Female Fertilization: Effects of Sex-Specific Density and Sex Ratio Determined Experimentally for Colorado Potato Beetles and Drosophila Fruit Flies

If males and females affect reproduction differentially, understanding and predicting sexual reproduction requires specification of response surfaces, that is, two-dimensional functions that relate reproduction to the (numeric) densities of both sexes. Aiming at rigorous measurement of female per capita fertilization response surfaces, we conducted a multifactorial experiment and reanalyzed an extensive data set. In our experiment, we varied the density of male and female Leptinotarsa decemlineata (Colorado potato beetles) by placing different numbers of the two sexes on enclosed Solanum tuberosum (potato plants) to determine the proportion of females fertilized after 3 or 22 hours. In the reanalysis, we investigated how the short-term fertilization probability of three Drosophila strains (melanogaster ebony, m. sepia, and simulans) depended on adult sex ratio (proportion of males) and total density. The fertilization probability of female Leptinotarsa decemlineata increased logistically with male density, but not with female density. These effects were robust to trial duration. The fertilization probability of female Drosophila increased logistically with both sex ratio and total density. Treatment effects interacted in m. sepia, and simulans. These findings highlight the importance of well-designed, multifactorial experiments and strengthen previous experimental evidence for the relevance of sex-specific densities to understanding and prediction of female fertilization probability.WKV, MEH, and HK acknowledge financial support from the Academy of Finland. GB and PM acknowledge financial support from Agriculture and Agri-Food Canada. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Female Fertilization : Effects of Sex-Specific Density and Sex Ratio Determined Experimentally for Colorado Potato Beetles and Drosophila Fruit Flies

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Food divisibility and interference competition among captive ruddy turnstones, Arenaria interpres

Interference competition among foraging animals arises from agonistic interactions among foragers. Interactions can concern single food items but also clumps of food. Food clumps consist of multiple food items, and are therefore easier to divide among foragers than food items. Theoretical studies indicate that differences in divisibility can be essential to the interference process. Empirically, however, little is known about effects of resource divisibility on interference competition. Therefore, we performed an experiment with captive ruddy turnstones. Turnstones foraged either alone or together with a competitor. We offered food at two so-called food pits and varied the divisibility of food in these pits by burying a fixed number of food items either in several layers (divisible) or in a single layer (indivisible). Additionally, we varied the distance between food pits. We accounted for differences in the social dominance status of foragers by using pairs of foragers as our experimental unit: each pair had both a dominant and a subordinate member. We found a strong asymmetry in the intake of birds of different dominance status. The strength of this asymmetry depended on both the divisibility of food and on the distance between food pits. Only when food was divisible did subordinate foragers get a share of the food; only when food pits were close to each other could dominant foragers monopolize food pits. These findings imply that to understand and predict interference competition we need to consider both the detailed characteristics of resources, and the determinants of dominance status. (c) 2007 The Association for the Study or Animal Behaviour. Published by Elsevier Ltd. All rights reserved

Interference competition, the spatial distribution of food and free-living foragers

Studies of interference competition among foraging animals generally assume that variation in the spatial distribution of food can be neglected. This assumption may be problematic as resource defence experiments suggest that such variation is of the essence in some interference mechanisms. Interpretation of the results of field experiments on this topic, however, is hard because most studies used univariate statistics to analyse multivariate data. Because in free-living foragers interference and patch selection are connected behaviours, treatment effects on these responses are best studied simultaneously, through multivariate analyses. We performed a field experiment in which we provided wild ruddy turnstones, Arenaria interpres, with experimental plots that varied in the distance between a fixed number of so-called food pits, and, using multivariate statistics, we studied effects on the combination of the turnstones’ behaviour and abundance. We found that when food pits were more spaced out, turnstones were present in higher numbers, while interacting less with each other. Nevertheless, turnstones spent about the same amount of time digging for food, our measure of intake rate, at each interpit distance. These findings imply that to reliably predict the combination of the number, intake rate and aggression of turnstones, the spatial distribution of food has to be known. We would not have reached this conclusion if we had used univariate statistics. In addition, we argue that multivariate statistics helps to clarify the way field experiments on the spatial distribution of food are to be interpreted.

Spatial clumping of food and social dominance affect interference competition among ruddy turnstones

In studying the success of foraging animals, studies of interference competition have put emphasis on effects of competitor density, whereas studies of resource defense have focused on the effects of the spatial distribution of food within patches. Very few studies have looked at both factors simultaneously, that is, determined whether the effects of competitor density on foraging success depend on the spatial distribution of food. We studied the behavior and the foraging success of ruddy turnstones (Arenaria interpres) using an experiment in which we varied both the presence of a competitor and the food distribution. Because turnstones may differ strongly in their relative dominance status, we also experimentally varied the foragers' relative dominance status. We found that the presence of a competitor only reduced the foraging success of subordinate birds foraging at the clumped food distribution. At this condition, dominant and subordinate birds differed markedly in their foraging success. Contrary to our expectations, we did not observe more agonistic behavior at the clumped food distribution. This indicates that the amount of agonistic behavior observed may be a bad indicator of interference effects. These findings have specific implications for models of interference competition. Most notably they show that the effects of competitor density on agonistic behavior and foraging success may well depend on the spatial distribution of food and the foragers' relative dominance status. Additionally, our results suggest that social dominance will not be fully understood without considering long-term processes such as the formation and maintenance of social dominance hierarchies. Copyright 2005.agonistic interactions; Arenaria interpres; density dependence; foraging behavior; foraging experiment; resource defense

The mechanisms of interference competition: two experiments on foraging waders

Models of population dynamics that include interference competition have often been applied to foraging waders and less so to other foragers, even though these models are, in principle, generally applicable. At present, however, it is still unclear whether interference competition is of importance for foraging waders. To support this idea experimental evidence and knowledge of the mechanisms underlying interference effects are required. We experimentally determined the relationship between forager density and foraging success in two wader species: the red knot (Calidris canutus) and the ruddy turnstone (Arenaria interpres). With each of the two species, we conducted an experiment consisting of 300 one-min trials. In these trials we scored the behavior and the foraging success of focal individuals at specific combinations of bird and prey density. Irrespective of prey density, individuals of both species discovered fewer prey items at higher bird densities. Despite this, only in turnstones did intake rates decline with increasing bird density. Knots compensated for a lower prey-discovery rate by rejecting fewer prey items at higher bird densities. In knots, bird density had a complex, nonmonotonic effect on the time spent vigilant and searching. In turnstones the main effect of increased bird density was a reduction in the prey-encounter rate, that is, the reward per unit search time. Effects on the time spent vigilant and the time spent searching were less pronounced than in knots. Thus, the mechanistic basis of the effects of bird density was complex for each of the two species and differed between them. Copyright 2005.Arenaria interpres; behavioral mechanisms; Calidris canutus; density dependence; exploitation competition; social dominance

Results of the (logistic) regression model of the ln-transformed odds of fertilization for female <i>Leptinotarsa decemlineata</i> (Colorado potato beetles)^†.

†<p>Parameter estimates ‘β’ and their standard error ‘s.e.’ were computed using the ‘lmer’ function in R. Confidence intervals ‘CI’ of parameter estimates were computed as β ± z_α,2 · se(β), the ‘odds ratio’, that is, the ratio of the odds of fertilization at two treatment values that differ exactly one unit, was computed as exp(β), and the ‘z-values’ were computed as β/se(β), all conform Collett <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0060381#pone.0060381-Collett1" target="_blank">[38]</a>. Effects that are substantial relative to their standard error are presented in bold to guide the eye. Random effects (σ²) of the block factors experimental day (nested within TD) and cage were 0.13 and 0.00, respectively.</p