Chimps Will Work Harder to Get Their Favorite Foods

An experimental setup shows that chimps will travel farther to get a more preferred reward

chimp eating
35-year old male chimp Frodo enjoying Mbula fruit he collected in Gombe National Park, Tanzania Anup Shah/Corbis

Grapes are better than carrots, at least according to chimpanzees at the Lincoln Park Zoo in Chicago. So much better in fact, that the chimps are willing to put in a bit of work to get the sweet, juicy fruits from researchers, reports a new study.

Intuitively, this makes sense to us. We’re willing to drive an extra fifteen minutes or take a few more subway stops to get to our favorite restaurants.  Wild chimp females will travel farther and earlier in the day to nab some tasty and scare foods like figs. But the idea hadn’t been studied before in a zoo setting. 

Other work shows that chimps carefully think about what food they’ll expend effort for—and they don’t conform to what the majority is doing. So researchers were eager to see how offering different foods for different efforts would effect chimps weighing of the pros and cons of treats. All this work can tell them about foraging behavior and learning in social creatures like chimps and potentially, early humans.

In the new study, six zoo-born chimps learned that they could exchange tokens—short lengths of PCV pipes—for food rewards. The catch was that they could collect tokens from one location, but the rewards were offered in two different locations. The closer one had carrots, the farther had grapes.

The chimps were willing to walk around climbing structures in their enclosure, climb up almost 20 feet to reach the location where grapes were held. Carrots were just a short walk away from the token station.

The researchers weren’t only interested in the problem-solving aspect of the puzzle, but also how the social dynamics of the group played out. In previous studies, different tokens got the apes different rewards. Chimps of low social status would exchange their "better" tokens for the one that gave a less preferred reward, just so they’d have a better chance of holding onto any food reward, at all. The new study’s results therefore were a little surprising. A press statement explains:

Interestingly, the first chimpanzee to discover the better reward being offered at the far location was a female named Chuckie, who is the lowest-status female in the group. The far location may have been preferred for not only it's food reward, but because it gave her an opportunity to avoid competition from higher-status chimpanzees at the close location. Additionally, alpha male chimpanzee, Hank, observed the other apes and learned the benefits of the far location, thus exchanging 100 percent of his tokens at the far, preferred food location.

Hank didn’t make his first exchange until 13 months after the tokens were first introduced. He had seen 181 successful exchanges between the researchers and his troop mates, the study, published in PeerJ, notes. The chimps also learned to carry more than one token at a time, to avoid multiple trips.

"All of the chimpanzees in this study demonstrated flexible foraging strategies with minimal scrounging from one another," says lead researcher Lydia Hopper in the statement. "Understanding the animals' preferences and exploration of their habitat is critical to caring for these animals."

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