Date Published: January 27, 2010
Publisher: Public Library of Science
Author(s): Christophe Boesch, Camille Bolé, Nadin Eckhardt, Hedwige Boesch, Laurie Santos. http://doi.org/10.1371/journal.pone.0008901
Abstract: In recent years, extended altruism towards unrelated group members has been proposed to be a unique characteristic of human societies. Support for this proposal seemingly came from experimental studies on captive chimpanzees that showed that individuals were limited in the ways they shared or cooperated with others. This dichotomy between humans and chimpanzees was proposed to indicate an important difference between the two species, and one study concluded that “chimpanzees are indifferent to the welfare of unrelated group members”. In strong contrast with these captive studies, consistent observations of potentially altruistic behaviors in different populations of wild chimpanzees have been reported in such different domains as food sharing, regular use of coalitions, cooperative hunting and border patrolling. This begs the question of what socio-ecological factors favor the evolution of altruism. Here we report 18 cases of adoption, a highly costly behavior, of orphaned youngsters by group members in Taï forest chimpanzees. Half of the adoptions were done by males and remarkably only one of these proved to be the father. Such adoptions by adults can last for years and thus imply extensive care towards the orphans. These observations reveal that, under the appropriate socio-ecologic conditions, chimpanzees do care for the welfare of other unrelated group members and that altruism is more extensive in wild populations than was suggested by captive studies.
Partial Text: In recent years, extended altruism towards unrelated group members has been proposed to be a unique characteristic of human societies –. Evolutionary theory predicts that altruistic interactions, which are costly to the actor and beneficial to the recipient, will be limited to kin or reciprocating partners –. In contrast to such predictions, economists adopting a rational maximizing approach were struck by the fact that experiments done in different human societies did not support such a model. Rather, humans were always willing to share or cooperate with others more than expected , –. This resulted in an effort to identify the mechanisms that would lead to such observations and, in the end, it was proposed that both punishment by one’s peers and reputation improvement will promote altruism towards unrelated group members in humans –. In a complementary approach, experimental studies done with captive chimpanzees showed limits in the way individuals were able to share or cooperate with others, especially when it came to food –. This dichotomy between humans and chimpanzees was proposed to indicate an important difference between the two species, and one study proposed that “chimpanzees are indifferent to the welfare of unrelated group members” . This difference was suggested to result from chimpanzees’ inability to think about others’ minds and therefore understand that others might need or could profit from help –.
We operationally defined adoption as any relationship between an adult and orphan infant or juvenile in which the adult shows species-specific maternal behavior towards the orphan for at least a two month period. Following a standard definition of adoption –, we required that the adult be permanently associated with the orphan, as well as, at the very least, wait during travel for, provide protection in conflicts to, and share food with the orphan. These behaviors are altruistic in the sense that they are costly for the adopting individuals and do not bring any visible benefit to them, while being beneficial to the orphans. Since juvenile chimpanzees remain associated with their mothers for over ¾ of their time and become clearly independent of their mothers only when they have reached adolescence, we included this period in our study. It is important to underline that adult chimpanzees at Taï do not wait for juveniles or infants, or react to their whimpering at being left behind until they are their mothers. In chimpanzees, orphans suffer tremendous costs in terms of reduced survivorship (orphans less than 5 years of age normally do not survive , ) or retardation in physical development (up to 6 years delay ). However, if adopted, such orphans may present almost normal physical development .