Rank and grooming reciprocity among females in a mixed-sex group of captive hamadryas baboons

In a mixed-sex, captive group of hamadryas baboons (Papio hamadryas hamadry as) we investigated whether female grooming relationships are affected by t heir dominance ranks. Seyfarth's [1977] grooming for support model and Barr ett et al.'s [1999] biological market model both predict that in primate gr oups where competition for monopolizable resources is high, grooming among females is based, at least partly, on the interchange of grooming for rank- related benefits, and that rank thus influences the distribution of groomin g in females. Contrary to this prediction, our results show that despite th e existence of a linear dominance hierarchy, rather strict dominance relati onships, and high food-related aggression rates, grooming among female hama dryas baboons is not affected by rank and is only exchanged for itself. Thi s is understandable since rank differences in our study group only result i n differential access to limited, preferred food items that are not activel y shared. Although some females are more likely to tolerate one another at the food pile, this tolerance is not determined by their grooming efforts a nd interchange of grooming for rank-related benefits does not occur. We con clude that female hamadryas baboons groom others in order to be groomed by them, which is supported by our observation that grooming reciprocity withi n a dyad increases when more grooming occurs in this dyad. Our results indi cate that grooming is indeed a valuable commodity in itself, probably becau se of its stress- and tension-reducing effect. Based on our findings, the e xisting groom trade model is extended to include circumstances in which mon opolizable resources are available but are not traded for grooming. Am. J. Primatol. 55:25-42, 2001. (C) 2001 Wiley-Liss, Inc.