Chimpanzees can be vengeful, aggressively punishing a wrongdoer, but new research shows that they will not hurt another chimp just because they can. Spite, it seems, may be an exclusively human emotion.
Chimps have a distinct sense of right and wrong and dislike unfair occurrences, such as being denied food they were working towards acquiring. Evolutionary biologist Keith Jensen of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, wanted to know whether chimps can be spiteful. Jensen and his colleagues placed two chimps into separate cages facing one another, with a table in between them that held peanuts. One of the two chimps, which could not access the table, nonetheless had the power to deprive its compatriot of food, by pulling a rope in its cage and collapsing the table. But the chimp did that no more often than in another experiment where it was alone. Frustration at being unable to reach the food itself, not a petty desire to deny food to the other chimp, was behind its behavior, the team concludes this week online in The Proceedings of the National Academy of Sciences.
But would the chimp be this accommodating if food it was eating were stolen from it, and would it care whether a chimp or a human was the thief? To find out, the researchers offered the animals peanuts on a sliding tray on top of the table standing between them. As in the first experiment, one chimp could pull a rope to upset the food tray. Here, however, both chimps could access the food--but only one at a time could do so. While the chimp with the rope was chomping away, either the second chimp would slide the tray of food from it, or a researcher would take the food away and offer it to the chimp without the rope. The disgruntled chimp pulled the rope almost 50% of the time when the other chimp stole its food, showing a tendency to punish the offending chimp for theft by cutting off its access to the food. But when it was a human who took the food away to give to the other chimp, the chimp only pulled the rope about 20% of the time.
"It's like a kid with a big slice of cake, and then having an adult take it to give to another kid," says Jensen--something that may not prompt the same outrage as one child stealing cake from another. The chimp's behavior, he says, shows that it punishes the chimp only when it's the offender. Jensen's group is now interested in examining how far this nuanced behavior extends--for example, whether, if a chimp sees another do wrong, it will give up food to see that chimp punished.
"We normally focus on the positive aspects of chimps," says anthropologist Joan Silk of the University of California, Los Angeles. This study, she says, "is one of the few times where we look at how they respond to situations that are disadvantageous to them," and how the animals' behavior coincides with or diverges from that of humans.