Richardson's ground squirrel (Urocitellus richardsonii) alarm call receivers experience an auditory continuity illusion.

Auditory continuity illusions are perceptual illusions in which receivers perceive an auditory signal as continuous when part of the signal has been removed or obfuscated by a louder noise. Richardson's ground squirrels (Urocitellus richardsonii) communicate alarm acoustically in noisy natural settings. Thus, we tested whether they experience an auditory continuity illusion by presenting free-living squirrels with repeated whistle alarm calls within which every second syllable remained unmanipulated, was partially obfuscated by relatively brief (25 ins) bursts of white noise, or partially obfuscated by longer duration (100 ms) white noise bursts. Field playbacks of alarm calls with brief white noise bursts failed to elicit increased vigilance from receivers relative to unmanipulated calls, whereas calls with longer white noise bursts elicited increased vigilance relative to the other 2 call types. As with previous empirical demonstrations of the auditory continuity illusion in vertebrates, relatively short obfuscating noise resulted in receivers perceiving each syllable as a whole, while longer obfuscating noise caused receivers to perceive the beginning and end of each manipulated syllable as 2 distinct syllables, increasing the perceived syllable repetition rate. These results reveal that Richardson's ground squirrels experience an auditory continuity illusion allowing the extraction of the rate of syllable repetition from conspecific repeated alarm calls in the face of short duration environmental noise. Selection has acted upon receivers to refine the cognitive mechanisms underlying signal perception, facilitating the extraction of information masked by environmental noise.