Manual action expectation and biomechanical ability in three species of New World monkey

Being able to anticipate another's actions is a crucial ability for social animals because it allows for coordi-nated reactions. However, little is known regarding how hand morphology and biomechanical ability influ-ences such predictions. Sleight of hand magic capitalizes on the observer's expectations of specific manual movements,1,2 making it an optimal model to investigate the intersection between the ability to manually pro-duce an action and the ability to predict the actions of others. The French drop effect involves mimicking a hand-to-hand object transfer by pantomiming a partially occluded precision grip. Therefore, to be misled by it, the observer ought to infer the opposing movement of the magician's thumb.3 Here, we report how three species of platyrrhine with inherently distinct biomechanical ability4-6-common marmosets (Callithrix jac-chus), Humboldt's squirrel monkeys (Saimiri cassiquiarensis), and yellow-breasted capuchins (Sapajus xan-thosternos)-experienced this effect. Additionally, we included an adapted version of the trick using a grip that all primates can perform (power grip), thus removing the opposing thumb as the causal agent of the ef-fect. When observing the French drop, only the species with full or partial opposable thumbs were misled by it, just like humans. Conversely, the adapted version of the trick misled all three monkey species, regardless of their manual anatomy. The results provide evidence of a strong interaction between the physical ability to approximate a manual movement and the predictions primates make when observing the actions of others, highlighting the importance of physical factors in shaping the perception of actions.