LINCOLN PARK — Nine out of 10 chimpanzees prefer grapes over carrots, and researchers at Lincoln Park Zoo are using that information to see how far a chimp will go for the extra sweetness.
A study started earlier this year has created a sort of economy for the zoo's chimps, who use 6-inch pieces of PVC pipe as a currency that can be traded for either a grape or a carrot.
A researcher dispenses the carrots next to the box where the chimps get the pipes, while a grape-dispensing handler is a 20-foot walk around the bend.
"First of all, we were looking at their problem-solving skills," said Lydia Hopper, a research scientist at the zoo's Lester E. Fisher Center for the Study and Conservation of Apes. "They had to figure out what to do."
On the first day of the study in January, researchers placed a box of 150 of the plastic tokens within reach of the chimps, and a tub of carrots and a tub of grapes at either end of an enclosed station.
"There was a lot of just sitting around and staring," Hopper said. "They started exchanging other things like sticks, maybe pushing wood wool through."
Then by chance, Optimus Prime, one of the younger chimps in the enclosure, was playing with one of the tokens and pushed it through the cage. He was handed a carrot piece cut the same size as the grapes.
"He was like ohhh, and there was a light bulb moment," Hopper said.
Chimps, unlike humans, do not do active teaching but are very socially tolerant, according to researchers at the zoo.
Optimus, as the researchers call him, let the other chimps take close watch. On the eighth day of trials, Chuckie, the subordinate female of the group, figured out that a grape was just a walk and a token away.
"They would see him doing it and think, 'Maybe I can just exchange anything and I get a carrot,'" Hopper said. "They were really exploring the rules of the game and trying to learn how to play."
A similar situation occurs in the wild, as researchers have witnessed mother chimpanzees let others watch closely while they crack nuts with a stone tool.
Now that the study is about four months along, researchers are planning the next step, which will likely be moving the grapes to another harder-to-find location. They also plan on starting the study with gorillas sometime soon.
"To go back to the natural analogy of their natural habitat in Africa, it might be that they would go to this tree for fruit and all the sudden for whatever reason it's not fruiting this year," Hopper said. "You wouldn't just stick with that tree."
The current record for the number of carrots eaten by a chimp is 141 out of the 150 available tokens. During the last test Thursday, Chuckie ate 30 carrots and 25 grapes.
The study is conducted twice a week, and researchers hope to incorporate a narrator on the other side of the glass to educate the public during trials.
"They are doing it for more than just getting the food. It’s the game itself, I think," Hopper said.