Experimental evidence that density mediates negative frequency-dependent selection on aggression

1. Aggression can be beneficial in competitive environments if aggressive individuals are more likely to access resources than non-aggressive individuals. However, variation in aggressive behaviour persists within populations, suggesting that high levels of aggression might not always be favoured. 2. The goal of this study was to experimentally assess the effects of population density and phenotypic frequency on selection on aggression in a competitive environment. 3. We compared survival of two strains of Drosophila melanogaster that differ in aggression across three density treatments and five frequency treatments (single strain groups, equal numbers of each strain, and strains mixed at 3:1 and 1:3 ratios) during a period of limited resources. 4. While there was no difference in survival across single-strain treatments, survival was strongly density-dependent, with declining survival as density increased. Furthermore, at medium and high densities, there was evidence of negative freque...