Experimental evidence of condition-dependent sexual dimorphism in the weakly dimorphic antler fly Protopiophila litigata (Diptera: Piophilidae)

Sexually dimorphic traits have a history of directional selection for exaggeration in at least one sex. Theory suggests that traits targeted by persistent selection should evolve heightened condition dependence whereby their expression reflects the availability and processing efficiency of metabolic resources. This joint dependence of sexual dimorphism and condition dependence on directional selection should result in a positive correlation between the extent of sexual dimorphism and the strength of condition dependence. However, because direct phenotypic evidence is predominately from species with highly exaggerated male traits, it remains unclear whether condition-dependent sexual dimorphism is characteristic of species with more typical levels of dimorphism. We manipulated condition via larval diet and quantified sex-specific responses in adult body size and shape in a moderately dimorphic dipteran species, the antler fly Protopiophila litigata. While dimorphism did not increase with diet quality within any trait, among traits the extent of dimorphism was positively associated with the strength of condition dependence in males but not females, as previously reported for highly dimorphic species. This finding suggests that a shared developmental basis to condition dependence and sexual dimorphism can arise via sexual selection on males even in lineages lacking highly exaggerated male traits.