When Victor's mother died from anthrax, Fredy came along and adopted him. He shared his home with Victor every night, carried him on his back, and even gave him some of his precious food. Such altruistic behavior is one of the noblest attributes of our species. But Fredy and Victor aren't humans--they're chimps. A new study of these primates in the wild suggests that they are far more selfless than scientists have given them credit for, though some researchers have their doubts.
Researchers have seen evidence of altruistic-type behavior in several species, including marmosets, rats, and even ants. But it's unclear whether these behaviors fit the scientific definition of altruism: spontaneously helping others with no expectation of a reward.
Until very recently, scientists were hard put to detect altruistic behavior in chimps, at least in laboratory studies of captive animals. But with more refined experiments, that has started to change. In a 2007 study, for example, psychologist Michael Tomasello of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and colleagues put a chimp in a room with a clear view of tantalizing pieces of fruit locked behind a door. A second chimp was then presented with the opportunity to unlock the door, even though it would receive nothing for its trouble. Nearly 80% of the time, the second chimp did the right thing. But studies of chimps in captivity, where they are exposed to human behaviors, do not necessary reflect how they act in the real world.
So Tomasello's Max Planck colleague, primatologist Christophe Boesch, looked for evidence of altruism in the wild. For the past 3 decades, Boesch and his team have been watching the chimps living in Taï National Park in the Cote d'Ivoire. The team has observed 18 cases of adult chimps adopting young chimps whose mothers had died. The researchers defined adoption to mean that an adult showed behavior toward an orphan for at least 2 months--such as sharing food and waiting for the youngster to catch up while traveling to a new location--that normally only mother chimps display toward their offspring.
Ten of the 18 adoptions were by males, even though male chimps normally have little to do with their offspring. And in only one case was an adoptive male the father of the orphan. The duration of the adoptions varied from 3 months to more than 5 years, and the team found that taking care of the orphans was usually very costly in time and trouble for the adults. For example, on one occasion Fredy cracked open 196 Coula nuts, a favorite food at Taï, and shared most of them with Victor. This sharing behavior between an adopter and an orphan is a clear sign of altruism, the researchers conclude online this week in PLoS One. Boesch says altruism may not show up as often in captivity because captive animals don't need to stick together to survive.
Experts are hesitant to endorse the team's conclusions. Joan Silk, an anthropologist at the University of California, Los Angeles, says that there is "no doubt" that chimps engage in altruistic behavior in the wild. But it "remains unresolved," she says, whether they do so out of true concern for others or because they eventually expect some sort of payoff, such as gaining grooming partners or allies in chimp social wars. And Derek Penn, a cognitive psychologist at the University of Louisiana, Lafayette, says that 18 adoptions over 27 years of observations, some of which involved brothers and sisters, does not amount to hard evidence for altruism. Rather than selfless behavior, he says, the adoptions could be the result of adults mistaking the young chimps for their own offspring.