In vitro digestion and fermentation of Japanese macaque (Macaca fuscata) food: The influence of food type and particle size.

Chewing is critical for herbivores to obtain nutrients. Measuring digesta particle size as the outcome of chewing can improve our understanding of the relationship between food and digestion. Previous studies of feeds of domestic animals have shown that smaller digesta particle size leads to more efficient digestion. Increased digesta particle size-either due to animal factors (e.g., a senile dentition) or to feed factors (e.g., fracture resistance) could be a sign of an animal experiencing compromised nutritional intake. However, for some primates that are dietary generalists, digesta particle size has been shown to increase when consuming preferred foods, which raises doubts about the role of chewing in digesting such foods. This uncertainty makes it difficult to understand the connection between diet, chewing, and digestion through digesta particle size in dietary generalists. In this study, using five typical food items from the Japanese macaque (Macaca fuscata) diet, we conducted in vitro digestibility and fermentation assays to explore the effects of particle size on enzymatic and microbial digestion. For the fermentation assays, we used feces from captive Japanese macaques as inoculum. Among the five food items, we found that particle size has a stronger influence on the digestibility of seeds and mature leaves compared to young leaves and pulp. The influence of particle size on the fermentation rate was stronger in pulp and seeds compared to that in leaves. The differences in physical structure, texture, digestion barriers, and soluble components may play important roles in such differences. These results support the hypothesis that reducing food particle size is less important for consuming fruits than for consuming leaves. The limited effects of particle size on digesting fruits suggest that the two fruits examined in this study are cost-effective concerning food processing and chewing.