Becky Frank: UCLA Anthropology

The goal of this project was to examine the dynamics of exchange among female baboons and test
predictions derived from a biological market model of grooming. Evolutionary theory predicts that cooperation
among nonkin will be limited to reciprocating partners who monitor the balance of trade within their
relationships in order to prevent cheating. But primates may be cognitively limited to negotiating balanced
reciprocity over very short time scales. If so, then trade might be regulated in a biological market, where
supply and demand determines the value of an exchange, and individuals choose to trade with the partner
offering the highest value. Individuals maximize their immediate benefits without having to monitor the
balance of their exchanges over time. When demand for a partner or commodity is greater than the supply,
individuals compete for access to the preferred partner by raising the price they are willing to pay. Applied to
primate grooming exchanges, a market model predicts that females will balance the amount of grooming they
trade within single bouts when all partners offer similar value. In some cases, partners can offer other
valuable benefits and will trade those with who ever offers the most grooming in return. Thus, females are
predicted to trade grooming for access to resources when feeding competition is elevated and rank
differences translate to differential foraging success. Females are also predicted to trade grooming with
mothers of young infants in exchange for access to the infant. I tested these predictions in a group of 16 adult
female olive baboons in Chololo, Kenya. In this troop, females only reciprocated within the same grooming
bout 34% of the time and balanced their grooming more evenly over many bouts than within single bouts.
Non-mothers preferentially groomed mothers of young infants, but did not compete for access to infants by
raising their grooming offers to mothers as the availability of infants declined. When feeding conflict within
dyads was high, females provided additional grooming to their higher ranking partners, but they do not spend
more time co-feeding in return. These results suggest that female baboons are capable of monitoring their
exchanges over time and can trade across some currencies, but it is not clear that market pricing explains the
observed patterning of exchange.