Associations among animals of different species vary from parasitism to mutualism. The primate order is not well represented by studies on this subject, with scarce data concentrated on few species. This study is the first attempt to describe interactions between Japanese macaques (Macaca fuscata) and sika deer (Cervus nippon), in an area where no natural predators exist.\ud Large quantities of food drop on the ground when Japanese macaques forage on the trees and deer were often observed to forage on this food items (i.e. gleaning). This observation may indicate that sika deer associated with Japanese macaque while feeding. Food quantity on a tree was the best predictor of this apparent feeding association. Yet, tree size, food quantity, and number of monkeys foraging on a tree significantly predicted the number of deer gleaning under that tree. Monkeys were rarely aggressive towards deer that were gleaning, although agonistic interactions among deer were frequently observed. Food dropped from the trees represents an additional, otherwise unavailable, source of energy for deer. Therefore, deer probably improved their foraging efficiency by gleaning. These results suggest that deer gain some benefits by this behaviour while they apparently do not inflict any costs to monkeys. As such, interactions between sika deer and Japanese macaques may represent a case of commensalisms, or of asymmetrical mutualism
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