They saying "You scratch my back and I'll scratch yours" may not extend to chimpanzee groups, which largely rely on one another to groom and clean the places of their thick coats they can't reach on their own. In a recent study from the University of Kent, researchers found the number of bystanders largely influences the grooming decisions of adult make chimps. 

Grooming is an important social interaction for chimps, used to maintain friendly ties among family and community members. It involves removing pieces of dirt, plants, dried skin, and insects from the hair of another chimpanzee or of one's self. The recent study, however, challenges the long-held belief that such cooperative behaviors are built on trust and prior interactions, according to a news release.

Dr. Nicholas E. Newton-Fisher and Dr. Stefano Kaburu of Kent's School of Anthropology and Conservation studied grooming interactions among wild chimpanzees of the Mahale Mountains National Park, Tanzania. Their observations revealed that with more bystanders, or a larger audience, male chimps offered less grooming at the beginning of individual interactions and were less inclined to even initiate one.  Also, their grooming efforts were less likely to be reciprocated when numerous social partners entered the scene.

Therefore, the study findings suggest grooming and other social interactions among chimpanzees are largely driven by the evaluation of direct benefits rather than relationships based on trust. In other words, if there are numerous potential social partners nearby, chimpanzees don't need to rely on only trusted partners to groom them -- they can explore other options.

Their study was recently published in the journal Scientific Reports

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