Researchers performed a famous magic trick for three species of monkey with differing hand structures and discovered that they could only deceive monkeys with a similar hands to humans. The psychologists used a sleight-of-hand trick called the French drop, in which an object appears to vanish when a spectator assumes it is taken from one hand by the hidden thumb of the other hand.
The study, carried out at the University of Cambridge’s Comparative Cognition Lab, found that monkeys lacking opposable thumbs did not fall for the trick staying wise to the whereabouts of tasty treats a magician tried to make disappear. The research suggests that sharing similar limb structures may be necessary for accurately anticipating the movements of those same limbs in other individuals.
Eight capuchins were dazzled with peanuts, eight squirrel monkeys with dried mealworms, and eight marmosets with marshmallows. The capuchins, who like humans have opposable thumbs, were regularly fooled by the French drop (81% of the time). Squirrel monkeys are much less dextrous than capuchins, with limited thumb rotation, but can oppose their thumbs. They were also fooled - 93% of the time. Marmosets do not have opposable thumbs and rarely taken in by magic (just 6% of the time). They simply chose the hand in which the marshmallow was initially placed, and stuck with it.