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Communicating species identity is a key component of many animal signals. However, whether selection for species recognition systematically increases signal diversity during clade radiation remains debated. Here we show that in woodpecker drumming, a rhythmic signal used during mating and territorial defense, the amount of species identity information encoded remained stable during woodpeckers’ radiation. Acoustic analyses and evolutionary reconstructions show interchange among six main drumming types despite strong phylogenetic contingencies, suggesting evolutionary tinkering of drumming structure within a constrained acoustic space. Playback experiments and quantification of species discriminability demonstrate sufficient signal differentiation to support species recognition in local communities. Finally, we only find character displacement in the rare cases where sympatric species are also closely related. Overall, our results illustrate how historical contingencies and ecological interactions can promote conservatism in signals during a clade radiation without impairing the effectiveness of information transfer relevant to inter-specific discrimination.

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Calls of male rock ptarmigans show individual vocal signatures.These signatures allow acoustic censusing of the number of males present in an area.Acoustic sampling is more accurate than point-count protocol.

Small animals typically localize sound sources by means of complex internal connections and baffles that effectively increase time or intensity differences between the two ears. However, some miniature acoustic species achieve directional hearing without such devices, indicating that other mechanisms have evolved. Using 3D laser vibrometry to measure tympanum deflection, we show that female lesser waxmoths (Achroia grisella) can orient toward the 100-kHz male song, because each ear functions independently as an asymmetric pressure gradient receiver that responds sharply to high-frequency sound arriving from an azimuth angle 30°contralat-eral to the animal's midline. We found that females presented with a song stimulus while running on a locomotion compensation sphere follow a trajectory 20°-40°to the left or right of the stimulus heading but not directly toward it, movement consistent with the tympanum deflections and suggestive of a monaural mechanism of auditory tracking. Moreover, females losing their track typically regain it by auditory scanning-sudden, wide deviations in their heading-and females initially facing away from the stimulus quickly change their general heading toward it, orientation indicating superior ability to resolve the front-rear ambiguity in source location. X-ray computer-aided tomography (CT) scans of the moths did not reveal any internal coupling between the two ears, confirming that an acoustic insect can localize a sound source based solely on the distinct features of each ear.

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