Maintaining social cohesion is a more important determinant of patch residence time than maximizing food intake rate in a group-living primate, Japanese macaque (Macaca fuscata).

Animals have been assumed to employ an optimal foraging strategy (e.g., rate-maximizing strategy). In patchy food environments, intake rate within patches is positively correlated with patch quality, and declines as patches are depleted through consumption. This causes patch-leaving and determines patch residence time. In group-foraging situations, patch residence times are also affected by patch sharing. Optimal patch models for groups predict that patch residence times decrease as the number of co-feeding animals increases because of accelerated patch depletion. However, group members often depart patches without patch depletion, and their patch residence time deviates from patch models. It has been pointed out that patch residence time is also influenced by maintaining social proximity with others among group-living animals. In this study, the effects of maintaining social cohesion and that of rate-maximizing strategy on patch residence time were examined in Japanese macaques (Macaca fuscata). I hypothesized that foragers give up patches to remain in the proximity of their troop members. On the other hand, foragers may stay for a relatively long period when they do not have to abandon patches to follow the troop. In this study, intake rate and foraging effort (i.e., movement) did not change during patch residency. Macaques maintained their intake rate with only a little foraging effort. Therefore, the patches were assumed to be undepleted during patch residency. Further, patch residence time was affected by patch-leaving to maintain social proximity, but not by the intake rate. Macaques tended to stay in patches for short periods when they needed to give up patches for social proximity, and remained for long periods when they did not need to leave to keep social proximity. Patch-leaving and patch residence time that prioritize the maintenance of social cohesion may be a behavioral pattern in group-living primates.

A more detailed seasonal division of the energy balance and the protein balance of Japanese macaques (Macaca fuscata) on Kinkazan Island, northern Japan.

Nakagawa (Am J Primatol 41:267-288, 1997) reported that both the gross energy and gross protein intakes of an adult female Japanese macaque (Macaca fuscata) on Kinkazan Island, northern Japan, were high in spring (March-May) and fall (September-November) and low in summer (June-August) and winter (December-February), and that these values reflected the seasonal differences in nutritional conditions (defined as whether the intakes of energy and protein satisfy the requirements). We estimated the energy balance (energy intake minus its expenditure) and the protein balance (protein intake minus its requirement) of the monkeys on Kinkazan Island every month over the course of 1 year (2004--2005) in order to verify Nakagawa's conclusions. Like Nakagawa, we found that the energy balance of the monkeys in the fall was higher than in the summer and winter, whereas the protein balance in the fall was higher than in the winter. However, we did not find that spring energy and protein values were greater than summer and winter values. We also did not find that summer protein values were low. Both the energy balance and the protein balance changed rapidly within the same season. The energy intakes and the energy balances were higher in mid-spring and mid- and late fall and lower in late spring and early summer, whereas the protein intakes and the protein balances were higher in mid-spring and mid-summer and lower in early and mid-winter. Since Japanese macaques respond to seasonal changes in food supply by changing their foods, continuous data collection with short intervals is recommended in order to accurately document the energy and protein balances of the monkeys.