"What this experiment has done is formalize the problem and demonstrate ... what everyone suspected: chimpanzees judge others by how useful they are or can be as cooperators," primatologist Frans B.M. De Waal of Emory University's Yerkes Primate Center wrote in an e-mail.
What can you do for me?
Cooperation is not altruism, of course. Melis' study found that the chimps recruited a partner when they needed help, but almost never when they didn't need assistance. That would have meant sharing the food.
"The chimps in our study were clearly behaving totally selfishly," Melis wrote. "Why should they share the bananas if they can get them all for themselves?" But she added that food is "a serious matter" for chimps, and may be too critical a resource to share.
That's where Warneken of the Planck Institute and his collaborator, Mike Tomasello, come in. They decided to ratchet up the stakes and compare the capacity of chimps and human infants for altruism, which they also call "helping behavior."
In one experiment, Warneken pretended to accidentally drop clothespins with an 18-month-old infant nearby. He would stop hanging up clothes, wait a few seconds, stare at the child for a few seconds more and finally blurt out: "My pin!"
Warneken had never met the infants before, and never asked for help or promised a reward. Still, almost all the children picked up the pin and handed it to him. In fact, he reported, 84 percent did so even before he looked at them.
The study is the first, Warneken said, to demonstrate this kind of helping behavior in children too young to have language skills.
Warneken then tested chimpanzees that were raised by humans. Again, he reported, they helped him fetch objects that were out of reach without the promise of reward.
It was, Warneken told Science, "the first experiment showing altruistic helping toward goals in any non-human primate."
Melis said chimps may be willing to make sacrifices for the greater good, though only up to a point.
"If food is not involved and what they have to do is not too much ... they might be willing to help others," she said.