Interspecific and Intraspecific Variation in Diet of Two Forest Guenons, Cercopithecus ascanius and C. mitis

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Abstract
(1) A log-linear model of variance in the consumption of fruit, leaves, insects and other plant foods by sympatric Kenyan guenons showed that species identity, age-sex class, time of year, and all two-way interactions of these factors were significant sources of dietary variation. (2) Blue monkeys were more folivorous and less frugivorous and insectivorous than redtails. The two species used different plant species and substrates as sources of insect prey. Redtails used capture methods more appropriate for mobile prey more often than blue monkeys. Both species showed a bimodal annual seasonality in their consumption of fruit, leaves and insects, but differed in how the proportions of these items in the diet inter-relate. (3) In both species, adult males were highly frugivorous, whereas adult females were more insectivorous and/or folivorous. Juvenile size was directly related to the proportions of leaves and of insects in the diet, but inversely related to the proportion of fruit. The largest juveniles were more folivorous and insectivorous than adults. Some age-sex differences were found in the plant species and substrates used for insect capture, and in capture methods. (4) The diets of some heterospecific age-sex classes were more similar than those of some conspecific age-sex classes in degree of frugivory and insectivory, and in how the monthly proportions of the three major dietary components covaried. Dietary similarity of conspecifics tended to exceed that of heterospecifics, however, when plant species-specific items and the number of individuals in each species-age-sex class were taken into consideration. (5) These results and comparisons with other Cercopithecus communities suggest that dietary variation between species and between species-age-sex classes is related to body size, the physiological demands of reproduction and growth, social constraints, and competition, especially for fruit.