The evolution of vocal alarm communication in rodents

On encountering a predator, many species emit potentially risky vocalizations known as alarm calls. We evaluated the relative importance of two adaptive hypotheses on the evolution of calling: (1) communicating to predators, which may function by deterring pursuit and hence increasing individual survival, and (2) an alternative nepotistic hypothesis for alarm calling whereby callers obtain direct and indirect fitness by warning relatives. Focusing on 209 species of rodents, we found significant associations between diurnality and alarm calling, living socially and alarm calling, and diurnality and sociality. Diurnality, however, accounted for nearly three times as much variation in whether or not a species alarm called than did sociality. Phylogenetic tests revealed that the evolution of diurnality preceded the evolution of alarm calling, and that the evolutions of diurnality and sociality were unrelated. Our results are consistent with the hypothesis that alarm communication evolved to communicate to predators. If so, then nepotistic benefits, although important for the maintenance of alarm calling in some rodents, may be relatively less important in its evolution.