The first cut is the deepest: primary syllables of Richardson's ground squirrel, Spermophilus richardsonii, repeated calls alert receivers

Richardson's ground squirrels produce alarm calls that warn conspecifics of potential predatory threat. Variation in the acoustic structure of individual syllables within a call carries information regarding the extent of threat. Given this variation, along with the production of multiple syllables within a repeated call, the syntactical structure of the call could also encode meaningful information. To test for evidence of syntax in Richardson's ground squirrel alarm communication, we presented adult and juvenile subjects with repeated alarm calls with unaltered syllable order and those same calls with syllable order randomized. There was limited evidence that syllable order affected receiver response, but squirrels were more responsive to primary syllables of repeated calls even when these syllables were embedded within randomized calls. This suggests that primary syllables serve a general alerting function, possibly priming receivers for information that follows. Distinctive structural elements of primary syllables, including reduced syllable duration and reduced harmonic durations relative to the fundamental frequency, were correlated with the heightened vigilance responses that these syllables evoked.