Trade-off between warning signal efficacy and mating success in the wood tiger moth

Abstract

The coloration of species can have multiple functions, such as predator avoidance and sexual signalling, that directly affect fitness. As selection should favour traits that positively affect fitness, the genes underlying the trait should reach fixation, thereby preventing the evolution of polymorphisms. This is particularly true for aposematic species that rely on coloration as a warning signal to advertise their unprofitability to predators. Nonetheless, there are numerous examples of aposematic species showing remarkable colour polymorphisms. We examined whether colour polymorphism in the wood tiger moth is maintained by trade-offs between different functions of coloration. In Finland, males of this species have two distinct colour morphs: white and yellow. The efficacy of the warning signal of these morphs was tested by offering them to blue tits in the laboratory. Birds hesitated significantly longer to attack yellow than white males. In a field experiment, the survival of the yellow males was also higher than white males. However, mating experiments in the laboratory revealed that yellow males had lower mating success than white males. Our results offer an explanation for the maintenance of polymorphism via trade-off between survival selection and mating success.

Figure 1.

(a) The white (left) and yellow (right) male morph of wood tiger moth. (b) Predators' hesitation (in seconds) to attack against colour morph.

Figure 2.

(a) Survival plot of the yellow (solid line) and white (dashed line) colour morph survival in the field. The lines are the probability of surviving avian predation as a function of time (hours) based on Cox regression estimates to account for censored data during the 5 day experiment. (b) Chromatic contrast values of hind wing colour compared against different backgrounds and their relation to predicted probability of survival. Squares represent white males and circles represent yellow males.

Figure 3.

(a) The mean mating probability of wood tiger moth males. On the x-axis, the ‘control’ represents individuals, which were not in the defensive fluid treatment, and ‘manipulated’, stands for the defensive fluid (i.e. droplet) treatment group. Left bars within the group (white) stands for the white males, and right bars (yellow) represent yellow males. On the y-axis is the mating probability of males, when females have no alternative choice. (b) The mean volume (mm³) of defensive fluid between two male colour morphs of wood tiger moth.