Monkey see, monkey do? Sleight of hand only fools monkeys with opposable thumbs

When performing magic tricks, it’s important to choose your audience carefully.

And according to a new study, it would be wise not to try sleight of hand with a group of marmosets.

Researchers have discovered deceptive tricks that only fool monkeys with opposable thumbs, since a magician must have an anatomy similar to that of their audience.

So while capuchin and squirrel monkeys are fooled by “tricky hands,” their research shows that species without opposable thumbs aren’t as easily fooled.

Psychologists from the University of Cambridge performed a famous magic trick on three species of monkeys with different hand structures.

They used a trick maneuver called the French Drop, in which an object appears to disappear when a spectator assumes it is being taken from one hand with the other hand’s tucked thumb.

The trick — often the first thing a budding magician tries to master — is to hold a coin with two fingers of one hand.

The other hand reaches over with the palm facing the magician and the thumb tucked behind her fingers.

The audience knows the thumb is lurking, ready to grab, and so assumes the coin has been taken when the hands part and it is no longer visible.

Your attention follows the seconds hand, only to find it blank as it “reveals”. In fact, the magician secretly dropped the coin into the palm of the original hand.

The researchers performed the trick on 24 monkeys, including capuchins and marmosets, which both have opposite thumbs but with different dexterity, and marmosets, who do not have opposite thumbs.

Bites of food replaced coins and were given out as a reward – but only if the animals guessed the right hand.

The analysis found that monkeys with opposable thumbs were fooled more than 80 percent of the time — much like human audiences — into assuming the hidden thumb grabbed the treat and chose the wrong hand.

Meanwhile, the monkeys without an opposable thumb were fooled only 6 percent of the time.

dr Elias Garcia-Pelegrin, one of the study’s authors, said: “Magicians use intricate techniques to make the observer experience the impossible.

“By examining how primate species experience magic, we can understand more about the evolutionary roots of cognitive defects that expose us to the wiles of magicians.”

The team wrote in the journal Current Biology that their research suggests that sharing a biomechanical ability might be necessary to accurately predict the movements of the same limbs in other people.

Lead author Professor Nicola Clayton said: “There is growing evidence that the same parts of the nervous system that we use when we perform an action are also activated when we observe that action being performed by others.

“This mirroring in our neural motor system could explain why the French drop worked in capuchins and squirrel monkeys, but not in marmosets.”