Defense against predators incurs high reproductive costs for the aposematic moth Arctia plantaginis

To understand how variation in warning displays evolves and is maintained, we need to understand not only how perceivers of these traits select colour and toxicity, but also the sources of the genetic and phenotypic variation exposed to selection by them. We studied these aspects in the wood tiger moths Arctia plantaginis which has two locally co-occurring male colour morphs in Europe; yellow and white. When threatened, both morphs produce defensive secretions from their abdomen and from thoracic glands. Abdominal fluid has shown to be more important against invertebrate predators than avian predators and the defensive secretion of the yellow morph is more effective against ants. Here we focused on the morph-linked reproductive costs of secretion of the abdominal fluid and quantified the proportion of phenotypic and genetic variation in it. We hypothesized that if yellow males pay higher reproductive costs for their more effective aposematic display, the subsequent higher mating success of white males could offer one explanation for the maintenance of the polymorphism. We first found that the heritable variation in the quantity of abdominal secretion was very low (h2 = 0.006) and the quantity of defensive secretion was not dependent on the male morph. Secondly, deploying the abdominal defensive secretion decreased the reproductive output of both colour morphs equally. This suggests that potential costs of pigment production and chemical defence against invertebrates is not linked in A. plantaginis. Furthermore, our results indicate that environmentally induced variation in chemical defence can alter an individual’s fitness significantly.
Methods
Data sets include:

- Pedigree data used to analyse h2 of the abdominal defensive secretion of Arctia plantaginis male moths (Pedigree data for the h2 analyses of abdominal defensive secretions of Arctia plantaginis males).

-Experimental data to test the reproductive costs of responsive defence in the Arctia plantaginis male moths (A. plantaginis_Cost of defence experiment_Phenotypic data).

-Behavioural data for the Suppelementary information of the manuscript about the defensive behaviour of male moths (Supplementary Data for the Figure S1_The amount of defensive fluid released under repetitive attacks; Supplementary Data for the Figure S2_Quantity of deployed fluid under repetitive attacks). 

- Natural history data about the defensive behaviour from the wild A. plantaginis populations from Southern Finland populations (Volumes of defensive fluid collected from the wild Arctia plantaginis populations).