 Animal representation of approximate quantities. If this numerical system (what Dehaene has called our number sense) is part of our innate endowment, might it be evident in other primate species? Hauser and his colleagues (Hauser et al. 2003) presented cotton-top tamarins with auditory sequences of syllables of different numerosities. Like humans, monkeys orient their attention to unexpected stimuli. When they hear a sequence of syllables of an unexpected numerosity, they turn their heads toward the audio speaker from which the sounds are emanating, providing a reliable indicator of their discrimination of the novel number. The results are similar to those of the infant studies: cotton-top tamarins discriminated between sequences of syllables based on approximate numerosity alone. Moreover, discriminability depended on the ratio of the numbers, just as it does in humans. Indeed, adult tamarins showed comparable discrimination abilities to nine-month-old human babies.