Healthier or bigger? Trade‐off mediating male dimorphism in the black scavenger fly S epsis thoracica ( D iptera: S epsidae)

1. Life history trade‐offs emerge when limited resources are allocated to multiple functions of an organism. Under highly competitive conditions trade‐offs can result in alternative phenotypes that differ morphologically and physiologically. Such is the case in insect species that grow under high densities, where competition for resources but also the risk of disease contagion is high, prompting important adjustments in immune response and melanic cuticular pigmentation, with consequent sacrifices in other fitness‐related traits. 2. In the present study, the potential trade‐offs between total‐ and active phenoloxidase ( PO ), body size and body pigmentation in S epsis thoracica black scavenger flies that show alternative male morphs differing in cuticular pigmentation, and body size were evaluated. 3. As expected, small/dark ( obsidian ) males showed higher total‐ PO activity than larger/orange ( amber ) males. A negative relationship was found between total‐ PO activity and body size in females and obsidian but not amber males, suggesting that growth and immunity are more costly for the former. In contrast, density did not affect PO activity, as predicted by the density‐dependent prophylaxis hypothesis, which had not been tested in dipterans before. However, rearing density did affect the body size negatively in females and amber but not obsidian males, showing that male morph is largely determined by condition‐dependent plasticity rather than genes. 4. This study provides good evidence that trade‐offs between different life‐history traits can result in alternative resource allocation strategies, even within one species. These strategies can produce strikingly different alternative phenotypes, evincing that there is not only one optimal solution to address fitness optimisation.