Data from: Intrasexual selection favours an immune-correlated colour ornament in a dragonfly

Sexual signalling is predicted to shape the evolution of sex-specific
ornamentation, and establishing the costs and benefits of ornamentation
and the information that ornamentation provides to receivers is necessary
to evaluating this adaptive function. Here, we assessed the adaptive
function of a common colour ornament in insects, melanin wing
ornamentation, using the dragonfly Pachydiplax longipennis. We
hypothesized that greater ornamentation would improve territory-holding
success by decreasing aggression that males receive from territorial
rivals, but that more ornamented males may have shorter lifespans. Using
mark–recapture field observations, we found that more ornamented males had
greater territory-holding success and that viability selection did not act
on wing melanization. We then compared the aggression of territorial
rivals to decoy males before and after experimentally augmenting wing
melanization, finding that males significantly reduced aggression
following the manipulation. We next hypothesized that wing melanization
would signal fighting ability to territorial rivals by reflecting
condition via investment in the costly melanin synthesis pathway. We
observed a positive relationship between ornamentation and the likelihood
of winning territorial disputes, suggesting that wing melanization
provides information about fighting ability to rivals. We also found a
positive relationship between melanin-based immune defence and
ornamentation, supporting a link between the signal and condition. We
conclude that wing melanization is a condition-related signal of fighting
ability and suggest that this may be a common mechanism promoting the
evolution of melanin ornamentation.