Life-history constraints and warning signal expression in an arctiid moth

1. Traditionally, large pattern elements in conspicuous warning signals are assumed to be selected by predation because increasing signal strength enhances education and avoidance of predators. However, variation in the colour pattern of a warningly coloured species often exists even within a location, and many warning signals have relatively small pattern elements. This suggests that there must be opposing selection pressures that can constrain warning signal expression, resulting in variation in warning signals as well as suboptimal signals for predator education. 2. We quantified the variation in the warning signal of Parasemia plantaginis-moth larvae. With a full-sib rearing experiment, we tested whether this variation has a genetic as well as an environmental component, by measuring the warning signal size of the developing larvae that were reared on either Rumex or an artificial diet. We also measured whether signal development was correlated to other life-history variables. 3. We show that the relative size of the warning signal varies among families but increases with larval age. The signal development also has an environmental component, since signal size develops more slowly and remains smaller in larvae feeding on the artificial diet than in those feeding on Rumex. Also, larvae with a short development time produced large warning signals. We also found that the larvae with a large orange signal were more likely to survive to adulthood. 4. Our results suggest that diet and life-history correlates can constrain development of the warning signal. Our study underlines the importance of considering life-histories of animals, not only predation, when studying the evolution of warning signals. Constraints of multiple selection pressures should be incorporated into the theory of the evolution of aposematism.