Chimpanzees remember the results of one-by-one addition of food items to sets over extended time periods.

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Abstract
Four chimpanzees were highly accurate in selecting the larger of two concurrent accumulations of bananas in two opaque containers over a span of 20 min. One at a time, bananas were placed into the containers, which were outside the chimpanzees' cages. The chimpanzees never saw more than one banana at a time, and there were no cues indicating the locations of the bananas after they were placed into the containers. The performance of these animals matched that of human infants and young children in similar tests. The chimpanzees were successful even when the sets to be compared were sufficiently large (5 vs. 8, 5 vs. 10, and 6 vs. 10) to cast doubt on the possibility that the chimpanzees were using an object file mechanism. These chimpanzees are the first nonhuman animals to demonstrate extended memory for accumulated quantity.