The monopolization of key resources via intraspecific competition is essential for survival and achieving reproductive success. The best predictor of the patterns of competition for mates is likely the operational sex ratio (OSR), defined as the number of competitors of one sex to the number of ready-to-mate members of the opposite sex, which predicts an increase in intrasexual competition as the availability of mates decreases. OSR theory has been generalized to other resources (e.g. food) under the term competitor-to-resource ratio (CRR), defined as the ratio of individual competitors to the number of resource units available, which predicts the same relationship between competition and resource availability for any type of resource. However, selection pressures are likely to act differently on each sex. Due to their higher reproductive rates, males are usually the most competitive sex in terms of reproduction, whereas females tend to be more aggressive than males for food. In addition, the cost of predation forces prey individuals to trade-off fitness-related activities for antipredator behaviour. As imminent predation risk increases, individuals should invest more energy into avoiding predation at the cost of foraging and mating opportunities. This trade-off is shaped by ambient predation risk as suggested by the risk-allocation model. When imminent predation risk is low, individuals experienced with high ambient predation risk should perform more fitness-related activities to compensate for lost opportunities during period of higher imminent predation risk. Conversely, low ambient predation-risk individuals should respond to imminent risks with an 'all-or-nothing' response at the cost of short-term benefits. These various factors influencing intraspecific competition have been studied, but in isolation from one another in most cases. The aim of this thesis was to explore the potential interactions of these factors in a single species using Trinidadian guppies. To test the above predictions, we observed wild-caught individuals from two populations (i.e. high versus low ambient predation pressures) under different CRRs and imminent risk (i.e. conspecific alarm cue) levels. Surprisingly, there was no difference in aggression rates between sexes overall, independent of the type of resource. The availability of mates had the largest effect on aggression rates with an increase in intrasexual competition as the number of mates decreased. Food was more defendable than mates in our experiment, likely because prospective mates are mobile and can be choosy about their sexual partners. Finally, due to the cost of predation, individuals exposed to high ambient predation risk tended to be less aggressive and more cooperative. This work highlights the complexity of competitive interactions in natural prey populations, and the various selective pressures affecting wild populations.

This book celebrates the guppy's unique contribution to evolutionary ecology. Ever since Caryl Haskins described guppy populations as a 'natural experiment' because of the way predation pressure varies over a small geographical area, generations of researchers have been drawn to Trinidad to investigate evolution in the wild. The species continues to provide classic examples of natural selection in action and elegantly illustrates how ecology, evolution, and behaviour are interlinked. Anne Magurran's account of the evolutionary ecology of the guppy integrates historical breakthroughs with new research in this fast-moving field. She reveals how guppies provided some of the first evidence of sperm competition and sexual selection, and how they continue to inform scientific thought on mating systems and cryptic choice. The consequences of variation in predation risk—as well as a host of other biotic and abiotic factors—are described and evaluated at all life stages from conception to death. The book discusses behavioural responses to ecological conditions alongside life history patterns. It examines the potential for ecological speciation and discusses new research into how reproductive isolating mechanisms become established in promiscuous mating systems. Conservation issues are also considered, both in terms of protecting the irreplaceable Trinidadian guppy system and in the context of invasion ecology. This timely synthesis of research into a species that has raised key questions in evolutionary ecology will be of great interest to graduate level students as well as professional researchers in the fields of behavioural ecology and evolutionary biology.

The Trinidadian guppy represents a uniguely tractable vertebrate system, which has raised key questions in evolutionary ecology and supplied many of the answers. This work discusses this study and incorporates significant new findings and insights.

Trinidadian guppies, Poecilia reticulata, have adapted differently to their habitats with and without predators. One by product of these adaptations is behavioral differences in schooling and aggression. Those who live in high predation environments tend to aggregate and find safety in numbers to avoid being eaten, while those who live in low predation environments are inclined to be more aggressive towards one another to compete for limited food. We have developed behavioral assays to quantify the tendency to school and the number of aggressive acts shown toward each other. As a part of the design, we have incorporated a repeatability measure to test the variation within our populations. We found that males from populations that live with and are adapted to predators have a significantly higher tendency to school than males from populations that do not have predators. Females from one high predation environment were also more inclined to school but those from the second population were not. It may be that the sex difference is caused by males being brightly colored and more susceptible to predation than females, so females are less exposed to predators and gain less from schooling. We also found a relatively high repeatability of approximately 40% to 50% for all four populations for the schooling behavior. The aggression assay is still in progress. Once completed the assays will address the gap in knowledge of the interaction of cooperative and aggressive behaviors as well as the inheritance of complex traits.