Overlapping vocalizations produce far-reaching choruses: a test of the signal enhancement hypothesis

Many animals gather in large groups to mate. When these animals produce sexual signals, their signals may overlap. The signal enhancement hypothesis proposes that overlapping signals exhibit enhanced transmission properties, increasing the active space and potency of the signal. We tested this hypothesis using multispeaker playback to simulate a chorus of explosively breeding Neotropical Yellow Toads (Incilius luetkenii). We varied the number of simulated males and the frequency of their vocalizations and we rerecorded the choruses at different distances through this species' native habitat in Costa Rica. Our results support the signal enhancement hypothesis: transmission distance increased with the number of simultaneous calls. Call frequency varies inversely with body size in many animals, including Yellow Toads, and our results reveal that the signal enhancement effect of overlapping calls is heightened when the calls are low in frequency (i.e., a chorus of large-bodied animals) compared to medium or high frequency (i.e., a chorus of smaller-bodied animals). Our findings represent the first experimental demonstration of chorus-level signal enhancement in the vocalizations of vertebrates.