What is the sound of fear?: behavioral responses of white-crowned sparrows (Zonotrichia leucophryus) to synthesized nonlinear acoustic phenomena
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Fear and anxiety may be adaptive responses to life-threatening situations, and animals may communicate fear to others vocally. Nonlinear phenomena—sounds produced by the desynchronization of vibrations in a sound production system—are commonly found in stress-induced animal vocalizations, such as in alarm calls, mobbing calls, and fear screams. There are several functional hypotheses for these nonlinear phenomena. One specific hypothesis is the unpredictability hypothesis, which suggests that because nonlinear phenomena are somewhat unpredictable, animals are less likely to habituate to them. Animals should, therefore, have a greater response to sounds with nonlinear phenomena than sounds without them. Most studies involving nonlinear phenomena have used mammalian subjects and conspecific stimuli. My study focused on white-crowned sparrows (Zonotrichia leucophrys ssp. oriantha) and used synthesized stimuli to investigate behavioral responses to stimuli with and without nonlinear phenomena. I predicted that birds would be less relaxed after hearing a stimulus with a nonlinear phenomenon. I calculated the difference from baseline of proportion of time spent in relaxed behaviors. In the 30-60 s after the playback experiment, birds were significantly less relaxed after hearing stimuli with an abrupt frequency jump down an octave or white noise but not after hearing a stimulus with an abrupt frequency jump up an octave or a pure tone. Nonlinear phenomena, therefore, may be generally arousing to animals.