Citizen science supports ecological predictions of phenology in North American mimetic butterflies

Phenological synchrony enables species to occur when conditions are optimal for survival. While phenological synchrony between butterflies and their host plants has been extensively documented, the importance of phenology in maintaining interspecies interactions, such as mimicry, is less understood. Mimicry occurs when a species (i.e. the mimic) evolves a phenotypic resemblance to an unpalatable species (i.e. the model), resulting in protection against predation for the mimic. Theory predicts that in Batesian mimicry systems, models should appear seasonally before their mimics to give predators sufficient time to learn, recognize, and avoid their aposematic signal (i.e. model-first hypothesis). Here, we use citizen science data from iNaturalist to test these long-standing predictions. To understand how mimicry influences the evolution of different phenological strategies, we estimate onset phenology in two systems: the defended model species Battus philenor and its classic Batesian mimic Limenitis arthemis astyanax, and the more complex system consisting of Mullerian co-mimics Danaus plexippus and Limenitis archippus. Our results support the model-first hypothesis and demonstrate that unpalatable models appear significantly before their mimics across large geographical scales. This research highlights a new avenue for utilizing large-scale citizen science datasets to address long-standing questions about how phenology impacts complex ecological interactions.