Abstract

Although animals of many species have been shown to discriminate between visual-spatial arrays or auditory-temporal sequences on the basis of numerosity, most of the evidence for numerosity discrimination comes from experiments involving extensive laboratory training. Under these conditions, animals' discrimination of two numerosities depends on their ratio and is independent of their absolute value. It is an open question whether any untrained nonhuman animal spontaneously represents number in this way as do human children and adults. Here we present the results of habituation-discrimination experiments on cotton-top tamarin monkeys (Saguinus oedipus) that provide evidence for numerosity discrimination in the absence of training. Presented with auditory stimuli (speech syllables) controlled for the continuous variables of sequence duration, item duration, inter-stimulus interval, and overall energy, tamarins readily discriminated sequences of 4 vs 8, 4 vs 6, and 8 vs 12 syllables. In contrast, tamarins failed to discriminate sequences of 4 vs 5 and 8 vs 10 syllables, providing evidence that their numerosity discrimination is approximate and shows the set-size ratio signature of numerosity discrimination in humans and trained non-human animals. These results provide strong support for the hypothesis that representations of large, approximate numerosity are evolutionarily ancient and spontaneously available to non-human animals.

Citations