Condition dependence and trade-offs of sexual versus non-sexual traits in an insect

Abstract

Sexual traits often communicate male condition and so are known to be highly condition-dependent. Thus, it is expected that, under restricted environments, sexual traits will be more heavily impacted than non-sexual traits, and so a negative covariation will be expected between sexual traits and non-sexual traits as only high-quality males will sustain the costs of producing both trait types. Such covariation will not necessarily appear in non-restricted environments. We tested these predictions using males of the American rubyspot, Hetaerina americana. First, fully mature males from different seasons were collected and their sexual [a wing red spot and body size (this corrected for body mass using residuals)], and condition-indicating, non-sexual (phenoloxidase and protein concentration) traits were measured. Second, larvae were reared under different food quantities and the same traits plus another non-sexual trait [pro-phenoloxidase (proPO)], were measured in recently emerged males. Contrary to expected, non-sexual traits showed larger expression variance than sexual traits. We found a significant covariation between body size and proPO for experimental males. Both rich and poor diet groups showed a negative slope for body size and proPO. This supposes a resource allocation trade-off between these two traits for recently emerged animals. On the other hand, the presumed signaling function between sexual traits, such as spot size, and physiological indicators of condition in this species, is not supported.