Colour alone matters: no predator generalization among morphs of an aposematic moth

Abstract

Local warning colour polymorphism, frequently observed in aposematic organisms, is evolutionarily puzzling. This is because variation in aposematic signals is expected to be selected against due to predators' difficulties associating several signals with a given unprofitable prey. One possible explanation for the existence of such variation is predator generalization, which occurs when predators learn to avoid one form and consequently avoid other sufficiently similar forms, relaxing selection for monomorphic signals. We tested this hypothesis by exposing the three different colour morphs of the aposematic wood tiger moth, Arctia plantaginis, existing in Finland to local wild-caught predators (blue tits, Cyanistes caeruleus). We designed artificial moths that varied only in their hindwing coloration (white, yellow and red) keeping other traits (e.g. wing pattern and size) constant. Thus, if the birds transferred their aversion of one morph to the other two we could infer that their visual appearances are sufficiently similar for predator generalization to 2 take place. We found that, surprisingly, birds showed no preference or aversion for any of the three morphs presented. During the avoidance learning trials, birds learned to avoid the red morph considerably faster than the white or yellow morphs, confirming previous findings on the efficacy of red as a warning signal that facilitates predator learning. Birds did not generalize their learned avoidance of one colour morph to the other two morphs, suggesting that they pay more attention to conspicuous wing coloration than other traits. Our results are in accordance with previous findings that coloration plays a key role during avoidance learning and generalization, which has important implications for the evolution of mimicry. We conclude that, in the case of wood tiger moths, predator generalization is unlikely to explain the unexpected coexistence of different morphs.

Multiple modalities in insect warning displays have additive effects against wild avian predators

“…Yellow models were made in the same way, using the same picture to keep the pattern constant, but modifying the hue to match the colour of a yellow male. Similarities between the actual moths and the paper models were corroborated with reflectance measurements as described in Rönkä et al (2018) (Fig. 1c).…”

“…For example, the low number of birds that actually ate the prey might indicate that, although the artificial wings accurately resembled the appearance of the real moths, our models lacked some extra cues involved in rapid prey recognition. A previous study carried out in a similar setting showed that some birds were reluctant to attack and eat these models at the very first encounter, but the majority ate them after some training (Rönkä et al 2018). Because we were interested in bird reaction to the first encounter with these prey, training was not a possibility within the scope of our study.…”

“…This visual signal is coupled with two types of chemical defences, one of which (hereafter, neck fluids) is particularly deterrent against birds (Rojas et al 2017a). Experiments done with real moths and dummies, both in the lab and in the wild, indicate that yellow individuals tend to be less attacked by birds, suggesting that yellow is a better warning signal (Nokelainen et al 2012(Nokelainen et al , 2014; but see Rönkä et al 2018), yet from those studies, it is not possible to infer which of the components of the warning display is conferring this advantage. Recent work where natural predators were exposed to the moths' chemical defences only (i.e.…”

An aposematic colour‐polymorphic moth seen through the eyes of conspecifics and predators – Sensitivity and colour discrimination in a tiger moth

“…Both adult males (Nokelainen et al, 2012;Rojas et al, 2017) and females (Brain, 2016;Lindstedt, Reudler Talsma, Ihalainen, Lindström, & Mappes, 2010) are unpalatable to bird predators, which learn the moths' hindwing coloration as a warning signal (Lindstedt et al, 2011;Nokelainen, Valkonen, Lindstedt, & Mappes, 2014;Nokelainen et al, 2012;Rönkä, De Pasqual, Mappes, Gordon, & Rojas, 2018). Although the coloration of A. plantaginis has mostly been studied within the context of aposematism and predator-prey interactions (i.e.…”

Multimodal Aposematic Signals and Their Emerging Role in Mate Attraction

“…They, too, produce chemical defenses that deter birds effectively, but are costly to produce (Brain, 2016;Furlanetto, 2017). Red females are slightly more conspicuous (Lindstedt et al, 2011;Henze et al, 2018), and less frequently attacked by avian predators (Lindstedt et al, 2011) than orange ones, and experiments with wild-caught birds have demonstrated that the red coloration is learned faster than white and yellow (Rönkä et al, 2018a).…”