Long-term reciprocation of grooming in wild West African chimpanzees

Abstract

Humans are well known for their ability to keep track of social debts over extended periods of time, and for their tendency to preferentially cooperate with closely bonded partners. Non-human primates have been shown to cooperate with kin and non-kin, and reciprocate helpful acts. However, there is ongoing debate over whether they keep track of previous interactions and, if so, whether they can do it over extended periods of time, or are constrained to finalize exchanges within a single encounter. In this study, we used 3000 hours of all-day focal follows of wild chimpanzees (Pan troglodytes verus) to investigate whether both females and males reciprocate grooming within a single interaction, throughout the day, or over longer periods of time. We found that grooming was reciprocated more symmetrically when measured on a long-term, rather than on an immediate or short-term basis. Random giving, general allocation of grooming efforts, similarities among individuals and kinship do not appear to explain these highly reciprocal exchanges. Previously collected consecutive focal follows of single individuals revealed that dyads groomed an average of once every 7 days. Our findings strongly suggest that chimpanzees, similar to humans, are able to keep track of past social interactions, at least for a one-week period, and balance services over repeated encounters.

Urinary oxytocin and social bonding in related and unrelated wild chimpanzees

“…Contingent reciprocity, where one remembers a service given by another and then offers a service in return at a later date, offers a possible explanation, although this mechanism is rare in animals [16,17] and has been found among individuals that interact at low rates [18]. By contrast, individuals that interact at high rates and have strong, stable social bonds typically show short-term imbalances in services that are more equitable when calculated over months [19][20][21]. Whether or not some animal species have the cognitive capacity to remember social exchanges over time is currently unclear [17,21,22].…”

“…Because the dataset contained incomplete values (not all subjects could be tested in all conditions) and because different individuals appeared a different number of times as subjects or grooming partners, we included identity of subject as a random factor in all models. In addition, identity of grooming partner and identity of dyad were included as random factors in models that examined factors on a dyadic level [19,51]. Across the three grooming conditions, there were 36 grooming partners, with each partner occurring with a frequency of 2.05 + 1.36 (mean + s.d.)…”

Adaptations for social cognition in the primate brain

“…Individuals rely on each other to form close social bonds, and the fitness of partners therefore becomes interdependent [48]. Indeed, among non-human primates, the affiliative interactions of individuals who share a close social bond tend to become highly reciprocal over time (female baboons [14]; male chimpanzees [49,50]). Such reciprocity is evident even among kin, where the opportunity for indirect as well as direct benefits should be expected to result in high skew.…”

Food transfers in capuchin monkeys: an experiment on partner choice

“…Although recent events do affect the likelihood to behave cooperatively [9], animals also accept shortterm imbalances in their within-dyad exchanges. Overall, long-term exchanges are often more balanced than short-term exchanges [10,11].…”