The effects of competition on fitness depend on the sex of both competitors

In intraspecific competition, the sex of competing individuals is likely to be important in determining the consequences of competition, both for the immediate outcome of competitive interactions, and for long-term effects of competition during development on adult fitness traits. Previous studies have explored differences between males and females in their response to intraspecific competition. However, few have tested how the sex of the competitors, or any interactions between focal and competitor sex, influence the nature and intensity of competition. We set up larval seed beetles Callosobruchus maculatus to develop either alone or in the presence of a male or female competitor, and measured a suite of traits: development time, emergence weight; male ejaculate mass, copulation duration and lifespan; and female lifetime fecundity, offspring egg-adult survival and lifespan. We found effects of competition and competitor sex on the development time and emergence weight of both males and...