No abstract

Mating signals of many animal species are difficult to produce and thus should indicate signaler quality. Growing evidence suggests that receivers modulate their behaviour in response to signals with varying performance levels, although little is known about if and how responses are affected by receiver attributes. To explore this topic we conducted two experiments with swamp sparrows, Melospiza georgiana, in which we challenged territorial males with playback of songs with trill rates that were natural, digitally reduced, or digitally elevated (control-, low-and highperformance stimuli, respectively). In our first experiment, we found that males responded more aggressively to control songs than to low-performance stimuli, that low-performance stimuli with the most severe trill-rate reductions elicited the weakest aggressive responses, and that the subjects' own trill rates predicted aggressive responses. In our second experiment, we found that male responses to high-performance stimuli varied significantly, in ways predicted by two factors: the degree to which we had elevated stimulus performance levels of high-performance stimuli, and subjects' own vocal performance levels. Specifically, males were less aggressive towards stimuli for which we had elevated performance levels to higher degrees, and subject males with higher vocal performances themselves responded more aggressively. These findings together offer a novel illustration of how responses to aggressive signals may rely not just on signal attributes, but also on attributes of responding animals themselves.

Anthropogenic noise imposes novel selection pressures, especially on species that communicate acoustically. Many animals—including insects, frogs, whales and birds—produce sounds at higher frequencies in areas with low-frequency noise pollution. Although there is support for animals changing their vocalizations in real time in response to noise (i.e. immediate flexibility), other evolutionary mechanisms for animals that learn their vocalizations remain largely unexplored. We hypothesize that cultural selection for signal structures less masked by noise is a mechanism of acoustic adaptation to anthropogenic noise. We test this hypothesis by presenting nestling white-crowned sparrows (Zonotrichia leucophyrs) with less-masked (higher-frequency) and more-masked (lower-frequency) tutor songs either during playback of anthropogenic noise (noise-tutored treatment) or at a different time from noise playback (control treatment). As predicted, we find that noise-tutored males learn less-masked songs significantly more often, whereas control males show no copying preference, providing strong experimental support for cultural selection in response to anthropogenic noise. Further, noise-tutored males reproduce songs at higher frequencies than their tutor, indicating a distinct mechanism to increase signal transmission in a noisy environment. Notably, noise-tutored males achieve lower performance songs than their tutors, suggesting potential costs in a sexual selection framework.