Mechanisms and constraints underlying acoustic variation in rodents

Like passerines among birds, rodents among mammals have experienced an enormous radiation. Similar to songbirds, this radiation is associated with the evolution of a critical innovation in how vocal signals are produced. Recent studies on the anatomical and physiological bases of vocal production provide an opportunity to understand how proximate mechanisms shape acoustic variation to influence signal function and evolution. Acoustic variation ranges from subtle changes within individuals in different social contexts to extreme divergence among closely related species. Despite increased appreciation of this modality, relatively few studies have assessed variability of acoustic features within individuals over time and/or the consequences of such variation. Moreover, our understanding of the mechanisms that promote and constrain acoustic variation is limited. Herein, we review ontogenetic, social and endocrine factors associated with intraspecific acoustic signal variation in murid and cricetid rodents, summarize the functional consequences of such variation and describe morphological and physiological adaptations underlying vocal displays. We then suggest a framework for developing mutually informative mechanistic and functional studies to explore the evolution of signal elaboration. Clarifying the causes and consequences of interspecific and intraspecific acoustic variation is critical to understanding the evolution and function of vocalizations in the largest radiation of mammals.