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Psychophysics measures the relationship between a stimulus's physical magnitude and its perceived magnitude. Because decisions are based on perception of stimuli, this relationship is critical to understanding decision-making. We tested whether psychophysical laws explain how female túngara frogs (Physalaemus pustulosus) and frog-eating bats (Trachops cirrhosus) compare male frog calls, and how this imposes selection on call evolution. Although both frogs and bats prefer more elaborate calls, they are less selective as call elaboration increases, because preference is based on stimulus ratios. Thus, as call elaboration increases, both relative attractiveness and relative predation risk decrease because of how receivers perceive and compare stimuli. Our data show that female cognition can limit the evolution of sexual signal elaboration.

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Females often prefer to mate with males who produce complex signals. It is not clear why they do so. Females might prefer complexity if it predicts mate quality, or signals might evolve complexity to exploit females' sensory or cognitive biases. We tested whether complexity increases active time, the period over which a signal influences a receiver's response to that signal. Mating signals are often ephemeral, yet their active time has largely been ignored. Here we demonstrate that signal complexity influences active time in túngara frogs. Male advertisement calls consist of frequency sweeps (whines) followed by 0-7 high-frequency bursts (chucks). Females preferentially approach complex (whines with chucks) over simple (whines alone) calls but do not consistently prefer greater complexity, so the function of multiple chucks has been uncertain. We found that females remember which speaker previously broadcast complex calls when choosing between simple calls broadcast after a delay. This effect occurred for calls with multiple chucks, but not with single chucks. Neither motivation nor orientation behavior differed with chuck number, suggesting that results are due to differential memory. Thus, female memory could favor the evolution of increasing signal complexity through sexual selection.