A survey of the context and structure of high-and low-amplitude calls in mammals

Quantifying how animal vocalizations vary is central to understanding their function and evolution. One commonly documented feature of mammalian calls is the systematic variation in amplitude across call types. Despite a large body of existing data, there has been little attempt to assess how common calls of varying amplitude are in mammals, how broadly they are distributed at the taxonomic level, and whether similarities in context and structure across species can inform our understanding of the selective pressures promoting the evolution of amplitude variation. Here, we perform a comparative survey of amplitude variation in adult vocal repertoires from 47 species belonging to nine mammalian orders. Our data set demonstrates that low-, medium- and high-amplitude calls are not restricted to certain mammalian groups but occur widely across taxa. Furthermore, contextual analyses indicate that there are consistent differences in the contexts that accompany low-, medium- and high-amplitude calls. Specifically, we found that high-amplitude calls are reported to occur more often in agonistic and alarm-related contexts and less often in affiliative social contexts compared to low- and medium-amplitude calls. In addition, acoustic comparisons indicate that calls of varying amplitude are divergent in terms of underlying call structure. Our findings suggest that low-amplitude calls are shorter in duration and lower in frequency than medium- and high-amplitude calls. We compare and contrast our findings with similar recent approaches investigating amplitude variation in birds and discuss the implications our findings have for unpacking the adaptive significance of amplitude variation in animals more generally