Ultimate causes of antipredator vocalizations in a nonhibernating squirrel

Communication plays an integral role in facilitating intra- and interspecific interactions. The study of signal function and content reveals the rules that govern such interactions, informing hypotheses in behavioural ecology and evolution. The ubiquitous nature of antipredator vocalizations in ground squirrels provides a useful model for studying the evolution of communication. Conspecifics in many ground squirrel species respond to antipredator vocalizations, and sociality functions as a strong selective force favouring more informative antipredator vocalizations. However, studies of a single antipredator vocalization system in both social and nonsocial contexts are relatively scarce, preventing diagnosis of selective forces other than sociality. We conducted a 2-year study to test two alternative hypotheses relating to the function of antipredator vocalizations in a nonhibernating squirrel, the Harris’ antelope squirrel, Ammospermophilus harrisii. We hypothesized that if vocalizations function as a predator deterrent, callers should be of equal sex ratio and vocalize year-round. If vocalizations function primarily as a warning to offspring, callers should be predominantly female and vocalize only when juveniles are above ground. We found that spontaneous callers were predominantly female but vocalized throughout the year. We also found that call bouts varied in trill number, which could hold additional layers of information. Our results suggest that antipredator vocalizations function as both a predator deterrent and a warning to offspring. Antipredator vocalizations with multiple functions or receivers are subject to a greater compilation of selective forces that may induce communicative complexity to arise.