Only Monkeys With Opposable Thumbs Fell for This Classic Magic Trick

Only Monkeys With Opposable Thumbs Fell for This Classic Magic Trick

A team of researchers performed magic tricks for several species of monkey, and they found that the animals’ gullibility may depend on the structure of their hands.

Specifically, the team found that the monkeys were more likely to be duped by the sleight of hand if they had opposable thumbs. The research is published today in Cell Biology.

Magic tricks are a great way to test animal intelligence, perception, and cognition. The researchers used a classic trick called the French drop to test the cognition of three monkey species: yellow-breasted capuchins, squirrel monkeys, and common marmosets. The former two species have opposable thumbs, while marmosets do not.

The magic trick requires an opposable thumb, and the idea here was to test “whether having the manual capability to produce an action, such as holding an item between finger and thumb, is necessary for predicting the effects of that action in others,” said Elias Garcia-Pelegrin, a cognitive scientist at the National University of Singapore, in a University of Cambridge release.

After several performances of the trick, the research team found that the primates’ morphology determined whether they were hoodwinked, bamboozled, led astray, run amok, and flat-out deceived by the monkeyshines.

The French drop is old trick, one you may recall from birthday parties as a kid. The performer shows the audience a small object, like a coin, held with the back of their hand facing the crowd, with the fingers pinched together and pointed toward the sky. Then, the performer uses their other hand to conceal the object, and mimes taking it, but truly lets the object fall into the palm of the original hand. Then, the audience is typically asked to figure out where the object is.

The illusion leads people to believe the object has changed hands, when it hasn’t. Key to the deception is the concealment of the thumb of the second hand, hidden behind the other fingers.

For the recent experiment, the coin was replaced with a treat: peanuts for the capuchins, mealworms for the squirrel monkeys, and marshmallows for the marmosets. The capuchins and squirrel monkeys overwhelmingly fell for the trick (81% and 93% of the time, respectively), but the marmosets did not — they were only hoodwinked 6% of the time.

Only Monkeys With Opposable Thumbs Fell for This Classic Magic Trick

“This mirroring in our neural motor system might explain why the French drop worked for the capuchins and squirrel monkeys but not for marmosets,” said Nicola Clayton, a cognitive scientist at the University of Cambridge, in the same release.

To confirm that the marmosets weren’t merely more perceptive to the switch (perhaps they could smell the treat while the other species could not!), the researchers also performed a modified version of the French drop. This manoeuvre, called the “power drop,” was the same basic trick, but the treats were transferred using a grip all three species were capable of.

The power drop fooled the capuchins 81% of the time and the squirrel monkeys and the marmosets 94% of the time, suggesting that the marmosets’ ability to follow the trick was due to the positioning of the magicians’ digits.

“It’s about the embodiment of knowledge,” Clayton added. “How one’s fingers and thumbs move helps to shape the way we think, and the assumptions we make about the world – as well as what others might see, remember and anticipate, based on their expectations.”

The scientists made monkeys out of the monkeys with some simple sleight-of-hand. But don’t feel too bad for the animals: When they correctly guessed the location of the treats, they got to eat them.

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