WHO IS RESPONSIBLE FOR FISSION IN A FREE-RANGING TROOP OF BABOONS

The study troop of chacma baboons (Papio cynocephalus ursinus) at Mkuz i Game Reserve, Zululand, South-Africa, comprised of about 76 members that split into two new troops. The events leading to this troop fissi on will be described and its possible causes will be discussed. Troop fission among baboons is generally attributed to the withdrawal of low -ranking females from the main group, as a result of the cost of food competition and its effect on their reproductive success. At Mkuzi, no evidence for food competition among females was recorded in terms of rank-related time spent feeding or other time-budget components, feedi ng-bout length, diet composition or context of female aggression. More over, no evidence for rank-related differential reproductive success w as found in terms of inter-birth intervals or infant survival. Female mortality was, however, related to dominance rank, with circumstantial evidence suggesting that cause of mortality was predation by leopards . Rate of female disappearances, aggression levels among females, and the percentage of time they spent in proximity to other adult troop me mbers increased after fission. Relatively shea inter-birth intervals a nd extremely low infant mortality rate at Mkuzi resulted in a small nu mber of receptive females at any one time, and therefore in high costs of male sexual competition as expressed in the high levels of male ag gression and woundings, both reduced after fission. It is suggested th at this troop fission may have been initiated by the resident males, t riggered by the high cost of sexual competition, and forced on the fem ales, who were, consequently, subjected to higher risk of predation. T he troop fission was preceded by a long process of increasing tendency for sub-trooping. It was initiated by the four resident mates who kep t a large distance apart from each other, herded oestrous female assoc iates away from others and were followed by other females. The females generally tended to stay close to associates, males and females. Thes e parties were followed by the peripheral and immigrant males who had no female associates, and eventually two distinct daughter troops were formed.