When it comes to monkeys, all bark may signal all bite, too. A recent study found that male gelada monkeys, close relatives to baboons, assess the loud, long-range calls of rivals to determine their fighting strength relative to their own.

A team of researchers from Georgia State University, the University of Michigan, and Princeton University observed geladas in their natural habitat and studied their reactions to calls from potential challengers as well as during playback experiments using recordings. Male geladas either have to defend their “harem” of females or fight for one of their own. The “leader” males belt out vocal displays to show their strength and deter attacks from “bachelor” males trying to take over their positions.

The authors played various calls to 60 geladas — 20 “leader” males, 20 bachelor males, and 20 females — and  measured for various responses, such as how long a monkey stopped to look after hearing the call, and how long before it resumed its activities.

They found that gelada males responded to loud calls of different quality based on how strong they view themselves and their opponent. The monkeys analyzed a challenger’s fighting ability by assessing the vocal acoustic quality in their calls. In other words, if a rival can sing, he can fight.

“Previous studies in wild primates have shown that they use mutual assessment, but this was between animals that knew one other,” explains Jacinta Beehner, an associate professor of psychology and anthropology, in a university release. “They see Kevin and they remember that they beat him in a previous fight. The novelty of our finding is that we have shown that primates can do this even for completely unfamiliar individuals–using signals.”

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