Sex Differences in Larval Competition in Drosophila melanogaster: The Testing of a Competition Model and Its Relevance to Frequency-Dependent Selection

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Abstract
A model describing the competition of Drosophila larvae for a fixed amount of concentrated food was successfully tested using data from intrastrain competition experiments. The use of the model showed significant differences between the competitive abilities of male and female larvae in both of the strains used. The relative abilities of the 2 sexes were the same in each case, with males being more successful. This male superiority is not inevitable, since females were the better competitors. The time of eclosion was a poor predictor of competitive ability because the pupal period of females is generally .apprx. 6 h shorter than that of males. The model accounted for > 90% of the experimental variance in each of its 6 applications. This level of accuracy will enable effects, such as facilitation, that are caused by larval metabolites to be distinguished from the effects of pure exploitation since the former are specifically excluded from the model. This is important because of the increasing amount of data linking larval competition to the maintenance of genetic polymorphism. The competition model could not maintain variation. It could generate patterns of survival which suggest stability. Frequency-dependent changes in relative fitness, which in a fixed density model yield a stable equilibrium, no longer do so and overcompensation can occur together with apparently stabilizing frequency effects, but again the joint equilibrium is not stable. Caution in interpreting frequency-dependent survival data is suggested. In particular absolute rather than relative fitness values should be used in any analysis.