Why is that monkey making that weird noise? The function of boom calls in blue monkeys.

Communication is a key part of life. It is what allows us, and many other species, to function successfully as social animals. A world without communication is a world of quiet isolation. This is why it is so important to us to understand how communication works—in our own human world but also outside of it. Why do different species communicate the way they do? Animals communicate using various methods of getting across information—bursts of noise, body movements, emission of smells. These are all examples of different kinds of signals—and scientists have long been interested in knowing more about these signals. What do they mean? When are certain signals used over others? Who are they meant for? Why did the signals evolve they way they did? 

A recent study in Ethology answers several of these questions concerning a loud boom call made by adult male blue monkeys. This call is loud, tonal, and low frequency. What is the function of the boom call? And knowing that, how might we understand more about why this boom call evolved? This specific understanding could then be extrapolated to fit broader trends across species. 

Before beginning their observations, the researchers considered several hypotheses explaining the function of the boom call. Perhaps the boom was a mating call, attracting females to the caller? Or acted as warning, deterring rival males? Other possibilities were that the boom call alerted fellow monkeys of predation risk, or facilitated general social cohesion. The researchers used two strategies to explore these questions—looking at the context the calls were made in, and observing the behavioral responses of other blue monkeys. 

Most of the data collected in this study was purely observational—researchers observed blue monkey groups in the Kakamega rainforest in Kenya, documenting contextual and behavioral information surrounding the boom calls for a year. In addition to the observation, researchers also introduced simulated boom calls, measuring the response of males to this intruder boom. They also simulated the presence of predators, using audio and visual stimulus, testing to find a correlation between predators and the boom. 

At the conclusion of this observational and experimental period, the researchers found that males produced significantly more boom calls during the mating season, suggesting a potential mating function of the call. Supporting this, the booms occurred more frequently during interactions between the caller and a female, especially after the female produced her own mating calls. The behavioral responses of the females matched this finding—females spent more time with the boom caller, and were more likely to mate with him. So it seems like the boom call is helping the male monkeys attract mates by directly communicating with the female herself! 

Boom calls may have a second function as well, a way for the calling males to ensure their own mating success while maintaining group cohesion. Other males, after hearing the boom call, moved away from the caller and did not compete for the female. 

The booms had no evidence of predator alarm call correlation. So returning to the original set of hypotheses proposed, data matched all but the predator hypothesis, with the two hypotheses relating to mating most strongly supported. The boom calls appear to have evolved in the male monkeys as a strategy to attract mates while simultaneously ensuring a lack of competition. This indicates that the call is highly complex—targeting multiple audiences and sending a different message to each one! 

At the end of the study, many unanswered questions remained. Booms also appeared correlated to falling branches, and were audible over much longer distances than the observed responses required. The function of these components remains unknown. There is still so much we don’t understand about communication in the animal kingdom. The more that we learn, the more we realize that incredibly complex social structures exist outside of our human bubble. Perhaps by understanding more about the function of these interactions, we can simultaneously improve upon valuing these animal communities as much as we value our own. 


Fuller JL, Cords M. Versatility in a loud call: Dual affiliative and agonistic functions in the blue monkey boom. Ethology. 2019;00:1–14.

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