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Social categorization in capuchin monkeys ( Cebus apella): In-group vs. out-group

Pokorny, Jennifer Jean (2009)
Dissertation (114 pages)
Committee Chair / Thesis Adviser: De Waal, Frans B M
Committee Members: Hampton, Robert ; Hopkins, Bill ; Rochat, Philippe ; Wallen, Kim
Research Fields: Psychology, Psychobiology
Keywords: face recognition; individual recognition; primate; oddity; cognition; visual discrimination
Program: Laney Graduate School, Psychology (Neuroscience and Animal Behavior)
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Social categorization in capuchin monkeys ( Cebus apella): In-group vs. out-group By Jennifer J. Pokorny The social groups of most primates consist of individuals of different ages, sexes, ranks and relatedness. This complexity requires that group members recognize and remember the individuals within ones own group. Furthermore, it is necessary to distinguish these individuals from those outside the group as outsiders pose a threat to a group's food and mating resources. Humans typically rely on faces as one way to recognize individuals, allowing us to distinguish between familiar and unfamiliar individuals, as well as group individuals into other categories, such as friend, family, stranger, male or female, young or old. A previous study in our laboratory demonstrated that capuchin monkeys ( Cebus apella), a New World primate species, could discriminate faces of conspecifics using an oddity task (Pokorny & de Waal, in press). In the current study, we examined capuchins' ability to categorize the faces of conspecifics as belonging to the in-group or out-group, relative to the subject, using the oddity paradigm. Follow-up tests ruled out alternative explanations for how subjects could solve the task, primarily addressing possible color cues in the images. Subjects successfully transferred to novel sets of images, suggesting that they used knowledge of familiar individuals depicted in the images to solve the task of selecting either in-group or out-group individuals. We also examined whether subjects were using an oddity concept to perform the task or whether they were using rules, such as: 1) select the in-group member, 2) select the out-group member. Findings reveal that subjects are likely doing the latter and not applying the oddity concept to the dimension of group membership. Overall, this study provides supporting evidence that nonhuman primates can recognize conspecifics in two-dimensional images and differentiate in-group from out-group members.

Table of Contents

Introduction 1 -- Face discrimination and recognition in nonhuman primates 1 -- Knowledge of familiar and unfamiliar conspecifics 4 -- Natural concepts 5 -- Knowledge of social relationships 9 -- Preliminary studies 11 -- Current study 15 -- Characteristics of the oddity paradigm 15 -- Alternative explanations for the categorization performance of capuchin faces 18 -- Brown capuchin monkeys (Cebus apella) 19 -- General Methods 20 -- Subjects and housing 20 -- Apparatus 21 -- Stimuli 21 -- Procedure 22 -- Previous training and experience 25 -- Data collection and analysis 25 -- Experiment 1: Group Membership Task 27 -- Procedure 27 -- Stimuli 28 -- Results Experiment 1 28 -- Discussion Experiment 1 34 -- Methods Experiment 2: Alternatives for group membership categorization - Generalize the identity of the depicted individual 37 -- Procedure 38 -- Specific predictions 39 -- Results Experiment 2 39 -- Discussion Experiment 2 43 -- Methods Experiment 3: Alternatives for group membership categorization - Color cues - grayscale 45 -- Procedure 45 -- Specific predictions 47 -- Results Experiment 3 47 -- a) Analysis of color or luminance differences 47 -- b) Testing with grayscale images 52 -- Discussion Experiment 3 66 -- Methods Experiment 4: Alternative for group membership categorization - Color cues - self 67 -- Procedure 68 -- Stimuli 68 -- Specific predictions for before and after mirror exposure 69 -- Results Experiment 4 70 -- Discussion Experiment 4 73 -- Methods Experiment 5: Categorization or oddity? 75 -- Procedure 75 -- Results Experiment 5 77 -- Discussion Experiment 5 79 -- General Discussion 80 -- References 87 -- Appendix A 100


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