Multiple functions and signal concordance of the pyow loud call of blue monkeys

Abstract

Signals that occur in multiple contexts and evoke varied responses from receivers underscore the challenges of assessing a signal’s function (adaptive benefit to signaler) or relationships between receivers’ responses and their own fitness. Our study investigated the “pyow” loud call of male blue monkeys in a wild population in western Kenya. Using natural observation and playback experiments, we characterized pyow function based on behavioral responses by adult male and female receivers, and contextual, seasonal, and demographic variation in call usage. To identify associations between call features and traits of callers (signal concordance), we examined pyows’ usage patterns and acoustic structure in relation to attributes of callers. Pyows occurred in association with numerous stimuli and contexts, including predators, other males, intergroup aggression, and often “spontaneously,” and were more frequent in the mating season. Receivers consistently oriented toward calls and increased scanning, but other responses differed by sex—females moved toward and spent more time near the caller and groupmates, whereas males retreated. Pyows exhibited concordance with identity and social status, but were not associated with callers’ attention to any particular external entity. Results indicate pyows achieve multiple functions, including mate defense, group cohesion, and mate attraction. The call’s functional versatility appears best explained by its ability to draw attention and that receivers’ subsequent behavior is consistent with their own fitness priorities relevant to the caller’s identity, status, and context of calling. To understand how selection favors a signal, it is important to acknowledge all the ways the signal is used, responses by multiple receivers, and that a signal’s fitness relevance might be different for different receivers. Our investigation of blue monkey pyows used natural observations and experiments to characterize variation in how males use the call and responses by male and female receivers. We also identified relationships between pyow features and attributes of callers—proposing the term “signal concordance”—as an objective basis for exploring receivers’ responses in relation to their own fitness. Our results demonstrate that pyows achieve multiple functions by evoking varying responses from receivers. We suggest that variation in responses relates to the pyow’s multiple concordance elements. Our study demonstrates how “generalist” signals can achieve specific outcomes and highlights the additive influence of multiple selection factors.