Honest signal of male quality in the territorial contests of a Neotropical giant damselfly

In species whose males defend territories to attract females, competing males frequently challenge the ownership. Signals predicting contest outcomes are beneficial because they reduce the costs associated with actual fights. However, a challenge to the use of such signals is that males may signal dishonestly. One common resolution to the problem is to involve condition dependent costs for signals. In the Neotropical giant damselfly Megaloprepus caerulatus, males defend water-filled tree holes to attract females. Previous work has suggested that a male's white wing patches, made of nano wax-structures, are a signal assessed by contesting rivals. However, the male traits it reflects and the honesty of this signal remain unclear. We hypothesized that the wing patch indicates (1) body size, (2) fat reserves or (3) flight muscle mass of males. Furthermore, if the patch honestly signals male quality, we expected that (4) the wing patch exhibits higher allometry and variation than non-signaling traits, and that (5) the production cost is condition dependent. We manipulated food availability of full-sib larvae, and measured wing patch traits and the potential underlying qualities from males emerged. The results suggested that the wing patch size was indicative of both body size and fat reserves, but not flight muscle mass. As predicted for honest signals, the wing patch showed a higher isometric scaling and greater variation relative to homologous, non-signaling traits. Additionally, the proportion of total fat allocated to develop the wing patch decreased with body condition, suggesting a condition-dependent production cost of the signal.