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.