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.