‘ Friendship ’ for fitness or ‘ friendship ’ for friendship ’ s sake ?

Correspondence: P. Stopka, Department of Zoology, South Parks Road, Oxford OX1 3PS, U.K. (email: [email protected]). L. Barrett is at the School of Biological Sciences, University of Liverpool, Liverpool L69 3BX, U.K. Hemelrijk et al. (1999), using data from a captive chimpanzee, Pan troglodytes, colony, state that there is no evidence to suggest that individuals trade ‘social services’ for fitness benefits. While fitness benefits are indicative of the evolutionary success of a given strategy, we suggest that Hemelrjik et al. (1999) do not pitch their investigation at the correct level of analysis, nor do they control for the fact that individuals will differ in the amount they have to ‘pay’ to gain a benefit. In this particular study, and for this species, we argue that copulations are the tradable commodity rather than fitness per se. The concept of trade as a mechanism in so-called biological markets was formally defined by Noë et al. (1991; see also Noë & Hammerstein 1994) as the situation in which two collaborating individuals that differ in terms of the commodities that they possess behave in such a way that the symmetry of an exchange depends on the level of supply and demand (as in real-world economic markets). Such commodities can be, for example, resource-holding potential (RHP) or grooming provided to other individuals. Asymmetries in the possession of different commodities have resulted in trade documented in, for example, groups of baboons (Barrett et al. 1999) where grooming was traded for grooming and possibly for other social services such as tolerance around food sources. The latter was suggested to occur when the ‘demand’ for food was much higher than the ‘supply’ in their natural habitat. Hemelrijk et al. (1999) assume that if trading between males and females is to be evolutionarily stable, it must have positive fitness consequences. Therefore, according to Hemelrijk et al. (1999), if there is no correlation between realized paternity score per male and his invest-