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Abstract Animals often mimic the behaviours or signals of conspecifics of the opposite sex while courting. We explored the potential functions of a novel female-like signal type in the courtship displays of male Enchenopa treehoppers. In these plant-feeding insects, males produce plant-borne vibrational advertisement signals, to which females respond with their own duetting signals. Males also produce a signal type that resembles the female duetting responses. We experimentally tested whether this signal modifies the behaviour of receivers. First, we tested whether the female-like signal would increase the likelihood of a female response. However, females were as likely to respond to playbacks with or without them. Second, we tested whether the female-like signal would inhibit competing males, but males were as likely to produce displays after playbacks with or without them. Hence, we found no evidence that this signal has an adaptive function, despite its presence in the courtship display, where sexual selection affects signal features. Given these findings, we also explored whether the behavioural and morphological factors of the males were associated with the production of the female-like signal. Males that produced this signal had higher signalling effort (longer and more frequent signals) than males that did not produce it, despite being in worse body condition. Lastly, most males were consistent over time in producing the female-like signal or not. These findings suggest that condition-dependent or motivational factors explain the presence of the female-like signal. Alternatively, this signal might not bear an adaptive function, and it could be a way for males to warm up or practice signalling, or even be a by-product of how signals are transmitted through the plant. We suggest further work that might explain our puzzling finding that a signal in the reproductive context might not have an adaptive function. 

The social environment is often the most dynamicandfitness-relevant environment animals experience. Here we testedwhether plasticity arising from variation in social environments canpromote signal-preference divergence—a key prediction of recentspeciation theory but one that has proven difficult to test in natural sys-tems. Interactions in mixed social aggregations could reduce, create,or enhance signal-preference differences. In the latter case, social plas-ticity could establish or increase assortative mating. We tested this byrearing two recently diverged species ofEnchenopatreehoppers—sap-feeding insects that communicate with plant-borne vibrationalsignals—in treatments consisting of mixed-species versus own-speciesaggregations. Social experience with heterospecifics (in the mixed-species treatment) resulted in enhanced signal-preference species dif-ferences. For one of the two species, we tested but found no differencesin the plastic response between sympatric and allopatric sites, sug-gesting the absence of reinforcement in the signals and preferencesand their plastic response. Our results support the hypothesis that so-cial plasticity can create or enhance signal-preference differences andthat this might occur in the absence of long-term selection against hy-bridization on plastic responses themselves. Such social plasticity mayfacilitate rapid bursts of diversification.