Symbolic snacks

Monkeys learn to deal with arbitrary tokens as if they are different foods

Don’t write off capuchin monkeys as simpleminded. It turns out that they’re symbol-minded, wielding a mental capacity often regarded as unique to people, researchers say.

CLEVER CAPUCHINS Capuchin monkeys, like this one lounging in a tree, demonstrate a capacity for symbolic thought in a new study. Elisabetta Visalberghi
FRUIT TRADER A male capuchin monkey named Robot exchanges a plastic token for a piece of dried apricot during an experiment tracking his and four other monkeys’ ability to think symbolically. Elisabetta Visalberghi

Capuchins given laboratory training treat arbitrary tokens as symbols for different foods, according to primatologist Elsa Addessi of the Institute of Cognitive Sciences and Technologies in Rome and her colleagues. The animals then spontaneously choose certain tokens over others in ways that correspond to how they choose certain foods over others. Thus, monkeys typically pick a favored token or food over a less-desirable token or food. But that decision gets reversed if a less-desirable token or food is offered in an amount large enough to make its selection worthwhile.

“Capuchins display a rudimentary form of symbolic reasoning, but they are still far away from reaching the complexity of symbolic reasoning that characterizes humans,” Addessi says.

Symbolic thinking involves the use of an object to represent something other than itself. Armed with this mental capacity, humans developed spoken language and writing systems.

The new evidence, published online June 11 in PLoS ONE, illuminates a basic form of symbolic thought in capuchins. The South American monkeys diverged from primate ancestors of modern humans about 35 million years ago.

Addessi’s team first determined five captive-born capuchins’ preferences for pairs of food items. The monkeys showed a classic form of rational decision making when dealing with three actual edibles — if they chose food A over food B and food B over food C, then they also favored food A over food C. One animal, named Carlotta, favored Cheerios over Parmesan cheese, Parmesan cheese over sunflower seeds and Cheerios over sunflower seeds.

Like the other monkeys, Carlotta weighed quality against quantity when choosing a treat. If offered a choice between one Cheerio and five sunflower seeds, she opted for the sunflower seeds even though Cheerios tasted better to her.

The monkeys then underwent two to three weeks of training during which they learned to exchange distinctive tokens for each of the three foods that they had ranked from most- to least-liked. After training, Carlotta knew to give an experimenter a green poker chip to receive a Cheerio, a black plastic tube to obtain a piece of Parmesan cheese and a brass hook to collect a sunflower seed.

In a final experiment, the capuchins chose from pairs of tokens that represented different foods. Each token appeared in varying amounts. As with the foods, monkeys almost always chose token A over token B, token B over token C and token A over token C. Token quantity could override quality, but compared to corresponding foods, it required more non-preferred tokens to convince animals to bypass a favored token.

As a result, Carlotta reversed one of her food choices when using tokens. She selected a green poker chip — representing a single Cheerio — over five brass hooks, corresponding to five sunflower seeds.

The researchers suspect that capuchins grasp the symbolic meaning of tokens with difficulty, necessitating the use of more less-desirable tokens to override preferences for favored tokens. Other investigators have found that children also struggle with basic symbolic tasks until age 2 or 3.

Attempts to demonstrate symbolic understanding by nonhuman animals have generated controversy for 30 years. Chimpanzees have learned to press arbitrary symbols on a key pad to ask for various foods and tools. Monkeys have learned to trade tokens for desirable pieces of food. Still, such behavior might reflect simple conditioning, akin to a pigeon learning to push a lever to obtain food.

Psychologist Daniel J. Povinelli of the University of Louisiana at Lafayette argues that a sharp division exists between the symbolic capacities of people and nonhuman animals. In his view, no evidence suggests that apes, monkeys or any other creatures possess “higher-order” human thought, which includes reasoning with analogies and forming intuitive theories about unseen causal forces at work in the world.

Mental disparities between people and other animals are not as wide as is often thought, counters psychologist Sarah Brosnan of GeorgiaStateUniversity in Atlanta. If New World monkeys such as capuchins can reason symbolically, as shown by Addessi’s group, then many other animals — primates and non-primates alike — may share that ability, Brosnan says.

“Based on this new study and previous ones, it is quite plausible that capuchin monkeys will be shown to have even greater symbolic capabilities,” she remarks.

Watch a capuchin monkey make the symbolic link between food and tokens in the following two videos:

Monkeys use symbols to trade for food from Science News on Vimeo.

Carlotta, a capuchin monkey, can opt for one Cheerio (food A, on the left) or three pieces of parmesan cheese (food B, on the right). Carlotta goes for the three pieces of cheese, even though she prefers Cheerios. Video courtesy of Elisabetta Visalberghi

Grasping symbols from Science News on Vimeo.

Carlotta has a choice between one green poker chip (token A, corresponding to one Cheerio, on the left) and three brass hooks (token C, corresponding to one sunflower seed each, on the right). She knows that three brass hooks correspond to three of her least favorite food, so she chooses the green poker chip. Video courtesy of Elisabetta Visalberghi

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

More Stories from Science News on Humans

From the Nature Index

Paid Content