According to the results, the chimpanzees were no more likely to choose the second option, even though they could see it would help another chimpanzee at no personal cost. Begging gestures on behalf of some of the recipients had no effect.
As a control the researchers performed the experiment with only one chimpanzee present.
"What we know is they didn't differentiate between situations when another chimpanzee was there and was not there," Silk said.
Whether or not another chimpanzee was present, the chimp in charge chose to the two-food-item option about half the time.
Mitani, the Michigan anthropologist, said that while the result shows indifference among chimps, he wonders why solo chimps chose the two-food-item option so often.
"Put yourself in this experiment, and after you do it for a while you'd notice that that food is going to waste, so you'd say, I'll just do it for myself," he said. "You have to wonder what's going on in their minds."
Silk and colleagues said the finding suggests that chimpanzee behavior is not motivated by so-called other-regarding preferencesempathy and a concern for the welfare of others.
They note in Nature that previous research has shown chimpanzees participate in a variety of group activities such as patrolling territory, hunting, sharing food, and guarding mates.
This suggests that in at least some circumstances chimpanzees are cooperative. Anecdotal accounts suggest chimpanzees show empathy toward injured individuals.
But this cooperative behavior is selective, according to Mitani. Research also shows that chimpanzees will kill strangers they encounter in the wild.
"They form strong social bonds with specific individuals and preferentially direct their cooperative behaviors towards those chimpanzees with whom they have developed ties," he said.
What the current study fails to examine, Mitani added, is whether the chimpanzees would show more concern toward individuals with whom they have already formed a bond.
"I can imagine they might pull that bar to get two rewards more often if in fact one of their buddies is next door, compared to somebody they don't like or who they have a rivalry with," he said.
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