Increasing absolute prey density within a mimicry complex protects aposematic models and their Batesian mimics.

Abstract

Batesian mimicry is a defensive adaptation where predators learn to avoid aposematic prey and generalize their warning signals to phenotypically similar mimics. The phenotypic accuracy needed for mimics to benefit from this adaptation depends on the relative densities of models and mimics and the model's unpalatability. As aposematic models become more unpalatable or more common relative to their mimics, warning signals become stronger, allowing even poor mimics to benefit. However, few studies have disentangled the importance of relative frequencies of models and mimics from absolute density of the prey community (both models and mimics) in driving relaxed selection on mimicry. Here, we test the hypothesis that increasing model unpalatability and absolute prey community density accelerates predator avoidance learning and enhances protection for mimics. Using replicas of the model Adelpha iphiclus (Linnaeus, 1758), its mimic Adelpha serpa (Boisduval, 1836), and the palatable control Junonia evarete (Cramer, 1779), we conducted field experiments that enhanced model unpalatability and doubled absolute prey density while maintaining a constant ratio of model, mimic, and control phenotypes. We found that enhanced model unpalatability and increased absolute density significantly reduced predation on all species, highlighting absolute community density as an underappreciated mechanism shaping selection on Batesian mimics.