Are primates out of the market?

Biological Market Theory (BMT) has provided an elegant framework to study how commodities are exchanged among individuals. In primates, BMT predicts that individuals exchange grooming with other commodities based on the law of supply and demand. However, BMT still suffers some theoretical and methodological limitations. Our aim in this paper is to discuss some of these limitations, including the lack of consensus over the time frame in which exchanges take place, and over the commodities involved, the cognitive challenges imposed by biological markets (BMs), and the heterogeneity of methods used to test BMT across studies. In particular, we discuss (1) the importance of predetermining both the time frame over which exchanges take place and (2) the commodities that are exchanged in primate BMs, (3) the cognitive skills that primates need to navigate in a BM, and (4) other methodological issues arising when testing BMT. For each of these points, we propose an agenda with possible solutions and we show how the issues raised also apply to BMs in species other than primates.