Natural Sounds Alter California Ground Squirrel, Otospermophilus beecheyi, Foraging, Vigilance and Movement Behaviours

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Many animals rely on the acoustic environment for functions spanning mate attraction, navigation and predator and prey detection. Growing evidence focused on human-altered acoustic environments suggests that anthropogenic noise can strongly interfere with the reception of biologically relevant sounds,causing a variety of behavioural changes in response to the evolutionarily novel acoustic conditions created by humans. However, little is known about how background natural sounds, such as river noise and biotic choruses, alter behaviour. Using field-placed playback of predominantly low-frequency whitewater river rapids, higher-frequency cicada choruses (~7.0e15.0 kHz) or a silent control, we sought to determine whether background natural sounds influence vigilance and foraging behaviour in the California ground squirrel. We found that California ground squirrels exposed to low-frequency river sounds increased vigilance and decreased foraging and movement relative to ambient acoustic conditions during control trials and, to a lesser extent, acoustic conditions imposed by the cicada chorus playback. Additionally, vigilance increased with sound level regardless of whether the playback stimulus was the low-frequency river noise or the high-frequency cicada chorus. However, background sound level interacted with group size, such that increased sound levels were associated with a strong increase in vigilance in small groups but not in larger groups. To our knowledge these results are the first to demonstrate that the spectral content and amplitude of natural sounds can influence vigilance and movement behaviours. Yet our results match those from recent studies reporting increased vigilance in response to low-frequency anthropogenic noise, suggesting that many observed responses to anthropogenic sounds may be those that animals have used to cope with variable sound levels from natural sources throughout their evolutionary history. Determining how natural sounds influence other key behaviours is ripe for future studies and will likely prove useful for predicting behavioural adjustments in response to an increasingly noisy world.