Geographic mosaic of selection by avian predators on hindwing warning colour in a polymorphic aposematic moth.

Katja Rönkä,Swanne Gordon,Bibiana Rojas,Ossi Nokelainen,Johanna Mappes,Emily Burdfield‐Steel,Janne K Valkonen,Greg Grether

doi:10.1111/ele.13597

Abstract

Warning signals are predicted to develop signal monomorphism via positive frequency-dependent selection (+FDS) albeit many aposematic systems exhibit signal polymorphism. To understand this mismatch, we conducted a large-scale predation experiment in four countries, among which the frequencies of hindwing warning coloration of the aposematic moth, Arctia plantaginis, differ. Here we show that selection by avian predators on warning colour is predicted by local morph frequency and predator community composition. We found+FDS to be the strongest in monomorphic Scotland and lowest in polymorphic Finland, where the attack risk of moth morphs depended on the local avian community. +FDS was also found where the predator community was the least diverse (Georgia), whereas in the most diverse avian community (Estonia), hardly any models were attacked. Our results support the idea that spatial variation in predator communities alters the strength or direction of selection on warning signals, thus facilitating a geographic mosaic of selection.

Highlights

The survival strategy of aposematism, wherein prey use warning signals that predators learn to associate with their unprofitability and subsequently avoid, has stimulated biological studies for centuries (Wallace, 1867; Poulton, 1890; Cott, 1940; Mappes et al, 2005; Merrill et al, 2015; Skelhorn et al, 2016; Ruxton et al, 2018)
The morphs with intermediate local frequencies show corresponding levels of attack risk (Fig. 2). This effect did not depend on colour morph itself (Table 1, Table S2), as expected if the local predator avoidance depends more on local morph frequency than on morph colour
principal component 1 (PC1) was dominated by Sylvidae, Fringillidae and Muscicapidae, which loaded in the negative end, whereas the positive end of the axis was loaded with Paridae (Fig. 3)

Summary

Introduction

The survival strategy of aposematism, wherein prey use warning signals that predators learn to associate with their unprofitability and subsequently avoid, has stimulated biological studies for centuries (Wallace, 1867; Poulton, 1890; Cott, 1940; Mappes et al, 2005; Merrill et al, 2015; Skelhorn et al, 2016; Ruxton et al, 2018). Prey benefit from lowered costs of predator education by carrying a common signal, whereas predators reduce risks by not attacking defended prey. This results in selection for local similarity in warning signals, a view that has been corroborated by theoretical approaches (e.g. Mu€ller, 1878; Mallet and Joron, 1999; Sherratt, 2008; Aubier and Sherratt, 2015), laboratory experiments It is expected that selection on aposematism is positively frequency-dependent (+FDS), with predators avoiding the most common warning signal in a locality (Sherratt, 2008; Comeault and Noonan, 2011; Chouteau and Angers, 2011; Chouteau et al, 2016; Ruxton et al, 2018)

Methods

Results

Conclusion