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Hauser, M. D., Kralik, J., Botto-Mahan, C., Garrett, M., & Oser, J. (1995). Self-recognition in primates: phylogeny and the salience of species-typical features. Proc. Natl. Acad. Sci. U.S.A., 92(23), 10811–10814.
Abstract: Self-recognition has been explored in nonlinguistic organisms by recording whether individuals touch a dye-marked area on visually inaccessible parts of their face while looking in a mirror or inspect parts of their body while using the mirror's reflection. Only chimpanzees, gorillas, orangutans, and humans over the age of approximately 2 years consistently evidence self-directed mirror-guided behavior without experimenter training. To evaluate the inferred phylogenetic gap between hominoids and other animals, a modified dye-mark test was conducted with cotton-top tamarins (Saguinus oedipus), a New World monkey species. The white hair on the tamarins' head was color-dyed, thereby significantly altering a visually distinctive species-typical feature. Only individuals with dyed hair and prior mirror exposure touched their head while looking in the mirror. They looked longer in the mirror than controls, and some individuals used the mirror to observe visually inaccessible body parts. Prior failures to pass the mirror test may have been due to methodological problems, rather than to phylogenetic differences in the capacity for self-recognition. Specifically, an individual's sensitivity to experimentally modified parts of its body may depend crucially on the relative saliency of the modified part (e.g., face versus hair). Moreover, and in contrast to previous claims, we suggest that the mirror test may not be sufficient for assessing the concept of self or mental state attribution in nonlinguistic organisms.
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Santos, L. R., & Hauser, M. D. (1999). How monkeys see the eyes: cotton-top tamarins' reaction to changes in visual attention and action. Anim. Cogn., 2(3), 131–139.
Abstract: Among social species, the capacity to detect where another individual is looking is adaptive because gaze direction often predicts what an individual is attending to, and thus what its future actions are likely to be. We used an expectancy violation procedure to determine whether cotton-top tamarins (Saguinus oedipus oedipus) use the direction of another individual's gaze to predict future actions. Subjects were familiarized with a sequence in which a human actor turned her attention toward one of two objects sitting on a table and then reached for that object. Following familiarization, subjects saw two test events. In one test event, the actor gazed at the new object and then reached for that object. From a human perspective, this event is considered consistent with the causal relationship between visual attention and subsequent action, that is, grabbing the object attended to. In the second test event, the actor gazed at the old object, but reached for the new object. This event is considered a violation of expectation. When the actor oriented with both her head-and-eyes, subjects looked significantly longer at the second test event in which the actor reached for the object to which she had not previously oriented. However, there was no difference in looking time between test events when the actor used only her eyes to orient. These findings suggest that tamarins are able to use some combination of head orientation and gaze direction, but not gaze direction alone, to predict the actions of a human agent.
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Santos, L. R., Miller, C. T., & Hauser, M. D. (2003). Representing tools: how two non-human primate species distinguish between the functionally relevant and irrelevant features of a tool. Anim. Cogn., 6(4), 269–281.
Abstract: Few studies have examined whether non-human tool-users understand the properties that are relevant for a tool's function. We tested cotton-top tamarins (Saguinus oedipus) and rhesus macaques (Macaca mulatta) on an expectancy violation procedure designed to assess whether these species make distinctions between the functionally relevant and irrelevant features of a tool. Subjects watched an experimenter use a tool to push a grape down a ramp, and then were presented with different displays in which the features of the original tool (shape, color, orientation) were selectively varied. Results indicated that both species looked longer when a newly shaped stick acted on the grape than when a newly colored stick performed the same action, suggesting that both species perceive shape as a more salient transformation than color. In contrast, tamarins, but not rhesus, attended to changes in the tool's orientation. We propose that some non-human primates begin with a predisposition to attend to a tool's shape and, with sufficient experience, develop a more sophisticated understanding of the features that are functionally relevant to tools.
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Santos, L. R., Pearson, H. M., Spaepen, G. M., Tsao, F., & Hauser, M. D. (2006). Probing the limits of tool competence: experiments with two non-tool-using species (Cercopithecus aethiops and Saguinus oedipus). Anim. Cogn., 9(2), 94–109.
Abstract: Non-human animals vary in their ability to make and use tools. The goal of the present study was to further explore what, if anything, differs between tool-users and non-tool-users, and whether these differences lie in the conceptual or motor domain. We tested two species that typically do not use tools-cotton top tamarins (Saguinus oedipus) and vervet monkeys (Cercopithecus aethiops)-on problems that mirrored those designed for prolific tool users such as chimpanzees. We trained subjects on a task in which they could choose one of two canes to obtain an out-of-reach food reward. After training, subjects received several variations on the original task, each designed to examine a specific conceptual aspect of the pulling problem previously studied in other tool-using species. Both species recognized that effective pulling tools must be made of rigid materials. Subsequent conditions revealed significant species differences, with vervets outperforming tamarins across many conditions. Vervets, but not tamarins, had some recognition of the relationship between a tool's orientation and the position of the food reward, the relationship between a tool's trajectory and the substance that it moves on, and that tools must be connected in order to work properly. These results provide further evidence that tool-use may derive from domain-general, rather than domain-specific cognitive capacities that evolved for tool use per se.
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Santos, L. R., Rosati, A., Sproul, C., Spaulding, B., & Hauser, M. D. (2005). Means-means-end tool choice in cotton-top tamarins (Saguinus oedipus): finding the limits on primates' knowledge of tools. Anim. Cogn., 8(4), 236–246.
Abstract: Most studies of animal tool use require subjects to use one object to gain access to a food reward. In many real world situations, however, animals perform more than one action in sequence to achieve their goals. Of theoretical interest is whether animals have the cognitive capacity to recognize the relationship between consecutive action sequences in which there may be one overall goal and several subgoals. Here we ask if cotton-top tamarins, a species that in captivity uses tools to solve means-end problems, can go one step further and use a sequence of tools (means) to obtain food (end). We first trained subjects to use a pulling tool to obtain a food reward. After this initial training, subjects were presented with problems in which one tool had to be used in combination with a second in order to obtain food. Subjects showed great difficulty when two tools were required to obtain the food reward. Although subjects attended to the connection between the tool and food reward, they ignored the physical connection between the two tools. After training on a two-tool problem, we presented subjects with a series of transfer tests to explore if they would generalize to new types of connections between the tools. Subjects readily transferred to new connections. Our results therefore provide the first evidence to date that tamarins can learn to solve problems involving two tools, but that they do so only with sufficient training.
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Stevens, J. R., Wood, J. N., & Hauser, M. D. (2007). When quantity trumps number: discrimination experiments in cotton-top tamarins (Saguinus oedipus) and common marmosets (Callithrix jacchus). Anim. Cogn., .
Abstract: The capacity for non-linguistic, numerical discrimination has been well characterized in non-human animals, with recent studies providing careful controls for non-numerical confounds such as continuous extent, density, and quantity. More poorly understood are the conditions under which animals use numerical versus non-numerical quantification, and the nature of the relation between these two systems. Here we test whether cotton-top tamarins and common marmosets can discriminate between two quantities on the basis of the amount of food rather than on number. In three experiments, we show that when choosing between arrays containing different numbers and sizes of food objects, both species based their decisions on the amount of food with only minor influences of numerical information. Further, we find that subjects successfully discriminated between two quantities differing by a 2:3 or greater ratio, which is consistent with the ratio limits found for numerical discrimination with this species. These studies demonstrate that non-human primates possess mechanisms that enable quantification of total amount, in addition to the numerical representations demonstrated in previous studies, with both types of quantification subject to similar processing limits.
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Weiss, D. J., Kralik, J. D., & Hauser, M. D. (2001). Face processing in cotton-top tamarins (Saguinus oedipus). Anim. Cogn., 3(4), 191–205.
Abstract: Current research on face processing in primates has focused on a few species, mostly macaques and chimpanzees; to date, only one New World monkey, the squirrel monkey, has been tested. We explored face processing, and the inversion effect in particular, in a New World primate species, the cotton-top tamarin (Saguinus oedipus). In phase 1 of our study, we trained subjects to discriminate between two faces and two scrambled faces; we then presented the tamarins with a series of novel probes in order to determine the features underlying classification. Results showed that the tamarins relied on the external contour of the face for discrimination more than the internal features and their configuration. Statistical analyses revealed no differences in accuracy or response times to upright versus inverted stimuli, and thus no inversion effect. In phase 2, we provided subjects with additional training on the face versus scrambled face discrimination task in order to focus their attention on the configuration of the internal features. Accuracy data revealed individual differences in how tamarins classified these stimuli, even though each subject was trained in the same way. In phase 3, we tested for generalization to a new set of face stimuli, as well as for the capacity to show an inversion effect. For one subject who attended to the configuration of internal features, we found significant evidence of generalization, but no evidence for an inversion effect.
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Wood, J. N., Glynn, D. D., Phillips, B. C., & Hauser, M. D. (2007). The Perception of Rational, Goal-Directed Action in Nonhuman Primates. Science, 317(5843), 1402–1405.
Abstract: Humans are capable of making inferences about other individuals' intentions and goals by evaluating their actions in relation to the constraints imposed by the environment. This capacity enables humans to go beyond the surface appearance of behavior to draw inferences about an individual's mental states. Presently unclear is whether this capacity is uniquely human or is shared with other animals. We show that cotton-top tamarins, rhesus macaques, and chimpanzees all make spontaneous inferences about a human experimenter's goal by attending to the environmental constraints that guide rational action. These findings rule out simple associative accounts of action perception and show that our capacity to infer rational, goal-directed action likely arose at least as far back as the New World monkeys, some 40 million years ago.
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Wood, J. N., Glynn, D. D., Phillips, B. C., & Hauser, M. D. (2007). online material (Vol. 317).
Abstract: Humans are capable of making inferences about other individuals' intentions and goals by evaluating their actions in relation to the constraints imposed by the environment. This capacity enables humans to go beyond the surface appearance of behavior to draw inferences about an individual's mental states. Presently unclear is whether this capacity is uniquely human or is shared with other animals. We show that cotton-top tamarins, rhesus macaques, and chimpanzees all make spontaneous inferences about a human experimenter's goal by attending to the environmental constraints that guide rational action. These findings rule out simple associative accounts of action perception and show that our capacity to infer rational, goal-directed action likely arose at least as far back as the New World monkeys, some 40 million years ago.
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