Production of Loud and Quiet Calls in Howler Monkeys

Abstract

One of the most striking features of howler monkeys’ natural history is their loud call, which gives the genus Alouatta its common name in English. However, the disproportionate focus on functional aspects of those calls has driven attention away from other relevant issues related to their vocal behavior. In this chapter, we review the studies of acoustic structure conducted so far on these peculiar calls, highlighting the variation among and within the species of this genus. The variation we uncover runs against the notion of uniformity among howler monkeys, but we do find that the relationship between loud call structure and phylogeny compliments genetic work in this genus. We also show how the anatomy of howler monkey’s vocal organs can explain the unusual features of their loud calls and possibly the variation found between species, while also pointing to the various gaps that exist in our knowledge regarding the role of the several components of their highly specialized vocal apparatus. Additionally, we review some basic concepts about sound propagation and geographic variation in long-distance communication. Unlike loud calls, we know relatively little about the low-amplitude calls of howler monkeys. Such calls have received a great deal of attention in the literature, particularly in Old World monkeys, because they can offer insights into the social lives of these animals. Because few comparable studies have been conducted on howler monkeys, we propose some lines of future research that we deemed potentially interesting. We conclude with some methodological approaches to recording howler monkey calls in the field and for sharing vocalizations with other researchers.