Multimodal signal interactions in the ladybeetle, Hippodamia convergens, aposematic system

In an aposematic system, the warning display (e.g., color, odor, or behavior) advertises the presence and potentially the strength of the chemical defense. A strong coupling between the indicator trait and the defensive trait is therefore integral to the evolution and maintenance of aposematism. An ostensibly conflicting theory predicts that a negative correlation between the indicator and defensive trait can exist, depending on the selective pressure on defenses and the presence of multiple aposematic signals. The aposematic ladybeetle, Hippodamia convergens, displays a variety of aposematic signals, the combined effects of which have been rigorously tested in predator stimulus–response experiments. Intraspecific variation of investment in these multimodal aposematic displays, however, has not been explored thoroughly. In this study, we examine multimodal signal theory from the perspective of the signaler, and identify significant correlations between warning coloration, warning odor, and toxicity. We show that the red coloration of H. convergens functions as an honest signal of alkaloid toxicity, while methoxypyrazine warning odors are negatively correlated with alkaloid content. However, there is no correlation between red coloration and methoxypyrazine content. Thus, the reddest females contained the highest concentrations of alkaloids and some of the lowest concentrations of methoxypyrazines. The reddest males, although producing less alkaloids than their red female counterparts, on average produced significantly more alkaloids and methoxypyrazines than orange females and orange males. Within each of these groups, the negative correlation between methoxypyrazines and alkaloid toxicity remains. The relationship between variations in these signals may be affected by the interactions of color and odor on avoidance learning in predators, as well as the supplemental aggregative function of the aposematic methoxypyrazine signal.