Multitasking males and multiplicative females: dynamic signalling and receiver preferences in Copeʹs grey treefrog

The ‘multitasking hypothesis’ for complex signal function predicts performance trade-offs between signal components that negatively covary (e.g. due to energetic or mechanical constraints) and receiver preferences for more extreme values of the negatively covarying components that are difficult to produce simultaneously. We tested these two predictions in Copeʹs grey treefrogs, Hyla chrysoscelis. In a field study, we recorded and analysed 1000 advertisement calls from males calling in breeding choruses to test the prediction that two signal components important in female mating decisions (call rate and call duration) negatively covary under natural conditions. In a laboratory study, we conducted phonotaxis tests with female subjects to test the prediction that females prefer calls with higher overall ‘call efforts’ (the product of call rate × call duration). Consistent with predictions of the multitasking hypothesis, call rate and call duration were significantly negatively related and females preferred calls produced with higher call efforts, manifested through preferences for greater values of both call rate and call duration. We conducted an additional playback experiment to test the hypothesis that males increase their call effort in competitive situations to maximize their attractiveness to females. Compared to quiet conditions, male subjects increased their call duration and decreased their call rate, but did not alter call effort, in response to a simulated calling neighbour or broadcasts of chorus noise. Together our data have implications for understanding the function of multicomponent signals when signallers must balance performance trade-offs in mate attraction with dynamic signal modifications in other social contexts.