Mate choice and the ‘opposite miss’ to Weber's law: proportional processing governs signal preferences in a treefrog

According to Weber's law, discrimination between stimuli is based upon the proportional difference (ΔI/I), not the absolute difference (ΔI), between their magnitudes (I). How such nonlinear processing of signal information by receivers impacts the effectiveness of communication and drives patterns of signal evolution remains poorly understood. To investigate whether female preferences for exaggerated sexual signals follow Weber's law, we tested females of Cope's grey treefrog, Hyla chrysoscelis, in two-alternative choice tests, in which we varied both the absolute (ΔI) and proportional (ΔI/I) differences in duration between two advertisement calls. Consistent with Weber's law, female preferences for longer signals increased as ΔI/I increased; however, preferences were negatively related to both ΔI and absolute signal duration (I). This so-called ‘opposite miss’ to Weber's law (i.e. poorer discrimination performance than predicted at high stimulus magnitudes) has potential to shape signal evolution in distinct ways that differ in the extent to which further signal exaggeration is favoured. Our results highlight that animals use a diversity of approaches to information coding in perception, including coding that is neither linear nor strictly proportional according to Weber's law, and that perceptual processes have the potential to impact an important selective pressure – female preferences – and shape animal communication.