Intrasexual selection favours an immune‐correlated colour ornament in a dragonfly

Abstract

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.

Immune deployment increases larval vulnerability to predators and inhibits adult life‐history traits in a dragonfly

“…We experimentally induced melanotic encapsulation responses with sterilized, roughened, knotted nylon monofilaments (1.84 mm length, 0.18 mm diameter). We inserted each implant up to the knot between the fifth and sixth tergites of each larva's abdomen and then removed the implant after 24 h (Moore & Martin, ). This ‘24HR‐challenge’ treatment closely mimics a natural immune insult by a common and important source of parasitism in dragonflies, parasitic mites (reviewed in Forbes & Robb, ).…”

“…We kept larvae in pairs in 500‐mL plastic containers (one larva died during immune challenge). After the implant was removed, we removed and digitally photographed it at three angles against a standard brightfield lighting condition with a dissecting microscope (Moore & Martin, ). For each picture, we used ImageJ (Rasband, ) to calculate the difference in implant darkness (mean grey value) between the area that was vs. was not inserted into the larva.…”

“…Following emergence, males in this part of the range produce dark, intrasexually selected wing coloration. A male producing more wing coloration intimidates its rivals, reducing the aggression it receives, and enabling it to hold a territory for longer throughout each day (Moore & Martin, ). Males without wing coloration are largely unable to hold breeding territories.…”

“…After killing adults, we gently removed and photographed their wings against a standard white background (DGK Color Tools ® ) in a dark box that excluded any light other than the flash. Because of the very strong correlation between wing area and body size ( r = 0.834; Moore & Martin, ), we used the average area of the rear wings as an estimate of adult size in all analyses (Corbet, ). We analysed differences in adult size using a mixed‐effects model with immune treatment, predator treatment, and their interaction as fixed effects and tank nested within block as random effects.…”

“…Here, we experimentally tested how larval predation risk influences the costs of immune deployment within and across life stages using a dragonfly, Pachydiplax longipennis , that produces dark intrasexually selected wing coloration (Moore & Martin, ). We specifically addressed three questions.…”