Mirror self-recognition in nonhuman primates: A phylogenetic approach 1

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1 Japanese Psychological Research 1997, Volume 39, No. 3, Special Issue: Cognition and behavior of chimpanzees Short Report Mirror self-recognition in nonhuman primates: A phylogenetic approach 1 NORIKO INOUE-NAKAMURA Department of Psychology, Faculty of Letters, Kwansei Gakuin University, Uegahara, Nishinomiya 662, Japan Abstract: To explore the phylogenetic origin of mirror self-recognition, the present study examined behaviors toward a mirror during a 30-min session in 12 species of nonhuman primates: prosimians (ring-tailed lemur), New World monkeys (cottontop tamarin, squirrel monkey, and capuchin), Old World monkeys (bonnet macaque, rhesus macaque, and Japanese macaque), gibbons (white-handed gibbon), and great apes (orang-utan, gorilla, bonobo, and chimpanzee). Only the great apes exhibited self-directed behaviors that were thought to be the critical evidence of mirror self-recognition. These results suggest that there is a large difference between great apes and the other nonhuman primates in terms of the capacity for mirror self-recognition. Key words: mirror self-recognition, primates, phylogeny. Gallup (1970) reported that chimpanzees were able to recognize themselves in a mirror. He further claimed that the chimpanzees had selfconcept. The chimpanzees initially showed social behaviors to the reflected image in a mirror, such as head bobbing, vocalizing, and threatening. However, the social behaviors declined rapidly over a few days. The chimpanzees then began displaying self-directed behaviors, such as grooming parts of their own body that would otherwise be visually inaccessible without the mirror image, making faces at the mirror, blowing bubbles, manipulating food wads with the lips while watching the reflection, and so on. After 10 days of exposure to the mirror, the chimpanzees were anesthetized and marked with red dye above one eyebrow ridge and at the top half of the opposite ear. After recovering, the chimpanzees touched the marks more frequently in the presence of the mirror than in the absence of the mirror. Since this landmark study, mirror self-recognition has been studied in a wide variety of primate species, including humans, and also nonprimate animals (e.g., Parker, Mitchell, & Boccia, 1994; see also review articles by Anderson, 1984; Gallup, 1982). However, in all these studies, the test procedures were different, and there has been controversy with regard to the criteria, validity, and reliability of self-recognition (Gallup, Povinelli, Suarez, Anderson, Lethmate, & Menzel, 1995; Heyes, 1994, 1995). The following two points at least must be improved in the study of mirror self-recognition. One is to perform the experiments following a standardized procedure, for example adopting the same size of mirror, presented at the same distance from 1 The study was financially supported by a fellowship from the Japan Society of Promoting Science (#0801), and a Grant-in-Aid for Scientific Research, Ministry of Education, Science, Sports, and Culture in Japan (# ). I wish to thank Professor Matsuzawa for his guidance. Special thanks are also due to Dr. S. Kotera, Y. Tanabe, A. Tanahashi, and R. Horigome, at Japan Monkey Center, and the staff of the Monkey Care Laboratory of the Primate Research Institute, Kyoto University Japanese Psychological Association. Published by Blackwell Publishers Ltd, 108 Cowley Road, Oxford OX4 1JF, UK and 350 Main Street, Malden, MA 02148, USA.

2 Mirror self-recognition 267 Table 1. Information on subjects and mirror size Focal subject Captive condition Species Sex Age a Number Sex and age b Place Mirror size Prosimians Ring-tailed lemur (Lemur catta) M 14:10 3 A(1), a(2) PRI Middle New World Cottontop tamarin (Saguinus oedipus) f 2:09 2 a(2) PRI Small monkeys Squirrel monkey (Saimiri sciureus) M 7:06 2 A(1), a(1) PRI Small Black-capped capuchin (Cebus apella) M 9:11 12 A(1), a(4), AD(1), PRI Small J(2), j(1), I(2), i(1) Old World Bonnet macaque (Macaca radiata) f 5:09 10 A(1), a(2), ad(3), J(2), PRI Middle monkeys I(1), i(1) Rhesus macaque (Macaca mulatta) f 4:10 5 J(1), ad(4) PRI Middle Japanese macaque (Macaca fuscata) M 3:10 5 A(1), a(1), AD(1), j(2) PRI Middle Gibbons White-handed gibbon (Hylobates lar) M 20 2 A(1), a(1) PRI Middle Great Apes Orang-utan (Pongo pygmaeus) f 8:10 1 ad(1) JMC Large Gorilla (Gorilla gorilla) f 12:08 2 A(1), a(1) JMC Large Bonobo (Pan paniscus) M 9:06 1 AD(1) JMC Large Chimpanzee (Pan troglodytes) M 9:10 1 AD(1) JMC Large a The first and second numbers represent years and months of the subject s age, respectively. b A = adult male, a = adult female, AD = adolescent male, ad = adolescent female, J = juvenile male, j = juvenile female, I = infant male, i = infant female, with number of subjects in parentheses. PRI, Primate Research Institute, Kyoto University. JMC, Japan Monkey Center. the subject, and for the same duration of exposure, and so on. Another is to analyze the data using a unified coding system of behavioral categories. A common coding system of this type is essential for undertaking comparative studies. It is also important to investigate the ontogeny of mirror self-recognition by means of longitudinal observations. The present author has already investigated the development of self-recognition in 17 infant chimpanzees longitudinally and cross-sectionally (Inoue, 1995). The study clearly showed that the age of onset of mirror self-recognition was dependent both on maturation and on previous experience of mirror exposure. The developmental process was characterized by a fairly rigid sequence of mirror-related behaviors, from social behaviors to exploratory behaviors, and to self-directed behaviors. The present study aims to investigate the phylogenetic origin of mirror self-recognition by means of examining mirror-related behaviors in 12 species of primate. I used a standardized test procedure. The behavioral coding system was based on that adopted in the previous study (Inoue, 1995). Method Subjects The subjects were individuals belonging to 12 nonhuman primate species: ring-tailed lemur (Lemur catta), cottontop tamarin (Saguinus oedipus), squirrel monkey (Saimiri sciureus), black-capped capuchin (Cebus apella), bonnet macaque (Macaca radiata), rhesus macaque (Macaca mulatta), Japanese macaque (Macaca fuscata fuscata), white-handed gibbon (Hylobates lar), orang-utan (Pongo pygmaeus), lowland gorilla (Gorilla gorilla gorilla), bonobo (Pan paniscus), and chimpanzee (Pan troglodytes). Table 1 presents information on subjects and

3 268 N. Inoue-Nakamura test conditions for each species. All subjects were believed to be mirror naive, according to the information given by the caretakers. Apparatus The subjects were given mirror exposure and were tested in their home cage. The cage size of each species was different, according to body size and group size. The subjects in the Primate Research Institute, Kyoto University, were cared for according to the Institute s guidelines. The subjects in the Japan Monkey Center were kept in comparable conditions. I prepared three sizes of mirror, made of light and strong acrylic: large (60 cm 90 cm), middle size (45 cm 67.5 cm), and small (30 cm 45 cm). One of them was presented to subjects, depending on their body size. The mirror was mounted on two iron pedestals, 12 mm shaft diameter, 1 m high. The apparatus was placed in front of the subjects cage, just out of reach. A video camera (Sony, CCD-TR3) was positioned 45 in a diagonal position behind the mirror to record the subjects responses toward the mirror. The camera was sometimes moved to follow the subjects movements. Procedure All subjects received a 30-min session. Before the session, the subject was exposed to the back of the mirror while the apparatus was set up in front of the cage. The session consisted of three phases in an A-B-A design: 20 min of mirror exposure, 5 min of exposure to the back of the mirror, and again 5 min of mirror exposure. No marking tests were done in the present study. Each species was tested while the subjects were in their peer groups, except for the single-caged species. The entire experiment was videotaped. Direct observations were also made to obtain written protocols of the behaviors toward the mirror. Data analysis Focal animal sampling was performed on all subjects from the video-records. I analyzed all behaviors that the subject displayed toward the mirror. The onset and offset of each mirror-related behavior were recorded to calculate duration. The total time spent in mirror-related behaviors was calculated for each subject. Each mirror-related behavior was then categorized into one of five classes, according to a mutually exclusive and exhaustive coding system: looking into mirror; social behavior; exploratory behavior; contingent behavior; and self-directed behavior. The definitions of the classes are given in Table 2. Some subjects (orang-utan, bonobo, and chimpanzee) were housed singly, while subjects in the other species were housed in groups. Further analysis used the data of the subject representing each species with the longest total duration of mirror-directed responding. Results Figure 1 shows the total time spent in mirrorrelated behaviors for each species. Mirrorrelated behaviors included all the coded behaviors. For analysis, the 12 species were divided into three major groups: prosimian and New World monkeys; Old World monkeys and gibbon; and great apes. The first and the second groups spent significantly more time engaged in mirrorrelated behaviors than the third group, using Kruskal-Wallis test, H(2, N = 12) = 6, p.05. However, individuals of two non-great apes, the cottontop tamarin and the white-handed gibbon, showed relatively little interest in the mirror. Figure 2 shows the relative amount of time that each species spent in each of the behavioral categories. The percentage of time spent in each of the categories was calculated from the total spent in mirror-related behaviors. There was a significant difference in the amount of time spent in each behavioral category between the three groups of species, using the chi-square test, χ 2 (8, N = 7,443) = 1,770, p.001. As shown in Figure 2, the group of prosimian and New World monkeys spent more time in looking into the mirror and in social behaviors. The group of Old World monkeys and gibbon spent more time in exploratory behaviors and contingent behaviors.

4 Mirror self-recognition 269 Table 2. Behavioral categories and definitions of mirror-related behaviors Categories Definitions Examples of behaviors Looking into mirror Subjects eyes were oriented to the mirror for 3 s or more Social behavior Subjects directed social behaviors at the mirror Threatening, vocalizing, sexual presentations, lip smacking, or lip flips Exploratory behavior Subjects both actively and passively showed Attempting to look over or behind exploration of the mirror the mirror, or to reach the mirror Contingent behavior Subjects made two or more repetitions Waving a hand, licking the mesh, of a bodily movement while looking into the mirror or moving upper body slowly while looking into the mirror Self-directed behavior Subjects used fingers or hands to manipulate Picking teeth, or grooming forehead parts of the body otherwise not visible while looking into the mirror while looking into the mirror 1500 Total time (s) Lc So Ss Ca Mr Mm Mf HI Pp Gg Ppa Pt Prosimian New World monkeys Old World monkeys Gibbon Great Apes Figure 1. Total time spent in mirror-related behaviors in 12 species of primate. Lc, Lemur catta; So, Saguinus oedipus; Ss, Saimiri sciureus; Ca, Cebus apella; Mr, Macaca radiata; Mm, Macaca mulatta; Mf, Macaca fuscata; Hl, Hylobates lar; Pp, Pongo pygmaeus; Gg, Gorilla gorilla; Ppa, Pan paniscus; Pt, Pan troglodytes.

5 270 N. Inoue-Nakamura 100 % of each behavioral category Lc So Ss Ca Mr Mm Mf HI Pp Gg Ppa Pt Prosimian New World monkeys Old World monkeys Gibbon Great Apes Self-directed behavior Contingent behavior Social behavior Looking into mirror Exploratory behavior Figure 2. Percentages of time spent in each behavioral category in 12 species of primate. Lc, Lemur catta; So, Saguinus oedipus; Ss, Saimiri sciureus; Ca, Cebus apella; Mr, Macaca radiata; Mm, Macaca mulatta; Mf, Macaca fuscata; Hl, Hylobates lar; Pp, Pongo pygmaeus; Gg, Gorilla gorilla; Ppa, Pan paniscus; Pt, Pan troglodytes. The group of great apes was characterized by self-directed behaviors, which were not observed in the other two groups. Figure 3 shows the time spent in each of the behavioral categories as a function of blocks of 5 min for each species. All the species except the ring-tailed lemur exhibited a marked decrease in social behaviors across blocks. On the contrary, the ring-tailed lemur increased time spent in social behaviors as a function of blocks, but it declined in the final block, where it is less than for the squirrel monkey and the bonnet macaque. In the New World monkeys, the cottontop tamarin showed some social and exploratory behaviors even in the last block of the session. The squirrel monkey showed extremely persistent social behaviors, 578 s in total. Not only the focal subject but also the other individual continued vocalizing throughout the session. The black-capped capuchin exhibited an increase in contingent behaviors, accounting for 88 s in the final block. In this sense, the mirror-related behaviors of the capuchin were different from those of the other two species of New World monkeys, and were more like those of the Old World monkeys and gibbon. Among the Old World monkeys, the bonnet macaque showed social behaviors for 48 s of the last block of the session. The bonnet macaque also showed exploratory behaviors consistently across the session. The three other species of this group (rhesus macaque, Japanese macaque, and white-handed gibbon) showed higher peaks of contingent behaviors, but these declined toward the end of the session. Among the great apes, all four species (orangutan, gorilla, bonobo, and chimpanzee) exhibited

6 Mirror self-recognition 271 Lc Social behavior Exploratory behavior Contingent behavior Self-directed behavior So Ss Ca Duration (s) Mr Mm Mf Hl Pp Gg Ppa Pt Blocks of 5 min Figure 3. Changes in the time spent in each of the categories as a function of blocks of 5 min. The fifth block was the exposure to the back of the mirror. Lc, Lemur catta; So, Saguinus oedipus; Ss, Saimiri sciureus; Ca, Cebus apella; Mr, Macaca radiata; Mm, Macaca mulatta; Mf, Macaca fuscata; Hl, Hylobates lar; Pp, Pongo pygmaeus; Gg, Gorilla gorilla; Ppa, Pan paniscus; Pt, Pan troglodytes. self-directed behaviors. The chimpanzee spent most time 168 s in total in self-directed behaviors, although he spent a lot of time in social behaviors at the beginning of the session. Thus, his behavior toward the mirror changed drastically in the 30-min session, from social behaviors to self-directed behaviors. However, self-directed behaviors decreased near the end of the session. Table 3 summarizes the results of the present study, and gives an overview of previous studies (a selection only, due to space limits)

7 272 N. Inoue-Nakamura Table 3. Summary of responses to mirror image in primates Social Exploratory Contingent Self-directed Results of the present behavior behavior behavior behavior study* and References Prosimians Ring-tail Lemur (Lemur catta) X X *; Fornasieri et al., 1991 New World monkeys Pygmy marmoset (Cebuella pygmaea) X Eglash & Snowdon, 1983 Cottontop tamarin (Saguinus oedipus) X b *; Hauser et al., 1995 Squirrel monkey (Saimiri sciureus) X X X *; Maclean, 1964 Black-capped capuchin (Cebus apella) X *; Anderson & Roeder, 1989 Spider monkey (Ateles spp.) X Lethmate & Ducker, 1973 Old World monkeys Red-faced macaque (Macaca arctoides) a X Anderson, 1983 Bonnet macaque (Macaca radiata) X * Pig-tailed macaque (Macaca nemestrina) X b Boccia, 1994; Anderson, 1986 Lion-tailed macaque (Macaca silenus) X Lethmate & Ducker, 1973 Tonkean macaque (Macaca tonkeana) X Anderson, 1986 Crab-eating macaque (Macaca fascicularis) X Gallup, 1977; Anderson, 1986 Rhesus macaque (Macaca mulatta) X *; Gallup et al., 1980 Japanese macaque (Macaca fuscata) X b *; Platt & Thompson, 1985; Itakura, 1987 Patas monkey (Erythrocebus patas) X Hall, 1962 Hamadryas baboon (Papio hamadryas) X Lethmate & Ducker, 1973 Olive baboon (Papio anubis) a X Benhar et al., 1975 Mandrill (Mandrillus sphinx) X Lethmate & Ducker, 1973 Gibbons White-handed gibbon (Hylobates lar) X *; Lethmate & Ducker, 1973 Agile gibbon (Hylobates agilis) X Lethmate & Ducker, 1973 Great apes Orang-utan (Pongo pygmaeus) *; Suarez & Gallup, 1981; Miles, 1994 Gorilla (Gorilla gorilla) *; Patterson & Cohn, 1994; Ledbetter & Basen, 1982 Bonobo (Pan paniscus) *; Hyatt & Hopkins, 1994; Savage-Rumbaugh, 1986 Chimpanzee (Pan troglodytes) *; Gallup, 1970; Povinelli et al., 1993 Human (Homo sapiens) a Amsterdam, 1972; Bertenthal & Fischer, 1978 Circles ( ) represent positive data. Crosses (X) represent negative data. Dashes ( ) represent no available data. The subjects of the present study are represented in bold. a The subjects were infants. b This behavior has been reported in a few individuals.

8 Mirror self-recognition 273 on mirror self-recognition in primates. Other authors have already attempted to make a similar table (e.g., Mitchell, 1993). The table shows clearly that prosimians show only social behaviors and exploratory behaviors. The New World monkeys and the Old World monkeys show some contingent behaviors, but do not show any evidence of self-directed behaviors. Gibbons show no self-directed behaviors. Only hominoids, the great apes and humans, display self-directed behaviors. Discussion Since Gallup (1970) conducted his landmark study, mirror self-recognition has been studied in several primate species including humans, and also in nonprimate animals such as dolphins and birds (e.g., Parker et al., 1994). In the present study, I used a standardized test procedure and a unified coding system of behavioral categorization to measure mirror self-recognition. The study examined behavior in front of a mirror in 12 species of nonhuman primate to explore the phylogenetic origin of mirror selfrecognition. Of the species studied, only the great apes displayed any self-directed behaviors. In contrast, the gibbon and Old World monkeys showed exploratory behaviors and some instances of contingent behavior. Generally speaking, social behaviors were dominant in New World monkeys and the prosimian. A number of previous studies failed to find evidence of mirror selfrecognition in various species, including those that used periods of mirror exposure spanning several days, months or even years (e.g., Anderson & Chamove, 1986; Bayart & Anderson, 1985; Gallup, 1977; Lethmate & Ducker, 1973), thus supporting the case that in spite of only limited mirror exposure the present differences are real. There seems to be a qualitative gap in mirror self-recognition between hominoids and the other primate species. These results can be considered from the perspective of the evolutionary divergence of primate species, to estimate the evolution of mirror self-directed behavior (Povinelli, 1993). A plausible reconstruction is as follows. If the mirror experiment had been possible, the common ancestor of living primates would have had a strong tendency to show social behaviors to the reflected image. Exploration of the self-image in the mirror emerged in the common ancestor of Cercopithecidae. Finally, the capacities for selfrecognition emerged in the common ancestor of Pongidae. Previous studies have revealed that developmental processes of mirror self-recognition in humans and chimpanzees show a similar sequential emergence of mirror-related behaviors, from social behaviors to exploration, and to self-awareness (Amsterdam, 1972; Inoue, 1995; Lin, Bard, & Anderson, 1992). The present study suggests parallels between the phylogeny and the ontogeny of mirrorrelated behaviors among primates, including humans. The mirror self-recognition test has been applied to various species, but the meaning of self-directed behavior is still debated. Selfdirected behavior to the reflected image in a mirror does not necessarily imply the existence of self-awareness or the self-concept. Heyes (1994, 1995) suggested the possibility that selfdirected behavior might reflect the use of novel, displaced visual feedback to guide action for body inspection. Based on the present study, however, I want to point out the practical importance of the mirror self-recognition test. A comparative approach using several species is essential for the study of the evolution of cognitive capabilities. However, there have been very few studies directly comparing the cognitive behavior of various species using a unified test. The few exceptions using primates include object-manipulation (Parker, 1974; Torigoe, 1985) and head-cocking behavior (Menzel, 1980). In this context, the mirror selfrecognition test has at least three advantages. The first is that the procedure and apparatus are simple and easily applicable to various species. Second, the measured behavior is free from constraints such as species-specific morphology or motor patterns. The third is that the test provides a common index, like

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