Low-status chimps revealed as trendsetters

Chimps with little social status influence their comrades’ behavior to a surprising extent, a new study suggests.

In groups of captive chimps, a method for snagging food from a box spread among many individuals who saw a low-ranking female peer demonstrate the technique, say primatologist Stuart Watson of the University of St. Andrews in Fife, Scotland, and colleagues. But in other groups where an alpha male introduced the same box-opening technique, relatively few chimps copied the behavior, the researchers report online February 7 in the American Journal of Primatology.

“I suspect that even wild chimpanzees are motivated to copy obviously rewarding behaviors of low-ranking individuals, but the limited spread of rewarding behaviors demonstrated by alpha males was quite surprising,” Watson says. Previous research has found that chimps in captivity more often copy rewarding behaviors of dominant versus lower-ranking group mates. The researchers don’t understand why in this case the high-ranking individuals weren’t copied as much.

The spread of new behaviors in groups of monkeys and apes depends on a variety of factors — including an innovator’s social status, age and sex — that can interact in unpredictable ways. “That’s why social learning in groups is so interesting to study,” says Elizabeth Lonsdorf, a primatologist at Franklin & Marshall College in Lancaster, Pa., who did not participate in the research.

In the new investigation, perhaps animals were monitoring potential threats from the alpha males more than their box-opening skills. “Alpha male chimps are large, powerful and prone to temper tantrums, so it makes sense to be vigilant for signs of what they’ll do next,” Watson says.

It’s also possible that lower-ranking chimps are sometimes unwilling to copy rewarding behaviors in the presence of a dominant chimp, suggests Lonsdorf. Researchers have reported this form of social deference in capuchin monkeys and rhesus macaques.

Watson’s team studied 38 chimps housed at a research facility in Texas. A low-ranking female from each of two groups and a dominant male from each of two other groups were trained to open a box and remove a piece of fruit. They then performed this feat in front of their home groups during two 20-minute sessions on consecutive days. Trained chimps moved a sliding door on the box up or down until it locked to reveal one of two chambers containing the food. Following each demonstration, group members had eight hours to manipulate the box however they liked.

Individuals who watched low-ranking chimps open the box, but not those who observed dominant chimps do the same, used the demonstrated technique as their first choice more often than expected by chance.

Some, but not all, chimps could have figured out how to open the box on their own, Watson suspects. Of 15 chimps from nonexperimental groups — each given 20 minutes to manipulate the fruit box — five failed to open it.

Unexpectedly, two low-ranking female chimps from different groups discovered a way to game the experimental box after watching dominant males open it. These animals realized that the contraption held two pieces of fruit, one in an upper chamber exposed by sliding the door down until it locked and another in a lower chamber revealed by sliding the door up to lock. Each chimp managed to slide the door up and down just enough, without locking it, to snatch both snacks.

In one group, the alpha male, who had originally demonstrated the locking technique to his group, started copying the low-ranking female’s superior approach. In the other group, two high-ranking females adopted the innovative box-opening method after seeing it performed by their social inferior.

Watson doesn’t know if low-ranking chimps are particularly apt to devise clever behaviors that others copy. It wouldn’t be surprising, he says, since chimps low on the social totem pole typically get less food than others and need to supplement their diets in creative ways.

In wild chimp communities, it’s unclear why certain novel behaviors catch on, Lonsdorf says. For example, young females move to new groups at sexual maturity, so they may bring useful knowledge from one community to another. In one reported case, chimps apparently learned how to use branches and other probes to collect and eat ants after observing the behavior in a recently arrived young female (SN: 2/9/13, p. 20).