Sexual difference in defense can drive the evolution of imperfect Müllerian mimicry in the less defended sex

Müllerian mimicry is the convergent evolution of warning signals among sympatric prey driven by predator learning. Theory therefore predicts signal homogeneity both within communities and within species that participate in Müllerian mimicry. Though rare, sexual dimorphism in Müllerian species does occur, but the underlying eco-evolutionary mechanisms are still relatively unexplored. Basing on the biology of aposematic butterflies, this study uses a modeling approach to test the hypothesis that sexual difference in defense can lead to the evolution of imperfect Müllerian mimicry in the less defended females as the consequence of opposing demands to minimize the cost of automimicry while maximizing reproductive output. Additionally, both the occurrence and degree of sexual dimorphism would decrease when the less defended sex becomes more valuable for reproduction, for example when offspring sex ratio is male biased or when females can mate only once in their lifetime. Findings from this study could help explain the evolution of extreme sexual dimorphism in some Müllerian systems, in which each sex mimics different models. Moreover, through understanding this intriguing exception to the rule, we will be able to gain a more complete picture of how a multitude of selective forces might shape the diversity in prey phenotypes.