Restoration of call attractiveness by novel acoustic appendages in grey treefrogs

Ethologists have often reported preferences for novel signals, especially if they are more extravagant than normal signals. Such preferences presumably reflect sensory biases that may promote the evolution of both novel and complex signals. We tested behavioural responses of female grey treefrogs, Hyla versicolor, to novel complex calls in relation to the response properties of previously described temporally selective neurons in the auditory midbrain. As predicted by the selectivity of interval-counting neurons, females discriminated against synthetic advertisement calls containing a gap, a missed pulse or a pulse of abnormally short duration. The addition of a novel tonal appendage to such defective calls often resulted in partial restoration of the attractiveness of the signal. The restorative effect occurred only when an appendage with a higher amplitude followed (rather than led) the defective call. Our results show how the consideration of proximate mechanisms can provide insights about the evolution of complex signals; the behavioural results, in turn, suggest new ways of assessing the response properties of the auditory system.