Function of Loud Calls in Howler Monkeys

Abstract

Beyond the unique sound of howler monkey vocalizations, their vigorous loud calling displays are perplexing given the otherwise sedentary lifestyle of these primates. Here we provide potential explanations for this energetic investment by reviewing all available functional studies conducted to date. We highlight the variation among and even within species when we explore whether male loud calls are used in group cohesion, predator avoidance, attraction of females, or competition with other males or other groups over resources. In the competition scenario, we examine strategies of avoidance versus direct competition and whether contests are focused on defense of space, food, mates, or infants. We suggest that much of the debate surrounding the function of loud calls stems from methodological differences among researchers and from the varied levels of analyses used, although we also demonstrate that studies of form and function can be intertwined. We emphasize the need to examine different call types separately and discuss the role of howling in intragroup male relationships. Finally, we address the understudied role of female loud calling and the potential use of hybrid populations to examine the evolution of species-typical loud calls. We conclude with some practical hints for designing field tests to uncover functional significance.