CONICET | Buscador de Institutos y Recursos Humanos

In group-living primates, behavioural exchanges have been reported to occur according tomarket-driven patterns where grooming is used as a currency in exchange for different commoditiessuch as agonistic support and feeding tolerance. As in a marketplace, the law of supply and demand plays a crucial role in determining not only the specific trading dynamic but also the market value of trading partners. Primates pay more when partner value is higher (i.e. individualsgroom longer those partners that exert higher control over resources). In this regard, aprevious experimental study has showed that grooming received by two low-ranking females,trained to provide food to their group, shifted according to their artificially enhanced values, withthe increase in grooming received by the first provider being reduced by the addition of a secondprovider. By adopting a similar experimental paradigm, this study tests whether the market valueof experimentally induced food providers responds to variations in the supply/demand ratio inblack capuchin monkeys (Sapajus nigritus). To do so, we created an artificial food market in agroup of wild capuchins in Iguazú National Park, Argentina. In this market, a low-ranking female(provider 1) and a subadult male (provider 2) were trained to provide food to the entire groupby opening two boxes containing banana pieces. Grooming interactions were collected across3 phases: baseline (phase 0), only provider 1 (phase 1), and both providers (phase 2). We thenanalysed the duration of grooming received by the providers across the 3 phases, predicting that:i) grooming received will increase in favour of provider 1 from phase 0 to 1; and that ii) such anincrease will be reduced by the addition of provider 2 in phase 2. Results indicate that groomingreceived by provider 1 significantly increased during phase 1, as predicted by the BM theory;however, such an increase was not reduced by the introduction of a second provider.