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Certain magic tricks only fool monkeys with opposable thumbs 

Magicians and their ability to deceive have always been a subject of fascination for psychologists. In a recent study, researchers from the University of Cambridge’s Comparative Cognition Lab used a classic magic trick to investigate the link between deception and anatomy in monkeys. The research suggests that to deceive effectively, a conjuror needs a similar anatomy to their audience.

The researchers 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 experiment involved three species of monkeys: capuchins, squirrel monkeys, and marmosets, each with differing hand structures.

Capuchins, known for their dexterity, use stone tools to crack nuts in the wild. They have opposable thumbs and can waggle each finger, allowing them to perform a “precision grip” between thumb and forefingers. In the experiment, capuchins were repeatedly fooled by the French drop, selecting the empty hand 81% of the time.

Squirrel monkeys, on the other hand, are much less dexterous than capuchins, with limited thumb rotation but still familiar with a hidden thumb interacting with fingers. They can oppose their thumbs but cannot perform a “precision grip” in the same way as capuchins and humans. Yet, squirrel monkeys were routinely misled by mealworms that seemingly vanished (93% of the time).

Marmosets, the third species tested, do not have opposable thumbs. Their thumbs align with their fingers to make five equidistant digits, ideal for climbing thick tree trunks. In the experiment, marmosets were rarely deceived by the French drop (just 6% of the time). They simply chose the hand in which the marshmallow was initially placed, and stuck with it.

The results suggest that sharing a biomechanical ability may be necessary for accurately anticipating the movements of those same limbs in other individuals, even when those apparent accurate predictions end in befuddlement at the hands of an illusionist.

“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 Dr. Elias Garcia-Pelegrin, the lead researcher of the study.

Dr. Garcia-Pelegrin, who has practiced magic for a decade, explained that studying how species of primates experience magic can help understand the evolutionary roots of cognitive shortcomings that leave us exposed to the cunning of magicians.

“Magicians use intricate techniques to mislead the observer into experiencing the impossible. It is a great way to study blind spots in attention and perception,” he said.

According to the Cambridge team’s previous research, birds from the corvid family, specifically Eurasian jays, make similar choices to marmosets when presented with the French drop, despite having no hands at all.

To test the effectiveness of misdirection, the team conducted an experiment where they completed the hand-to-hand transfers instead of using the French drop. This time, capuchins and squirrel monkeys correctly anticipated the food’s location, while marmosets missed out.

The scientists created their version of the French drop, called the “power drop,” using a hand action that all monkey species could perform. The power drop successfully deceived the vast majority of monkey species.

Professor Nicola Clayton FRS, the study’s senior author from Cambridge’s Department of Psychology, said that there is mounting evidence that the same neural motor system is activated when we watch someone else perform an action as when we perform it ourselves. This “mirroring” may explain why the French drop worked for some monkeys and not others.

Clayton added that the way we move our fingers and thumbs shapes our thinking, assumptions, and the expectations of others. The research suggests that an individual’s physical capabilities heavily influence their perception, memory, and ability to predict others’ manual movements.

Clive Wilkins, a professional magician and member of the Magic Circle, is an author of the study and an Artist in Residence at Cambridge’s Department of Psychology.

The study provides insight into the cognitive mechanisms underlying the ability to deceive and anticipate the actions of others. The research could also have implications for the field of robotics, where the design of machines that interact with humans needs to take into account the biomechanical abilities of their users. The study was published in the journal Current Biology.

Opposable thumbs are a unique trait that allows some animals to grasp and manipulate objects with a higher degree of dexterity and precision. Here are some animals that have opposable thumbs:

  1. Primates: Primates such as humans, apes, and monkeys have opposable thumbs. This allows them to perform complex tasks such as tool-making, object manipulation, and grasping.
  2. Koalas: Koalas have opposable thumbs on their front paws that help them climb trees and grasp eucalyptus leaves, which are their primary food source.
  3. Pandas: Giant pandas have a “false thumb” made of an extension of their wrist bone that functions like an opposable thumb, allowing them to hold bamboo stalks and strip the leaves off.
  4. Opossums: Opossums have opposable thumbs on their hind feet that help them climb trees and hold onto branches while they sleep.

The ability to grasp and manipulate objects with opposable thumbs has a significant impact on these animals’ lives. For example, primates use their opposable thumbs to build nests, gather food, and use tools to solve problems. Koalas use their thumbs to climb trees and grasp their food, and pandas use their “false thumbs” to strip bamboo leaves, which are an essential part of their diet. Opossums use their opposable thumbs to climb trees and avoid predators. In short, opposable thumbs are a vital adaptation that has allowed these animals to thrive in their respective habitats.


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