treated as more or less stable population characteristics, recent theoretical evidence suggests that sex ratios fluctuate under many conditions, and that the amplitude of these fluctuations can be considerable. Few studies have attempted to quantify this variation in systems with prominent, sex ratio-dependent sexual conflict. One of the species with the greatest potential to integrate these factors in the wild is the Trinidadian guppy, Poecilia reticulata. In this study, we quantified natural sex ratio variation both as detailed longitudinal studies of focal guppy populations and as snapshot estimates across a range of freshwater habitats. In line with theoretical predictions, we expected to detect significant sex ratio variation over time. We also investigated the association between juvenile and adult sex ratios to quantify a possible compensatory feedback implied in standard models of sex ratio evolution. Our results confirm that population-level sex ratios in wild guppy populations have a range of dynamic features, with all four focal populations showing significant variation in sex ratio over time. The survey showed that juveniles were generally close to equal (50:50) sex ratios whereas 7 out of 11 adult sex ratios differed significantly from equality. We found no evidence that a surplus of juveniles of the locally rarer sex had been produced. The results indicate that sex ratios and hence the balance between sexual selection and sexual coercion is normally fluctuating in nature, despite juvenile ratios being close to equality.Communicated by J. Krause     

A simple voltammetric procedure for speciation and evaluation of As removal from water

Here, we show a fast and sensitive method for the determination of inorganic arsenic in natural waters using differential pulse cathodic stripping voltammetry. All the arsenite determinations were done in 2.0 mol L⁻¹ HCl + 3.15 × 10⁻⁴ mol L⁻¹ Cu(II) supporting electrolyte. 1 × 10⁻³ mol L⁻¹ sodium thiosulphate was used as As(V) reducing agent. The detection limit was 0.5 μg L⁻¹ for both species. The method has been applied to water samples collected in an arsenic-contaminated region of Brazil, in particular, to verify the efficiency of the solar oxidation and removal of arsenic process applied to these waters.     

巴西亚马孙河流域雅诺马马人体内的有机汞含量

位于巴西北部的Catrimani河流域是雅诺马马人的居住地,且自1980年以来,一直是采金场所.金矿开采活动释放入环境中的无机汞会被转化为有机汞,且在水域生态系统会发生生物放大作用.摄食被汞污染鱼的人群,如雅诺马马人有可能会受到危害.我们分别于1994年和1995年对居住在采矿河流附近和非采矿河流附近的雅诺马马村民进行了调查,并且对被其捕捞的水虎鱼体内的汞含量进行了分析.我们分别于1994和1995年对5个村庄的90名雅诺马马印地安人和3个村庄的62名雅诺马马印地安人进行了调查研究.于1994年所调查的5个村庄中,有4个位于Catrimani河沿岸,大约在距过去金矿开采活动场所下游140～160km处.另一个村庄位于没有开采活动的ajaraní河沿岸.于1995年所调查的3个村庄中,有两个位于Catrimani河沿岸的村庄再一次被调查,另一个位于没有采矿活动的Pacu河.在被调查的所有村民中,其血液中有机汞含量的变化范围为0～62.6μg/L(每个村庄的平均值在21.2～43.1μg/L之间).采自有采矿活动的Catrimani河的水虎鱼汞含量的变化范围为235～1084ppb.在所测的13个水虎鱼样品中,有9个体长在30cm或更长的水虎鱼样品总汞含量超过了世界卫生组织和巴西卫生部规定的鱼中汞消费限值(500ppb).不幸的是,在没有开采活动的Ajaraní和Pacu河附近的居民和鱼体内都呈现出了较高的汞水平,这表明间接的汞源也会对亚马孙河流域造成环境污染.     

Risk-based prenatal screening for trisomy 18 using alpha-fetoprotein,unconjugated oestriol and human chorionic gonadotropin

Nine centres collaborated to examine the feasibility of a screening method for trisomy 18 that was based on assigning individual risk, using a combination of maternal age and measurements of alpha-fetoprotein (AFP), unconjugated oestriol (uE3), and human chorionic gonadotropin (hCG). Second-trimester measurements of these analytes were obtained from 94 trisomy 18 pregnancies. In the 89 pregnancies without an associated open defect, the median levels for AFP, uE3, and hCG were 0.65, 0.43 and 0.36 multiples of the unaffected population median, respectively. The strongest individual predictor of risk for trisomy 18 was uE3, followed by hCG, AFP, and maternal age, in that order. Using a method of individual risk estimation that is based on the three markers and maternal age, 60 per cent of pregnancies associated with trisomy 18 would be detected at a risk cut-off level of 1:100, with a false-positive rate of about 0.2 per cent. One in nine pregnancies identified as being at increased risk for trisomy 18 would be expected to have an affected pregnancy. This risk-based screening method is more efficient than an existing method that is based on fixed analyte cut-off levels. Even though the birth prevalence of trisomy 18 is low, prenatal screening can be justified when performed in conjunction with Down syndrome screening and when a high proportion of women offered amniocentesis have an affected fetus.     

The impact of feedback and self-efficacy on performance in training

This field experiment examined the main and interactive effects of self-efficacy and feedback (i.e. the use of feedback versus no feedback) on performance in a speed reading class. Results showed that the provision of feedback was beneficial to the performance of all subjects, however as subject self-efficacy increased, the beneficial effects of feedback to the subject's performance also increased. Subjects who received feedback on their performance experienced significantly greater increases in self-efficacy than subjects who received no feedback. The more positive the performance feedback received, the greater the increase in individual self-efficacy. The implications of these results for training programs are discussed.     

Fine-scale observations of the predatory behaviour of the carnivorous copepod <Emphasis Type="Italic">Paraeuchaeta norvegica</Emphasis> and the escape responses of their ichthyoplankton prey,Atlantic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>)

Paraeuchaeta norvegica (8.5 mm total length) and yolk-sac stage Atlantic cod larvae (4 mm total length) (Gadus morhua) larvae were observed in aquaria (3 l of water) using silhouette video photography. This allowed direct observations (and quantitative measurement) of predator–prey interactions between these two species in 3-dimensions. Tail beats, used by cod larvae to propel themselves through the viscous fluid environment, also generate signals detectable by mechanoreceptive copepod predators. When the prey is close enough for detection and successful capture (approximately half a body-length), the copepod launches an extremely rapid high Reynolds number attack, grabbing the larva around its midsection. While capture itself takes place in milliseconds, minutes are required to subdue and completely ingest a cod larva. The behavioural observations are used to estimate the hydrodynamic signal strength of the cod larva’s tail beats and the copepod’s perceptive field for larval fish prey. Cod larvae are more sensitive to fluid velocity than P. norvegica and also appear capable of distinguishing between the signal generated by a swimming and an attacking copepod. However, the copepod can lunge at much faster velocities than a yolk-sac cod larva can escape, leading to the larva’s capture. These observations can serve as input to the predator–prey component of ecosystem models intended to assess the impact of P. norvegica on cod larvae.