Spontaneous number representation in semi-free-ranging rhesus monkeys

Previous research has shown that animals possess considerable numerical abilities. However, this work was based on experiments involving extensive training, a small number of captive subjects and relatively artificial testing procedures. We present the results of experiments on over 200 semi-free-ranging rhesus monkeys using a task which involves no training and mimics a natural foraging problem. The subjects observed two experimenters place pieces of apple, one at a time, into each of two opaque containers. The experimenters then walked away so that the subjects could approach. The monkeys chose the container with the greater number of apple slices when the comparisons were one versus two, two versus three, three versus four and three versus five slices. They failed at four versus five, four versus six, four versus eight and three versus eight slices. Controls established that it was the representation of number which underlay their successful choices rather than the amount of time spent placing apple pieces into the box or the volume of apple placed in the box. The failures at values greater than three slices stand in striking contrast to other animal studies where training was involved and in which far superior numerical abilities were demonstrated. The range of success achieved by rhesus monkeys in this spontaneous-number task matches the range achieved by human infants and corresponds to the range encoded in the syntax of natural languages.