Chimpanzees are selective about what and when they eat, suggesting the primates may give some thought to the quality of their food, a U.S. researcher says. "There is an association between the time of day primates eat certain resources and the nutritional quality of those resources, suggesting consumption may track nutrient content," said Bryce Carlson, a Purdue University professor of anthropology who studies primate ecology and nutrition in human evolution. Chimpanzees -- whose diet includes fruit, leaves, plant stalks, roots and insects -- frequently consume more leaves at the end of the day than at any other time, he said. "We can't say for sure if chimpanzees are consciously selecting the leaves when nutritional content is greatest, but this correlation presents an intriguing hypothesis to explain feeding behavior in this primate species and mechanisms for ingestive behavior in general," Carlson said in a Purdue release Thursday. Some researchers have proposed the animals prefer eating leaves late in the day to feel full and facilitate greater nutrient absorption overnight, however the exact reason remains unknown, Carlson said. "But we know there is a correlation between nutritional quality and daily feeding patterns for other animals, such as domesticated sheep," he said. "So we wanted to take a closer look at chimpanzees by comparing the primates' feeding habits to the nutritional composition of these leaves throughout the day." In one variety of leaf in the chimps diet, Pterygota mildbraedii, the leaves' hemicellulose -- a more digestible fiber -- and nonstructural carbohydrates such as simple sugars and starch increased from 15 percent to 100 percent from morning to evening, the study found. "If these sugars or total non-structural carbohydrates are increasing, then the leaves are returning more calories late in the day," Carlson said. "At this time, they may taste sweeter, and the chimpanzees may then learn and adjust their feeding behavior accordingly."