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Humans are supposedly the only animals capable of altruistic behavior, but whoever believes that hasn’t heard of Gia and Porthos, chimpanzees from the Taï forest of Ivory Coast. Gia’s mother died when she was two and a half years old, and Porthos, an older unrelated male, took care of her for more than a year until he died of anthrax. A recent study describes no less than eighteen cases of chimp adoption, many of them between nonrelatives.

With three colleagues,Christophe Boesch of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, observed the adoptions during twentyseven years studying Taï chimpanzees. Out of thirty-six orphaned youngsters, eighteen were adopted—eight by female chimpanzees, ten by males. Male chimpanzees don’t usually do much parenting, so their involvement was particularly surprising.

The adoptive parents carried their charges, often for miles; protected them during social conflicts; and shared sleeping nests and food with them—all costly forms of behavior that entailed sacrifice. Some of the bonds lasted five years or longer. Genetic testing revealed that seven of the adoptive pairs were unrelated. (Five pairs were family; the status of the others could not be determined).

Earlier studies concluded that chimps do not display altruistic behavior toward nonrelatives, but they were all conducted in captivity, Boesch notes. What goes on in the wild, where resources are often scarce, seems to be much closer to human behavior than primatologists had thought. (PLoS ONE)