Choice of food patches by Japanese monkeys (Macaca fuscata)

Food patch choice was investigated in the A-troop of wild Japanese monkeys (macaca fuscata) of Kinkazan Island, Japan. The monkeys visited a Zelkova serrata tree 16 times and fed on its seeds for 43.5 min on average during the 36 day study period. The proportion of fallen seeds to total fallen seeds, the feeding speed, and the number of monkeys feeding on seeds in the crown and on the ground were recorded to clarify the decision factors involved in food-patch choice: when the monkeys visit the tree, do they feed on seeds in the crown or on the ground? The monkeys appeared to the patch quality by feeding speed; they chose a high-quality patch and fed there. As a result, they seemed to maximize seed intake. However, some troop members (in many cases, low-ranking ones) avoided feeding in a high-quality but crowded patch. When the quality of the two patches was equal, the monkeys separated into two groups (in the crown and on the ground) in a ratio of 1:1 to feed. As the difference in patch quality became larger, more monkeys fed in the high-quality patch. The frequency of agonistic interactions per individual increased on the ground as the number of monkeys feeding there increased. When the cost of agonistic interactions is considered, the net benefit (energy intake minus energy consumption from agonistic interactions) per individual in a higher-quality but crowded patch may be equal to that in a lower-quality but uncrowded patch.