Monkeys with hands that resemble ours fall for a sleight-of-hand magic trick, but those without opposable thumbs aren’t fooled.
These differences illustrate how primates’ expectations of the actions of others’ depend on their own anatomy and abilities, says Elias Garcia-Pelegrin at the National University of Singapore. His research uses magic to reveal aspects of animals’ mental capacities, such as how their perceptive systems can be deceived.
Garcia-Pelegrin, a trained magician with 12 years of experience, performed a classic illusion for three species of New World monkeys: Humboldt’s squirrel monkeys (Saimiri cassiquiarensis), common marmosets (Callithrix jacchus) and yellow-breasted capuchins (Sapajus xanthosternos).
Capuchins are the only New World monkeys that can perform a precision grip by bringing the thumb towards the index or middle finger. Squirrel monkeys have opposable thumbs, but these aren’t freely movable and they can’t grip precisely. Marmosets, meanwhile, lack any semblance of opposable thumbs, as their hands are adapted for climbing thick, vertical tree trunks.
The monkeys were trained to watch Garcia-Pelegrin handling an item of food and select which of his closed fists contained the reward. Then, he showed them the food with one hand and either passed it to the other hand or performed a trick called the French drop by pretending to grab it but keeping it in the same hand.
Both capuchins and squirrel monkeys correctly chose the hand with the reward when a real transfer took place, but they were fooled by the French drop. In contrast, the marmosets didn’t fall for the trick, but got fooled when the reward was actually transferred to the other hand.
Garcia-Pelegrin and his colleagues conclude that since the capuchins and the squirrel monkeys were familiar with the conjurer’s anatomy, they predicted his hand movements and were fooled as a result. “This exemplifies how our internal biases about movements can be sometimes misleading, and we seem to share this with other primates as well,” says Garcia-Pelegrin. “This is what magicians seem to capitalise on by exhibiting these cues but changing the movement’s outcome. Magicians can trick us, and it appears that monkeys can be fooled as well.”
“The most important takeaway from the research is that hand movements are highly critical as social cues. Spectators model other individuals’ hand movements precisely down to the very way that the spectators’ own hands might move in the same circumstance,” says Stephen Macknik at SUNY Downstate Health Sciences University in New York.