Behaviour and ecology of grey-cheeked mangabeys (Cercocebus albigena) in the Lope reserve, Gabon
Grey-cheeked mangabeys (Cercocebus albigena) are distributed across Central Africa, but have previously only been studied in detail at the eastern edge of their range in Uganda. Hence, little is known about this species from the western African rain forests, where forest composition and primate species are different from those in eastern Africa. The behaviour and ecology of grey-cheeked mangabeys was studied in the Lope Reserve, Gabon over 18 months, between January 1991- June 1991 and September 1991- August 1992. Systematic data were collected mainly from one habituated group, and data were also collected opportunistically from other groups in the study area. The mangabeys' diet is diverse, with 100 items of plant food from 75 species recorded. Overlap in the mangabey's diet with the seven other diurnal primate species at Lope is high. Sixty-four percent of fruit-pulp, 51% of seed, 38% of leaf, 27% of stem and pith, and 15% of flower species in their diet are also eaten by at least one other species of diurnal primate. Mangabeys spent 36% of their time feeding, eating seeds. This is high, compared to studies in Uganda where seeds were relatively unimportant in the diet of grey-cheeked mangabeys. Seed-eating, may be a result of differences in forest composition, since there are a higher number of species from the family Leguminosae at Lope. Alternatively, seed-eating may be a strategy for competing with sympatric primate species. This is the first time grey-cheeked mangabeys have been studied in areas where they coexist with both gorillas and chimpanzees, which at Lope, both have a large proportion of succulent fruits in their diets. For more than half of the time mangabeys spent eating seeds, the seeds were taken from immature fruit. Mangabeys, therefore, may be eating unripe seeds as a form of exploitation competition. The overall home range size of the main group (18-23 members) was 225 ha, and a second group (18-20 members) had an estimated home range size of 156 ha. Use of different habitats was shown to be related to the availability in time (as assessed by phenological monitoring), and in space (as determined from strip sampling two 1 ha plots in two habitat types: savanna-edge and river-edge forests) of certain plant species. Comparisons with grey-cheeked mangabeys studied in Uganda revealed that home range size varied from about 10% to 200% of the size of those at Lope. Mangabeys spent an average of 80% of the time in association with at least one other primate species. Benefits of the associations are thought to be biased towards the Cercopithecus spp. since they followed mangabeys, but rarely vice versa. These species may benefit from decreased predation rates due to the mangabey's larger body and group size, and because mangabeys more actively defend against predators. Forests at Lope are highly seasonal, with periods of relative fruit scarcity in the long dry season. During this period, mangabeys spent a greater proportion of time feeding, their diet was less diverse consisting almost entirely of seeds, and mangabeys were observed in polyspecific associations less, than during the long rain season when fruit was relatively more abundant. The great variation in behaviour and ecology between the present study, and studies of grey-cheeked mangabeys in Uganda, highlights the ecological flexibility of this species, and emphasises the importance of both forest composition and primate community structure in shaping behaviour.