How the physical properties of food influence its selection by infant Japanese macaques inhabiting a snow-covered area.

Dietary differences exist between infant and adult female non-human primates. These differences are considered to be related to the low ability of infants to bite, handle, and obtain food items. This study aimed to provide a comprehensive examination of how the physical properties of food items (fracture toughness, size, processing, and height) influence food selection by infant primates. In this study, four mother-infant Japanese macaque (Macaca fuscata) pairs were investigated in a snow-covered area during winter. The feeding behavior of both mothers and infants was recorded. The size, need for processing, and height of food items were recorded by direct observation, while the fracture toughness of food items (evaluating the required bite force by incisors) was measured using a rheometer. On average, infants spent 14% less time feeding than their mothers, and there were dietary differences between mothers and infants. Compared to their mothers, infants fed relatively more frequently on food items that were small, at a low position, or that could be consumed without processing. In addition, infants spent less time feeding on food items that were tougher than 2,000 J/m(2) . Thus, infants fed relatively more frequently on food items that are easy to obtain. This food selection by infants reduced the costs of feeding and allowed them to avoid falling from high trees.