Background. In animal reproductive contexts, calling behaviour is mostly performed by males but in species in which females call, it is not known how vocal interaction occurs between sexes, particularly when sexual dimorphism in signals is low, as in cases in which call repertoire is identical but acoustic properties differ. In Darwin’s frog (Rhinoderma darwinii), a species in which males brood larvae inside their vocal sacs, females have higher dominant frequency and shorter calls and notes than males. Since in this species males persist calling after getting pregnant with larvae, different vocal interaction patterns are expected to occur among animals having dissimilar reproductive status.
Methodology. To explore the mechanisms underlying vocal recognition among the different sexual status of R. darwinii, we recorded natural duets between non-pregnant males (NPM), pregnant males (PM) and females (F) and evaluated their evoked vocal response to natural playback stimuli of each sexual status from November to February 2015-2016 in Chiloé island, Chile. Call rate, phase angles, sound pressure level (SPL), number of overlapping calls and delay of overlapping calls were measured to determine differential responses between natural duets and in response to stimuli consisting of natural calls of individuals of different sexual status.
Results. Spontaneous duet interactions occurred mainly between males and no clear differences between duets were detected. In playbacks, call ratios in response to calls of different sexual status were similar. Females decreased their SPL in response to F calls, while F and PM had longer call delays and lower call overlaps between each other. Major differences were observed in call overlap, as the occurrence of this phenomenon was larger in playback experiments than during natural duets. The number of calls overlapped during natural duets was fewer (10.9 %) than during playback experiments (36.8 %).
Conclusions. Our results suggest that in R. darwinii, PM and F signalize their sexual status by decreasing their call overlap and that NPM respond indistinctly to the other sexual status. In general, these differences in selective call overlap between Darwin's frogs arise as a novel mechanism for signal recognition between animal vocal interactions.