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.

Perhaps the best-known fact about developmental speech perception is that infants are remarkably adept at discriminating phonetic contrasts. In early infancy, this ability is unaffected by language environment, resulting in the surprising fact that infants can discriminate certain foreign contrasts that their parents cannot. For example, infants from English-speaking homes can hear the difference between [ž] and [ř], two fricatives used in Czech that Englishspeaking adults have difficulty discriminating (Trehub, 1976). Infants’ advantage at foreign contrast discrimination wanes over the course of the first year, though, as they gain experience with their native language; and by the age of 12 months infants no longer discriminate those foreign contrasts (Werker & Tees, 1984). Developmental speech perception, then, can largely be described as a process of paring down previously discriminable contrasts, to just that set of contrasts that is utilized in the native language. However, though infants ’ discrimination of many phonetic contrasts exceeds adults’, there are in fact some phonetic contrasts that are difficult for

One of the longstanding issues in language research has been the extent to which the mechanisms underlying language acquisition are uniquely human. The primary goal of this article is to introduce the reader to some of the recent developments in comparative language research that have shed new light on this issue. To appreciate the significance of the new developments, we begin with a brief historical overview of language studies that have adopted a comparative approach, and then discuss a subset of the relevant theoretical accounts that seek to explain why humans are the only species capable of acquiring language. We next focus on findings from behavioral studies comparing the performance of human infants and adults with nonhuman primates on tests that tap the perceptual and learning mechanisms that are fundamental to language acquisition. We argue that in cases where the behavioral data appear similar across populations, there is a need to investigate the underlying computational abilities and units of analysis to correctly specify the degree to which the mechanisms are truly shared or are uniquely specified. Dogbert: I once read that given infinite time, a thousand monkeys with typewriters would eventually write the complete works of Shakespeare.

Recent theoretical and empirical developments in human category learning have differentiated an analytic, rule-based system of category learning from a nonanalytic system that integrates information across stimulus dimensions. The researchers applied this theoretical distinction to pigeons ’ category learning. Pigeons learned to categorize stimuli varying in the tilt and width of their internal striping. The matched category problems had either a unidimensional (rule-based) or multidimensional (information-integration) solution. Whereas humans and nonhuman primates strongly dimensionalize these stimuli and learn rule-based tasks far more quickly than information-integration tasks, pigeons learned the two tasks equally quickly to the same accuracy level. Pigeons likely represent a cognitive system in which the commitment to dimensional analysis and category rules was not strongly made. Their performance suggests the character of the ancestral vertebrate categorization system from which that of primates emerged.

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