Dietary constraints and costs of melanin pigmentation plasticity

Understanding the evolution of adaptive plasticity requires considering the costs of producing a plastic trait. We test the hypothesis that diet can act as a constraint on melanin pigmentation and lead to costly resource allocation trade-offs in Hyles lineata, the white-lined sphinx moth. In a diet manipulation experiment, we found that two aspects of melanin pigmentation, percent melanic area and darkness, are relatively robust in the face of diet variation in environments where they are prioritized. Next, we tested whether larval melanin pigmentation is involved in resource allocation trade-offs with traits sharing the same dietary precursors: larval immune response, adult flight muscle mass, and adult wing pigmentation. Larval pigmentation trades off with both immunity and adult pigmentation. Contrary to many patterns reported in the literature, these trade-offs existed on the high resource diet, but not the low resource diet, indicating that certain traits may have a minimum threshold of expression. Larvae can plastically increase melanin pigmentation in contexts where melanin benefits outweigh costs, but otherwise reduce melanin pigmentation so that limited dietary precursors can be allocated to immunity and adult pigmentation. These context-specific costs and benefits of melanin can help explain the adaptive value of melanin plasticity in this species.