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

Pokorny, Jennifer Jean (2009)

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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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