COHORT SIZE AND THE ALLOCATION OF SOCIAL EFFORT BY FEMALE MOUNTAIN BABOONS

Dunbar (1992, Behav. Ecol. Sociobiol., 33, 35-49) argued that constrai nts on social time limited the size to which savannah baboon, Papio cy nocephalus, troops in any given population could grow before fissionin g. Since this should be reflected in population structure, we have els ewhere (Henzi et al. 1997a, Anim. Behav. 53, 525-535) constructed a mo del, based on a rising probability of fission, that fits the observed distribution of troop sizes of mountain baboons, P. c. ursinus, in the Drakensberg mountains of South Africa and which predicts that the pro bability of fission will rapidly increase once a troop has more than 2 3 members (or 8.7 females). We test this prediction in this paper. Sin ce Dunbar argued that females will drive fission once they cannot enga ge in the grooming necessary to sustain alliances, we compared the gro oming interactions of adult females from four troops in the Drakensber g mountains. The mean female grooming clique size reached an asymptote at 7.4 females, so that females in cohorts of eight or more no longer attempted to groom all other females, and mean grooming bout length d eclined as the cohort grew to 7.9 females and then increased again. Th ese values are coincident with the female cohort size predicted by our model of troop growth and fission. We argue that females attempt to g room all other females as well as sustain closer relationships with a few females through longer bouts of reciprocated grooming. When the de mands of grooming all other females reduce bout length to a point when no reciprocated bouts are possible, female clique size is capped. As a troop continues to grow, the mechanical difficulties involved in gai ning access to grooming partners leads to a reduction in the diversity of grooming relationships. This weakening of the total female network , as cliques become more differentiated, is likely to facilitate fissi on. We conclude that our data provide the first within-population vali dation of Dunbar's hypothesis concerning the mechanism underpinning fi ssion. In the Drakensberg, where there is no advantage to female coali tions, we propose, as an amendment, that females will leave a troop no t to escape local competition, but to follow a male with whom they hav e a close friendship. (C) 1997 The Association for the Study of Animal Behaviour.