Different Roles of Seeds and Young Leaves in the Diet of Red Leaf Monkeys (Presbytis rubicunda): Comparisons of Availability, Nutritional Properties, and Associated Feeding Behavior

Abstract

It is important to clarify the roles of different types of foods in the diet of primates that switch to other foods as preferred food items become rare. Using red leaf monkeys (Presbytis rubicunda) in Danum Valley, Borneo, as an example, we compare the availability and nutritional properties of young leaves and seeds, as well as the monkeys’ feeding behavior, to examine food choice and how such choices may affect their competitive regime. Over 21 mo, the number of flushing trees was larger than that of fruiting trees. When we compared nutritional content for all species, including those that the monkeys did not eat, seeds contained more lipids than young leaves but young leaves did not have any nutritional advantage over seeds. However, when we compared only food species, young leaves contained more crude ash than seeds. The proportion of food species to the total available species was significantly higher for seeds than for young leaves. In addition, red leaf monkeys selected young leaf species with higher digestible protein, but no nutritional component predicted seed selection. Young leaves are nutritionally low-quality in general, so monkeys may need to select species with higher quality leaves. However, seeds generally have high-quality nutritional values, so it may not be necessary to select species with marginally higher quality. When eating seeds, the number of individuals eating in the patch was larger, and this feeding in the patch tended to last longer than when monkeys ate young leaves, suggesting more intense feeding competition when feeding on seeds. In conclusion, seeds and young leaves have different nutritional advantages as foods and red leaf monkeys were more selective when eating young leaves than when eating seeds.