Unraveling Primate Faces: Smiles in the Macaca tonkeana

In the world of primates, facial expressions play a huge role in their social lives. One particular expression, called bared-teeth faces (BTFs), has been fascinating to scientists studying the Macaca tonkeana species of macaques. Researchers used a mix of advanced techniques to study these expressions in detail. They found two main types of BTFs: bared-teeth (BT) and open mouth bared-teeth (OMBT). These expressions didn't always happen in specific situations, but they had some interesting patterns. For instance, BT was common during peaceful times and didn't vary much, while OMBT was very consistent during playtime, which can be unpredictable.The ways these expressions changed depended on the situation. Sometimes, the macaques stuck out their tongues during play, and other times, their foreheads wrinkled during fights. So, while BT and OMBT aren't always tied to a certain situation, they can be important signals depending on what else is happening. Future studies should look at how these expressions change in other primates and even non-primates to understand more about how social signals evolve.