Animals must make dietary choices to achieve adequate nutrient intake, however it is challenging to study in the field such nutritional strategies in wild populations. We explored the nutritional strategy of a generalist social primate, the blue monkey (Cercopithecus mitis). We hypothesized that females balance the intake of nutrients, specifically non-protein energy and available protein (hereafter, protein), both on a daily and long-term basis. When balancing was not possible, we expected subjects to prioritize constant protein intake, allowing non-protein energy to vary more. To understand the ecology of nutrient balancing, we examined how habitat use, food availability, diet composition, social dominance rank and reproductive demand influenced nutrient intake. Over 9 months, we conducted 371 all-day focal follows on 24 adult females in Kakamega Forest, Kenya. Subjects exhibited short- and long-term nutritional strategies. On a daily basis, they balanced non-protein energy to protein intake but when balancing was impossible, monkeys prioritized protein intake. Over the long-term, they balanced non-protein energy:protein intake in a 3.8:1 ratio. The ratio related positively to fruit in the diet and negatively to time in near-natural forest, but we found no evidence that it related to food availability, reproductive demand, or dominance rank. Lower-ranked females had broader daily diets, however, which may reflect behavioral feeding strategies to cope with social constraints. Overall, females prioritized daily protein, allowing less variation in protein intake than other aspects such as non-protein energy:protein ratio and non-protein energy intake. The emerging pattern of nutrient balancing in primates suggests that diverse dietary strategies evolved to allow adherence to a balance of non-protein energy:protein despite various social and environmental constraints. The data from this study also add to a small but growing number of studies that document nutritional strategies in wild animal populations.

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