The behavioural ecology of green monkeys (Cercopithecus sabaeus) at Mt. Assirik, Senegal

Abstract

The aims of this study were to provide detailed information on the ecology of green monkeys in Senegal, to examine adaptive behavioural variation by comparing their behaviour with that of other populations of this widespread species-group (C. aethiops), and to use field-data to test hypotheses about adaptive strategies, particularly those concerned with how their foraging patterns changed with the seasonally variable availability and distribution of resources. Field-work was carried out at Mt. Assirik, in the Parc National du Niokolo-Koba, Senegal. The climate, vegetation, and fauna of the region were described. On most criteria, Mt. Assirik is vegetationally richer in density and diversity of species than two study-sites compared in Cameroon, and one in Kenya, where other populations of C. aethiops have been studied. The demographic structure of the population of green monkeys at Mt. Assirik was assessed. The mean size of groups was 19 monkeys, who lived at a comparatively low overall density of 4.4 per km2. This, the lowest density recorded for C. aethiops, is ascribed to the extensive areas of sparse, unsuitable habitat that constitute a large part of the vegetational mosaic of the region.A single group of green monkeys was studied in detail, over one complete annual cycle. Aspects of their feeding, ranging, activity-budgets, and territorial behaviour were recorded during 5-day sample-periods each month, in parallel with close monitoring of the changing composition, density, and distribution of important resources. The green monkeys' diet was omnivorous and diverse, including over 65 species of plants, many invertebrates, and some eggs and meat. Preference was given to fruits and flowers, although particular species were not selected; rather, these foods were eaten in proportion to their availability. Leaves, gum, seeds, and fungi were secondary choice foods. There was little overlap in the composition of the diet from month to month, indicating the strong seasonality of the environment. There was a fairly consistent intake of invertebrates each month. The monkeys spent between 35% and 55% of their time feeding. Diurnal rhythms of activity were strongly influenced by temperature: the monkeys stopped feeding and travelling when it was either too hot or too cold. On a finer time-scale, feeding was more closely synchronized between the monkeys when they fed on less common species. Several age and sex differences in feeding were found. In particular, females with very young infants fed less than other adults. No particular height-niche was occupied by the monkeys. The study group ranged over an area of 1.78 km2, the largest range recorded for any C. aethiops group. Their ranging patterns differed from month to month, and were significantly influenced by the availability and distribution of food, water, sleeping sites, and habitat-types, and by patterns of intergroup relations. Territorial behaviour itself was strongly influenced by the availability and distribution of key food sources, and the intensity of intergroup encounters varied accordingly. Many differences in patterns of feeding and ranging between populations of C. aethiops are related to the floristic composition of the vegetation, but comparisons were limited by lack of appropriate data on the availability and distribution of food at other sites. Data on the seasonally varied patterns of feeding, ranging, and activity-budgets, and changing patterns of resource availability, were drawn together to examine the adaptive strategies underlying the monkeys' behaviour. Several models in optimal foraging theory were tested. Time and energy spent in feeding and travelling increased as food-availability increased. Their choice of diet was optimal in that they were more selective when profitable food-items were common: higher proportions of the diet were given over to fruit and flowers when food-availability was high. In parallel with these strategies, a nutritive balance was maintained by consistent inclusion of at least some foliage and invertebrates in the diet, however much fruit was eaten.