A further study of choice and percentage reinforcement

In Experiment I four pigeons were trained in a concurrent chains procedure with fixed-ratio schedules (FR1) in the initial components and fixed-time schedules in the terminal components. Pecking one of the keys when both keys were white initiated a fixed time schedule on that key. A peck to the left key produced three stripes on the key. At the termination of the fixed-time component food always occurred. Pecking the other key produced either a circle or a triangle. If a circle appeared, reinforcement occurred. If a triangle appeared a brief timeout was given. Initially the stripes appeared on the left key and the circle and triangle on the left. This was reversed during the course of the experiment. In addition, sessions were conducted in which both circle and triangle sometimes preceded reinforcement and sometimes timeout. For most birds under most conditions there was a preference for the key that produced the circle and triangle. When these were uncorrelated with reinforcement and time out three of the birds preferred the key producing 100% reinforcement.

In Experiment II three factors were varied and VI 20 sec schedules were used in the initial links instead of FR1. The results showed that pigeons preferred the 50% condition more 1) the greater the duration of the terminal links, 2) the smaller the value on the initial link VI schedules and 3) the less the probability of food in the terminal link with stripes on the key.     

Psychological distance to reward: equating the number of stimulus and response segments

Psychological distance to reward, or the segmentation effect, refers to the preference for a terminal link of a concurrent-chains schedule consisting of a simple reinforcement schedule (e.g. fixed interval [FI] 30s) relative to its chained-schedule counterpart (e.g. chained FI 15s FI 15s). This experiment was conducted to examine whether the segmentation effect is due to the number of terminal-link stimulus and response segments per se. Three pigeons pecked under a concurrent-chains schedule in which identical variable-interval (VI) schedules operated in the initial links. In each session, half the terminal-link entries followed one initial-link key and the other half followed the other initial-link key. The initial-link keys correlated with the different terminal links were manipulated across conditions. In the first three conditions, each terminal link contained a chained fixed-time (FT) FT schedule, and in the final three conditions, each terminal link contained a chained FI FI schedule. In each condition, in one terminal link (alternating), the order of two key colors correlated with the different schedule segments alternated across terminal-link entries, whereas in the other terminal link (constant), the order of two other key colors was identical for each entry. With the chained FT FT schedule terminal links, there was indifference between the alternating and constant terminal links within and across pigeons, as indexed by initial-link choice proportions. In addition, terminal-link response rates were relatively low. With the chained FI FI schedule terminal links, for each pigeon, there was relatively more preference for the alternating terminal link and terminal-link response rates increased relative to conditions with the chained FT FT schedule terminal links. These data suggest that the segmentation effect is not due simply to the number of terminal-link stimulus or response segments per se, but rather to a required period of responding during a stimulus segment that never is paired with reinforcement.     

Systematic review and meta-analysis: rapid diagnostic tests versus placental histology, microscopy and PCR for malaria in pregnant women

Background

To prepare field sites for malaria vaccine trials, it is important to determine baseline antibody and T cell responses to candidate malaria vaccine antigens. Assessing T cell responses is especially challenging, given genetic restriction, low responses observed in endemic areas, their variability over time, potential suppression by parasitaemia and the intrinsic variability of the assays.

Methods

In Part A of this study, antibody titres were measured in adults from urban and rural communities in Ghana to recombinant Plasmodium falciparum CSP, SSP2/TRAP, LSA1, EXP1, MSP1, MSP3 and EBA175 by ELISA, and to sporozoites and infected erythrocytes by IFA. Positive ELISA responses were determined using two methods. T cell responses to defined CD8 or CD4 T cell epitopes from CSP, SSP2/TRAP, LSA1 and EXP1 were measured by ex vivo IFN-γ ELISpot assays using HLA-matched Class I- and DR-restricted synthetic peptides. In Part B, the reproducibility of the ELISpot assay to CSP and AMA1 was measured by repeating assays of individual samples using peptide pools and low, medium or high stringency criteria for defining positive responses, and by comparing samples collected two weeks apart.

Results

In Part A, positive antibody responses varied widely from 17%-100%, according to the antigen and statistical method, with blood stage antigens showing more frequent and higher magnitude responses. ELISA titres were higher in rural subjects, while IFA titres and the frequencies and magnitudes of ex vivo ELISpot activities were similar in both communities. DR-restricted peptides showed stronger responses than Class I-restricted peptides. In Part B, the most stringent statistical criteria gave the fewest, and the least stringent the most positive responses, with reproducibility slightly higher using the least stringent method when assays were repeated. Results varied significantly between the two-week time-points for many participants.

Conclusions

All participants were positive for at least one malaria protein by ELISA, with results dependent on the criteria for positivity. Likewise, ELISpot responses varied among participants, but were relatively reproducible by the three methods tested, especially the least stringent, when assays were repeated. However, results often differed between samples taken two weeks apart, indicating significant biological variability over short intervals.     

Varying initial-link and terminal-link durations in concurrent-chains schedules: a comparison of three models

In Experiment 1, pigeons responded on concurrent-chains schedules with equal variable-interval schedules as initial links and fixed delays to food as terminal links. One terminal-link delay was always three times as long as the other. As terminal-link delays increased, response percentages on the key with the shorter terminal link increased according to a curvilinear function. This result supported the predictions of the hyperbolic value-added model and the contextual-choice theory but not delay-reduction theory. In Experiment 2, the terminal links were always delays of 2s and 12s, followed by food, and the durations of the initial links varied across conditions. As initial-link durations increased, pigeons' response percentages on the key with the shorter terminal link decreased, but toward an asymptote greater than 50%, indicating a continued preference for the shorter terminal link with very long initial links. This result was more consistent with the predictions of the hyperbolic-value added model than with those of the contextual-choice model or of delay-reduction theory.     

Discriminative proprieties of Vary and Repeat contingencies

Pigeons were trained on an arbitrary matching-to-sample task in which Vary and Repeat contingencies served as sample stimuli. During the sample component, two keys were lit red and a four-peck sequence was reinforced if its frequency was less than a certain threshold (Vary sample) or if it comprised one of two target sequences (Repeat sample). During the comparison component, two keys were lit white and green, and correct choices depended on the previous sample contingency. Pigeons learned to emit high and low variability levels during the sample, and correct matching choices were obtained. In two discrimination testing phases, the requirement of variation (Vary sample) or of repetition (Repeat sample) was parametrically manipulated such that behavioral variability became undifferentiated between samples (low sample disparity) and then differentiated (high sample disparity) again. Accurate choices fell to chance under low sample disparity conditions, but improved under high disparity conditions. The results provide evidence that high and low variability levels can be produced in the absence of antecedent cues and that pigeons can accurately report whether they had experienced a Vary or a Repeat contingency, thus indicating that those contingencies may serve discriminative functions.     

Y chromosome variation of mice and men

DNA sequences from the nonrecombining portion of the Y chromosome were compared with autosomal and X-linked sequences from mice and humans to test the neutral prediction that ratios of polymorphism to divergence are the same for different genes. Intraspecific variation within Mus domesticus was compared with divergence between M. domesticus and Mus caroli for Sry, a region 5' to Sry, and four X-linked genes, Hprt, Plp, Amg, and Glra2. None of these comparisons revealed significantly reduced variation on the Y chromosome. Intraspecific variation within humans was compared with divergence between humans and chimpanzees for three Y-linked loci (Zfy, the YAP region, and the Sry region), seven X- linked loci (Il2rg, Plp, Hprt, Gk, Ids, Pdhal, and Dmd), and the beta- globin locus on chromosome 11. In these comparisons, the observed level of variation on the human Y chromosome was slightly lower than expected, but was significantly lower in only one case (Sry region vs. Dmd). These results suggest that the levels of variability on the Y chromosome in mice and humans are close to expected values given the effective population size and mutation rates for these loci. There is at most only a modest reduction in variability that may be attributed to natural selection (either genetic hitchhiking or background selection).      

Evolution of morphological diversity and resin secretion in flowers of Clusia (Clusiaceae): insights from ITS sequence variation

The taxonomically poorly known, neotropical genus Clusia (Clusiaceae) comprises over 300 species of trees, shrubs and hemiepiphytes. The flowers are morphologically highly diverse and offer either nectar, pollen or resin as a reward for pollinators. Resin production in flowers is a poorly studied phenomenon, known from only five angiosperm genera. Variation in sequences of the nuclear ribosomal ITS region was analysed cladistically in order to trace the evolution of floral resin production in Clusia , and as a first step towards a phylogenetically based reclassification of the genus. The most parsimonious trees show that the genera Havetia, Havetiopsis, Oedematopus and Quapoya are nested inside Clusia. Traditionally, Clusia has been divided into sections based on androecial variation, and most of these groups are supported as monophyletic based on the ITS data. Sections Retinostemon, Havetia and Cochlanthera together form one of very few well-supported suprasectional groups. Character optimisation experiments suggest that resiniferous flowers have evolved at least three times independently in Clusia , but a character-evolution model with a single origin for floral resin is only slightly less parsimonious. Clades of resiniferous species are morphologically the most diverse, and it is concluded that resin production has been a key innovation triggering floral morphological diversification. Secondary losses of resin and switches to apomixis seem to be correlated with colonisation of habitats and areas where resin-collecting bees are less frequent.     

Quantitative genetic variation and developmental clocks

It is well-known that most genetic variation affects quantitative traits, and natural or artificial selection can act to change quantitative features of organisms more rapidly than qualitative ones. Surprisingly, variability is not confined to outbred species, but also occurs in inbred mice at a much higher rate than expected from known mutation rates. The size and shape of organisms and their constituent parts are, at least in part, controlled by the number of cell divisions, and there is published evidence for the existence of developmental clocks, which may count cell divisions. A molecular model for a developmental clock was previously proposed. It depends on the DNA methylation of repeated sequences of DNA, where the methylation of each additional sequence is tied to DNA synthesis and therefore cell division. The number of repeats specifies the number of divisions which will occur before a signal is produced which can activate or inactivate one or more genes. It is known that crossing over occurs between sister chromatids, and where tandemly repeated sequences occur unequal exchange can generate a larger or smaller number of repeats. An example of this is seen in the well-known variability of "minisatellite" sequences in human DNA. Unequal sister chromatid exchange can occur in mitotic and meiotic cells in the germ line, and in the case of developmental clock sequences could generate variation in clock length which in turn would directly affect quantitative traits. These events can be regarded as a special case of molecular drive during evolution.     

Quantitative analyses of methamphetamine's effects on self-control choices: implications for elucidating behavioral mechanisms of drug action

The purpose of the present research was to utilize quantitative methods to identify behavioral mechanisms involved in the effects of stimulant drugs on choice in a self-control procedure. A logarithmic equation based upon a combination of the matching law and hyperbolic discounting was used to separate drug-induced changes in sensitivity to reinforcement delay from drug-induced changes in sensitivity to reinforcement amount. Pigeons responded under a concurrent-chains schedule. In the initial link, two keys were illuminated simultaneously and access to the terminal link was controlled by a single random-interval (RI) schedule; pecks on one or the other key lead to its terminal link with a 0.5 probability. In the terminal links, one alternative provided 1-s access to food (the smaller reinforcer) and the other alternative provided 4-s access to food (the larger reinforcer). The signaled delay to the smaller reinforcer always was 2s, whereas the signaled delay to the larger reinforcer increased from 2 to 40s within each session, across 10-min blocks. In general, intermediate doses of methamphetamine increased preference for the larger more delayed reinforcer. Quantitative analyses indicated that, in most cases, methamphetamine decreased sensitivity to reinforcement delay. In a few instances, concomitant decreases in sensitivity to reinforcement amount also occurred. These results suggest that a reduced sensitivity to reinforcement delay may be important behavioral mechanism of the effects of stimulants on self-control choices, and that this effect sometimes can be accompanied by a decreased sensitivity to reinforcement amount.     

Body size variation in aquatic consumers causes pervasive community effects,independent of mean body size

Intraspecific phenotypic variation is a significant component of biodiversity. Body size, for example, is variable and critical for structuring communities. We need to understand how homogenous and variably sized populations differ in their ecological responses or effects if we are to have a robust understanding of communities. We manipulated body size variation in consumer (tadpole) populations in mesocosms (both with and without predators), keeping mean size and density of these consumers constant. Size‐variable consumer populations exhibited stronger antipredator responses (reduced activity), which had a cascading effect of increasing the biomass of the consumer's resources. Predators foraged less when consumers were variable in size, and this may have mediated the differential effects of predators on the community composition of alternative prey (zooplankton). All trophic levels responded to differences in consumer size variation, demonstrating that intrapopulation phenotypic variability can significantly alter interspecific ecological interactions. Furthermore, we identify a key mechanism (size thresholds for predation risk) that may mediate impacts of size variation in natural communities. Together, our results suggest that phenotypic variability plays a significant role in structuring ecological communities.     

Genetic variation, predator-prey interactions and food web structure

Food webs are networks of species that feed on each other. The role that within-population phenotypic and genetic variation plays in food web structure is largely unknown. Here, I show via simulation how variation in two key traits, growth rates and phenology, by influencing the variability of body sizes present through time, can potentially affect several structural parameters in the direction of enhancing food web persistence: increased connectance, decreased interaction strengths, increased variation among interaction strengths and increased degree of omnivory. I discuss other relevant traits whose variation could affect the structure of food webs, such as morphological and additional life-history traits, as well as animal personalities. Furthermore, trait variation could also contribute to the stability of food web modules through metacommunity dynamics. I propose future research to help establish a link between within-population variation and food web structure. If appropriately established, such a link could have important consequences for biological conservation, as it would imply that preserving (functional) genetic variation within populations could ensure the preservation of entire communities.     

Exceptional among-lineage variation in diversification rates during the radiation of Australia's most diverse vertebrate clade

The disparity in species richness among groups of organisms is one of the most pervasive features of life on earth. A number of studies have addressed this pattern across higher taxa (e.g. 'beetles'), but we know much less about the generality and causal basis of the variation in diversity within evolutionary radiations at lower taxonomic scales. Here, we address the causes of variation in species richness among major lineages of Australia's most diverse vertebrate radiation, a clade of at least 232 species of scincid lizards. We use new mitochondrial and nuclear intron DNA sequences to test the extent of diversification rate variation in this group. We present an improved likelihood-based method for estimating per-lineage diversification rates from combined phylogenetic and taxonomic (species richness) data, and use the method in a hypothesis-testing framework to localize diversification rate shifts on phylogenetic trees. We soundly reject homogeneity of diversification rates among members of this radiation, and find evidence for a dramatic rate increase in the common ancestor of the genera Ctenotus and Lerista. Our results suggest that the evolution of traits associated with climate tolerance may have had a role in shaping patterns of diversity in this group.     

Responses of a specialist and a generalist seed predator to variation in their common resource

Fluctuations of resources in time and space will influence not only species abundance but also interactions among species. For plant–consumer interactions, the effects of resource variation have mostly been studied in systems with high resource variability. Systems with moderate variations are less studied, although ecological and evolutionary dynamics of plants and consumers are likely to be affected also by less extreme variability. The effects of variation in a particular resource should depend on consumer diet width.
We examined how spatial and temporal variation in seed production in the perennial herb Lathyrus vernus influenced population dynamics and resource utilization in two beetle pre-dispersal seed predators with different host ranges over six years. The monophagous Apion opeticum occupied fewer patches and had lower densities than the oligophagous Bruchus atomarius . The proportion of seeds attacked increased with increases in seed production between years for both seed predators. A possible explanation for these patterns is that population dynamics of beetles are driven largely by local factors and that the same factors influence both beetle performance and seed production. In B. atomarius , patterns may also be influenced by a more pronounced preference for L. vernus in years with a high seed production in L. vernus . We conclude that relatively modest variation in seed production may result in responses that differ in both direction and extent from those usually observed in systems with high variation in seed production.     

Animal personality as a cause and consequence of contest behaviour

We review the evidence for a link between consistent among-individual variation in behaviour (animal personality) and the ability to win contests over limited resources. Explorative and bold behaviours often covary with contest behaviour and outcome, although there is evidence that the structure of these ‘behavioural syndromes'' can change across situations. Aggression itself is typically repeatable, but also subject to high within-individual variation as a consequence of plastic responses to previous fight outcomes and opponent traits. Common proximate mechanisms (gene expression, endocrine control and metabolic rates) may underpin variation in both contest behaviour and general personality traits. Given the theoretical links between the evolution of fighting and of personality, we suggest that longitudinal studies of contest behaviour, combining behavioural and physiological data, would be a useful context for the study of animal personalities.     

Identification and size variation of terminal fragments of sheep pox virus genome

Terminal fragments of sheep pox virus DNA identified by snap-back analysis showed terminal covalent cross-links. Southern blot hybridization using a terminal fragment probe confirmed the termini and terminal repeats (common sequences) of the sheep pox virus genome. Terminal fragment length variability was observed between virus isolates.     

Phenological response of tundra plants to background climate variation tested using the International Tundra Experiment

The rapidly warming temperatures in high-latitude and alpine regions have the potential to alter the phenology of Arctic and alpine plants, affecting processes ranging from food webs to ecosystem trace gas fluxes. The International Tundra Experiment (ITEX) was initiated in 1990 to evaluate the effects of expected rapid changes in temperature on tundra plant phenology, growth and community changes using experimental warming. Here, we used the ITEX control data to test the phenological responses to background temperature variation across sites spanning latitudinal and moisture gradients. The dataset overall did not show an advance in phenology; instead, temperature variability during the years sampled and an absence of warming at some sites resulted in mixed responses. Phenological transitions of high Arctic plants clearly occurred at lower heat sum thresholds than those of low Arctic and alpine plants. However, sensitivity to temperature change was similar among plants from the different climate zones. Plants of different communities and growth forms differed for some phenological responses. Heat sums associated with flowering and greening appear to have increased over time. These results point to a complex suite of changes in plant communities and ecosystem function in high latitudes and elevations as the climate warms.     

Predation and food web structure along a habitat duration gradient

Food web statistics showed a complex relationship with measures of habitat variability in temporary ponds. Connectance was highest in short-duration, highly variable habitats, and lowest in habitats of intermediate duration and variability. The number of links and links/taxon increased with increasing duration. Much of the variation in the food web statistics could be explained by a strong linear relationship between number of taxa and number of links/taxon and a quadratic relationship of taxa number with the number of links. However, after accounting for this variation, there remained a relationship of duration with links and links/taxon. The relationship between the food web statistics and duration corresponded to experimental evaluations of predation in these habitats that showed an increasing importance of predation in long-duration habitats. The food web statistics, however, missed threshold effects in the relationship between predation and habitat duration. Differences in food web statistics before and after a regional drought could be explained by a decrease in taxa number after the drought. Connectance was the most robust statistic in relation to taxa number, but was also the least sensitive to changes in habitat characteristics. Received: 11 March 1996 / Accepted: 8 January 1997     

Spatial variation in population growth rate and community structure affects local and regional dynamics

1. Theory predicting that populations with high maximum rates of increase (r(max)) will be less stable, and that metapopulations with high average r(max) will be less synchronous, was tested using a small protist, Bodo, that inhabits pitcher plant leaves (Sarracenia purpurea L.). The effects of predators and resources on these relationships were also determined. 2. Abundance data collected for a total of 60 populations of Bodo, over a period of 3 months, at six sites in three bogs in eastern Canada, were used to test these predictions. Mosquitoes were manipulated in half the leaves partway through the season to increase the range of predation rates. 3. Dynamics differed greatly among leaves and sites, but most populations exhibited one or more episodes of rapid increase followed by a population crash. Estimates of r(max) obtained using a linear mixed-effects model, ranged from 1 x 5 to 2 x 7 per day. Resource levels (captured insect) and midge abundances affected r(max). 4. Higher r(max) was associated with greater temporal variability and lower synchrony as predicted. However, in contrast to expectations, populations with higher r(max) also had lower mean abundance and were more suppressed by predators. 5. This study demonstrates that the link between r(max) and temporal variability is key to understanding the dynamics of populations that spend little time near equilibrium, and to predicting and interpreting the effects of community structure on the dynamics of such populations.