Thursday, March 12, 2009

Almonds or pizza? Capuchins are even smarter than we thought

White-faced capuchin eating fruit at
Manuel Antonio Parque Nacional, Costa Rica
Photo by Sally Kneidel

I'm thinking about monkeys again tonight. (See my last post about capuchins.) Since that last post, I've read another Science News article about capuchins; this one says they've been shown to use "symbolic thought." At one time, symbolic thought was believed to be a unique and defining attribute of humans. But now capuchins have joined the growing ranks of animals that can think symbolically. Specifically, capuchins have been shown to recognize tokens as symbols for different kids of foods. I was thrilled to see that bit of information, but not too surprised. Capuchins are smart, and I know a little about the power of tokens.

My freshman year at Roanoke College, I had an internship with a clever psychiatrist who worked at a nearby VA hospital. He too experimented with tokens as symbols. He had devised a "token economy" program to motivate severely ill psychiatric patients who couldn't behave acceptably in the hospital. If the patients managed an entire day without eating cigarette butts or smearing feces on the wall or attacking somebody, etc, then they were rewarded with tokens. They could trade the tokens for valued prizes, such as candy bars, or TV time, or even a visit with family off the hospital grounds. The tokens worked well. Even the most troubled of the patients got the idea, and took advantage of it. Violent and noxious misbehavior plummeted on the token-economy ward. My mentor published paper after paper about the success of his model.

So what's the deal with monkeys and tokens? In behavioral experiments, capuchins have learned to regard arbitrary tokens as symbols for different foods, according to primatologist Elsa Addessi of the Institute of Cognitive Sciences and Technologies in Rome. These capuchins then spontaneously choose certain tokens over others in ways that correspond to how they choose particular foods over others. The exception occurs when a less preferred token or food is offered in big quantities, which increases its attraction. Yes, we can all relate to that. One toasted almond versus a whole pesto pizza? Easy.

Adessi's team began by establishing a preference ranking for 3 foods among the experimental capuchins. The foods were Cheerios, Parmesan cheese, and sunflower seeds. The monkeys then learned to exchange distinctive tokens for each of the three foods. For example, the capuchin "Carlotta" knew to give the experimenter a green poker chip to receive a Cheerio, a black plastic tube to get a piece of Parmesan cheese, and a brass hook to obtain a sunflower seed.

In the final experiment, the capuchins were allowed to choose from among the tokens that they had learned to associate with particular foods. They almost always chose the tokens that represented the foods they preferred, demonstrating that they understood that the tokens symbolized the food items.

The experiment, published online June 11 in PLoS ONE, shows that capuchins have at least a rudimentary capacity for symbolic thinking, agree the researchers. Symbolic thinking involves the use of an object to represent something other than itself. The finding is significant to evolutionary biology because these South American monkeys diverged from primate ancestors of modern humans about 35 million years ago - which suggests that symbolic thinking has been around for a lot longer than we thought. After all, Homo sapiens has existed as a distinct species for only about 200,000 years. Symbolic thinking is pretty significant in our own development. Humans have used symbolic thinking to develop spoken and written languages, among other things.

Experimenters have been trying to demonstrate symbolic learning in nonhumans for 30 years. Chimps long ago learned to press artibrary symbols on a key pad to ask for various foods and tools. When I was in graduate school at the University of Oklahoma, I worked under the guidance of Dr. Roger Fouts, teaching American Sign Language to chimps and orangutans, which of course involves complex use of symbols.

me at 22, teaching ASL to Pancho the chimp, at University of Oklahoma's Institute for Primate Studies

me with Foots the orangutan, another ASL pupil at OU
Align Center
The chimps taught ASL to their own youngsters, used it among themselves, and used signs to ask us for things they wanted, like a snack, a drink, a ball, or a "tickle." There's no question that apes can understand and teach others the use of symbols. African grey parrots have also been shown to use symbols. If capuchins can reason symbolically too, then many other animals - primates, birds, and others - may also be able to. As I've said before, we're not as unique as some of us like to think. The more we learn about our furred and feathered relatives, the more blurred the distinctions become. Which, for me, makes our monopoly over the world's resources even more untenable and distressing! When I read about research like this, I pine for solutions to the habitat loss and climate change that threaten wildlife everywhere.

By Sally Kneidel, PhD

Bruce Bower. Monkeys learn to deal with arbitrary tokens as if they are different foods. Science News Vol. 174 #1.

Kate Wong. An interview with Alex, the African grey parrot. 60-Second Science Blog.

The Alex Foundation.

Keywords:: capuchins use of symbols primate intelligence symbolic thinking

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