Functional morphology of Richardson's ground squirrel, Spermophilus richardsonii, alarm calls: the meaning of chirps, whistles and chucks

Repetitive alarm vocalizations of Richardson's ground squirrels vary in terms of the acoustic structure of their primary syllables and the inclusion of brief, lower-amplitude, frequency-modulated trailing elements we term 'chucks.' Chucks are included in calls of both males and females and increase in prevalence with the proximity of the caller to the alarm-evoking stimulus. Furthermore, chuck presence is not independent of primary syllable type: chucks follow primary syllables that have constant frequency and diminishing amplitude ('whistle'), but do not trail primary syllables with diminishing frequency and nondescending amplitude spectra ('chirps'). Playbacks to free-living squirrels of repeated alarm calls having whistle- or chirp-like primary syllables factorially combined with chuck presence or absence revealed that chirp-like syllables elicited greater vigilance from call recipients during signal propagation. The addition of chucks to the end of primary syllables of either type, however, increased initial vigilance duration and the proportion of time devoted to vigilance during and after signal reception. Chucks thus promote increased and lasting vigilance on the part of call recipients. Beyond enhancing vigilance, however, the inclusion of frequency-modulated chucks and chirps facilitates the orientation of receivers to the signaller. Multiple acoustic parameters of Richardson's ground squirrel alarm vocalizations thus interact to communicate information regarding several aspects of a predator encounter. Receivers use this information to their advantage, affording greater attention to calls that would be more readily located by predators, and hence are more costly for signallers to produce.