Positive frequency-dependent selection (FDS) is a selection regime where the fitness of a phenotype increases with its frequency, and it is thought to underlie important adaptive strategies resting on signaling and communication. However, whether and how positive FDS truly operates in nature remains unknown, which hampers our understanding of signal diversity. Here, we test for positive FDS operating on the warning color patterns of chemically defended butterflies forming multiple coexisting mimicry assemblages in the Amazon. Using malleable prey models placed in localities showing differences in the relative frequencies of warningly colored prey, we demonstrate that the efficiency of a warning signal increases steadily with its local frequency in the natural community, up to a threshold where protection stabilizes. The shape of this relationship is consistent with the direct effect of the local abundance of each warning signal on the corresponding avoidance knowledge of the local predator community. This relationship, which differs from purifying selection acting on each mimetic pattern, indicates that predator knowledge, integrated over the entire community, is saturated only for the most common warning signals. In contrast, among the well-established warning signals present in local prey assemblages, most are incompletely known to local predators and enjoy incomplete protection. This incomplete predator knowledge should generate strong benefits to life history traits that enhance warning efficiency by increasing the effective frequency of prey visible to predators. Strategies such as gregariousness or niche convergence between comimics may therefore readily evolve through their effects on predator knowledge and warning efficiency.Müllerian mimicry | aposematism | warning signal | predation | butterflies F requency-dependent selection (FDS) occurs when the fitness of a phenotype depends on its frequency in the population (1). Under negative FDS, the fitness of a phenotype will decrease as its frequency increases. This supposedly common mechanism (2) maintains adaptive polymorphism and has been documented in a variety of natural systems, ranging from foraging behavior (2, 3) and pollination syndromes (4) to predator-prey (5-7) and parasite-host coevolution (8). On the other hand, positive FDS is a selection regime where the fitness of a phenotype increases with its frequency and does not readily maintain polymorphism. However, positive FDS is thought to be a mechanism central to the evolution of a large diversity of adaptive strategies. Notably, traits used in signaling and communication, where efficiency depends on local frequency, such as languages and social signals (9), flower coloration for pollinator attraction (10), and warning signals of prey unpalatability (11), should be subjected to positive FDS. However, although the principles of positive FDS may be well understood from a theoretical point of view, the extent to which it is operating in nature remains largely unknown.Warning coloration advertising ...