On the expression of male harm in <em>Drosophila melanogaster</em>: impacts of density and structural complexity of the mating environment

Male harm occurs when traits in males that increase their reproductive success incidentally reduce female fitness. In Drosophila melanogaster, many lab studies have revealed the presence of male harm, but recent work has shown that its expression can be dramatically reduced, even eliminated, when sexual interactions and mating occur in an environment that differs from traditional lab rearing vials. Here we follow up on this to separately test the effect of fly density and structural complexity of the mating environment in mediating the expression of male harm. We performed separate two-way factorial assays that measured the fitness of females while manipulating their exposure to males and the density of flies or the structural complexity of the environment during exposure. Male harm, quantified as the relative reduction in female fitness under increased male exposure, was not affected by density, but was significantly reduced –essentially eliminated– by increased structural complexity. Our results demonstrate that seemingly simple choices, like the environment used in a laboratory model system, can have profound impacts on the expression of harm and hence views on the prevalence of sexual conflict. This is noteworthy because conflict can shape other fundamental evolutionary processes including adaptation, purging, and speciation.