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.