FISSION AND TROOP SIZE IN A MOUNTAIN BABOON POPULATION

Baboon, Papio cynocephalus, populations show a circumscribed range of troop sizes, dependent on features of the habitat that set limits on t he ability of animals to balance their time budgets. To assess the pro posal that a rising probability of troop fission is the mechanism unde rlying this it is first shown that (1) the distribution of troop sizes in two very different baboon populations, in the Drakensberg and at A mboseli, are not described by the null model of random allocation of i ndividuals to troops (the Poisson process); and (2) data on troops und ergoing or attempting fission support the hypothesis that the probabil ity of fission increases with increasing troop size. A model was const ructed that generated a distribution of troop size from an underlying, 'pre-fission' distribution by varying two parameters: the probability that a troop of given size will split, and the relative sizes of the two daughter troops that result if fission occurs. Both the Drakensber g and the Amboseli populations were adequately described by the model and it is concluded that the distribution of troop size in both popula tions is governed by fission in the manner proposed by Dunbar (1992, B ehav. Ecol. Sociobiol., 33, 35-49). The model also indicates that, rel ative to Amboseli, the probability of fission in the Drakensberg is ve ry high, with all troops of more than 28 members eventually splitting. The hypothesis that low food availability accelerates fission in moun tain baboons through the decreased foraging efficiency of larger troop s was tested, Troops of a wide range of sizes allocated very similar a mounts of time to foraging, foraged with the same efficiency, and trav elled approximately the same distances to do so. It is concluded that fission in mountain baboons, being exacerbated neither by predation no r by inefficient foraging, may be caused directly by the inability of individuals to service social relationships in growing troops in the f ace of small amounts of time available for social interaction. (C) 199 7 The Association for the Study of Animal Behaviour.