Title: Diversity of Form, Content, and Function in the Vocal Signals of Adult Male Blue Monkeys (Cercopithecus mitis stuhlmanni)
Description: In species across virtually every vertebrate taxonomic division, vocal signals play key roles in predator avoidance, reproduction, competition, and mediating social interactions. Understanding signaling systems, and the various selection factors relating to their evolution and maintenance, therefore provides unique insight into species' behavior, social dynamics, and evolution. Decades of research has greatly improved knowledge of animal signals and how they are used, yet understanding of the mechanisms by which entire communication systems operate and evolve remains incomplete. The research presented in this dissertation examined the vocal repertoire of adult male blue monkeys (Cercopithecus mitis stuhlmanni). Specifically, I examined three elements of vocal signals - acoustic structure, signal content, and adaptive function - across the entire male repertoire, and used results to infer mechanisms of selection on signal usage and divergence. During 12 months of fieldwork in the Kakamega Forest, Kenya, assisted by a team of trained research assistants, I used a combination of natural observation, playback experiments, and digital audio recordings to examine vocal behavior of 32 adult males and responses to their calls by males and 62 adult females from 12 social groups and the surrounding area.

Analyses of digital recordings identified six distinct call types used by adult males: ant, boom, ka, katrain, nasal scream, and pyow. The repertoire is best described as discrete, though some gradation occurs between pyows and ants. To identify signal content - attributes of signalers reliably indicated by features of signals - I investigated each call types' relationship to callers' identity, social status, body size, and attention to external variables (e.g. predators). Results showed that at least three call types (boom, katrain, pyow) were reliable indicators of identity, and features of at least one call type (pyow) were correlated with body size. Resident males used all call types whereas "bachelors" used only nasal screams, indicating social status is content in all calls except nasal screams. Two calls (ka, katrain) were strongly associated with and essentially exclusive to aerial predators, and ants had a similar relationship to terrestrial predators. The pyow and boom were each associated with multiple external variables, demonstrating that these two calls do not include any specific external stimulus in content.

Lastly, the content of nasal screams, used exclusively during aggression with other males, included presence of another male. I tested four separate, non-exclusive functional hypotheses for each call type, using predictions relating to receiver response to hearing calls, as well as variation in temporal, demographic, and contextual patterns of usage. The ka, katrain, and ant each clearly functions in predator avoidance, with the first two relating specifically to aerial predators and the latter specifically to terrestrial threats such as snakes and dogs. Notably, the katrain also caused rival males to move away from callers, consistent with a mate defense function. The pyow, best described as a general alerting signal, demonstrated a clear role in repelling rival males, yet also functioned in facilitating within-group cohesion. The boom showed a clear role in affiliative interactions between callers and females in their groups, possibly functioning as a signal of benign intent, and was the only call type associated with proceptive interactions and an increase in number of estrous females, indicating a function in mating. Like pyows and katrains, booms also have a secondary function of repelling rival males.