Vocalizations, facial expressions, body postures, and other types of movement are ways that primates communicate with others. Communication is key to living in large social groups in which conflicts arise that require prevention, mitigation, or de-escalation. Being particularly social, as well as hierarchical, macaques have developed many nuanced ways by which they actively communicate with one another. While many similarities exist, such behaviors may vary between species—possibly even between groups of the same species! No doubt have booted macaques, too, evolved these sorts of behaviors. However, the specific ways in which they do so are not well-studied.

The communicative behaviors of other Sulawesi macaque species in general are better recorded and can provide insight into how booted macaques might communicate. Altogether, the social repertoire of Sulawesi macaques is known to be especially complex compared to other macaque species, consisting of almost 150 vocalizations, facial expressions, body postures, and other significant movements or behaviors. While only a few of these have been observed in booted macaques, comparisons of several other better-studied Sulawesi species suggests that all Sulawesi species are likely to have repertoires that are more similar than they are different.

Where their ranges overlap, groups of booted macaques are known to mingle with groups of Tonkean macaques. Tonkean macaques are one of the better-studied species of Sulawesi macaques and are known to make a wide range of vocalizations, which individuals use to consciously communicate their inner emotional states and share other important information. For instance, a mother interacting lovingly with her infant might makes a series of loud and expressive calls that serve to reinforce and nurture their bond. During episodes of aggression, such as intense staring or chasing, a male might form an “O” with his lips and let out a loud bark-like sound in an attempt to intimidate his opponent.

A Tonkean macaque also uses behavior that notifies others of her wants or needs. After a rest, a macaque might get up from her spot and move away from the group in the direction of the foraging spot she hopes to visit next. She looks back to see who will follow her. Those that gather around her communicate that they also wish to go to that foraging spot next. But another in the group might suggest a different direction, and others join his party. In the end, the group with the most individuals “wins,” and they set off in the respective direction. Researchers have determined that the macaques are not influenced by members of their immediate family, suggesting that this process is, in fact, as democratic as it seems.

A consistent facial expression used by all macaque species, including the booted macaque, is what researchers dub the “silent bared teeth” expression. Theorized by some to be a sort of evolutionary prototype of the human smile, a macaque who bares his teeth is one who comes in peace. By announcing that he means others no harm, he assuages any potential tensions and creates opportunity for more intimate interactions to take place. The expression is also in popular use among macaque infants, who use it to signal when they are playing.

Perhaps the most important take-away from Sulawesi macaque social repertoire is how so many behaviors are affiliative and not aggressive. Clearly, these macaques have vested interests in keeping the peace.