Abstract Self-recognition in front of a mirror is used as an indicator of self-awareness. Along with humans, some chimpanzees and orangutans have been shown to be self-aware using the mark test. Monkeys are conspicuously absent from this list because they fail the mark test and show persistent signs of social responses to mirrors despite prolonged exposure, which has been interpreted as evidence of a cognitive divide between hominoids and other species. In stark contrast with those reports, the rhesus monkeys in this study, who had been prepared for electrophysiological recordings with a head implant, showed consistent self-directed behaviors in front of the mirror and showed social responses that subsided quickly during the first experimental session. The self-directed behaviors, which were performed in front of the mirror and did not take place in its absence, included extensive observation of the implant and genital areas that cannot be observed directly without a mirror. We hypothesize that the head implant, a most salient mark, prompted the monkeys to overcome gaze aversion inhibition or lack of interest in order to look and examine themselves in front of the mirror. The results of this study demonstrate that rhesus monkeys do recognize themselves in the mirror and, therefore, have some form of self-awareness. Accordingly, instead of a cognitive divide, they support the notion of an evolutionary continuity of mental functions.

Citation: Rajala AZ, Reininger KR, Lancaster KM, Populin LC (2010) Rhesus Monkeys (Macaca mulatta) Do Recognize Themselves in the Mirror: Implications for the Evolution of Self-Recognition. PLoS ONE 5(9): e12865. https://doi.org/10.1371/journal.pone.0012865 Editor: Jan Lauwereyns, Kyushu University, Japan Received: June 9, 2010; Accepted: August 26, 2010; Published: September 29, 2010 Copyright: © 2010 Rajala et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by grants from the National Science Foundation (NSF) (IOB-0517458) and National Institutes of Health (NIH) (DC003693). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction Mirror self-recognition, measured with the mark test [1], is thought to be an indicator of self-awareness [2], [3], the capacity to comprehend that one exists as an individual separate from thoughts, other individuals, and the environment. Some chimpanzees [1] and orangutans [4], like humans [5], pass the mark test and, therefore, are self-aware. Macaques, on the other hand, are thought to lack self-awareness because, with few exceptions [6], they have consistently failed the mark test and have shown persistent social responses towards mirrors [1], even after prolonged exposure [7] and training [8]. The mark test [1], the standard test for self-recognition, is performed after first exposing an animal to the mirror, during which time the behavior may change from social interactions directed towards the reflection to self-directed behaviors [1], indicating that it may have learned to recognize its reflection as its own. The actual test consists of the application of marks on the animal's face while anesthetized, then exposure to a mirror after recovery. If the animal touches the marks, acknowledging their presence on its face, it is concluded that it has passed the test and thereby verifies the observations that suggested that it recognizes itself in the mirror [1] and, therefore, is self-aware [3]. Determining that an individual of a given species, an ape or a monkey for instance, can recognize itself constitutes a monumental problem because one cannot know objectively what is the creature's cognitive process; for a human, one cannot know what he or she is thinking. The mark test is thought to provide an objective solution to this problem. By touching the mark on its face, not the mark on the mirror, the animal is thought to show not only that it has detected the presence of the mark on its face but, fundamentally, to have judged the mark as foreign to the image of itself, demonstrating, therefore, that if has a concept of self. The results of the mark test have been used to delineate a fundamental divide in cognitive function between hominoids and all other species [9], [10], but recent evidence has called this assertion into question. Some elephants [11], dolphins [12], and magpies [13] have passed the mark test thereby demonstrating that the ability to learn to recognize one's self in a mirror has evolved independently along different branches of the evolutionary tree [13]. It is important to note that despite its objectivity and the fact that it has become a benchmark, the mark test is not free from controversy [14]. For instance, it may fail to properly measure the cognitive abilities of species that do not self-groom or rely heavily on senses other than vision [15]. Furthermore, it may share the limitations of comparative studies of cognitive function that fail to distinguish between differences in ability and differences in performance [16]–[18]. It may be possible, therefore, that the monkey has fallen on the wrong side of the cognitive divide. Observations of two rhesus monkeys that had been prepared for behavioral/electrophysiological studies with a head implant led us to question the assertion that monkeys do not recognize themselves in the mirror and, therefore, lack self-awareness. These monkeys held mirrors and looked into them while grooming. The results of two experiments with mirrors of different sizes, reflectivity, and location confirmed our initial observations that indicate that these animals do in fact recognize themselves in the mirror.

Discussion Here we have shown that rhesus monkeys, though failing the mark test, demonstrate behaviors indicative of mirror self-recognition. They use the mirror to groom their head implants and inspect unseen areas of their bodies such as their genitals. Though we cannot objectively claim that these animals are self-aware, all the pieces are there to suggest that, in some form, they are. If the ability to demonstrate self-recognition were innate, as suggested by Gallup [7], and could be explained solely on evolutionary grounds, one would expect that most, if not all members of a given species would or would not pass the test [21]. As it turns out, only a fraction of chimpanzees shows signs of mirror self-recognition [20], [21]. Furthermore, one would not expect a phylogenetic gap in the expression of this ability, a conclusion derived from the fact that gorillas fail to show signs of mirror self recognition and fail the mark test [20], while orangutans, though lower evolutionarily, do [22]; but see [23], [24] for positive evidence from two different gorillas. Note: even in children the proportion that exhibit self-recognition at a particular point in development varies as a function of intelligence level, cultural background, and type of self-recognition test administered [25]–[27]. A more likely explanation, however, is that behaviors indicative of mirror self-recognition are learned by establishing a contingency between self-produced movement and the reflection. The capability to learn and establish such a contingency and the form in which it is expressed is likely to vary across species. The question arises, therefore, as to the conditions that facilitate the establishment of the contingency. Overall, the data are consistent with the saliency hypothesis [16], which postulates that an alteration in an individual's body must be highly salient to draw attention to the mirror image. Accordingly, the changes imposed on the appearance of monkeys in the standard mark test, as with more extensive markings in cotton-top tamarins [28], are not sufficient to draw the animal to touch the marks while looking in the mirror. The head implant, on the other hand, constitutes a relevant change that motivates the subject to use the mirror to inspect the area around it. The sudden onset of self-directed behaviors in front of the mirror suggests that the monkeys either developed this ability de novo as a result of the surgery or were aware that they could see themselves in the mirror but were unable, perhaps due to gaze aversion, or uninterested in looking at themselves until a sufficiently relevant change took place - implantation of the head cap, therefore, simply triggered the display of this ability. Based on the length of the exposure to mirrors of these monkeys before they received the implants (all grew up with mirrors and were exposed to them constantly throughout their lives as part of their enrichment program), we conclude that the data are consistent with the latter. We hypothesize that for the monkeys in this study the implant constituted a “super mark” that, coupled to their prior experience with the small mirror, the mobility of both mirrors, and the monkey's direct access to them, facilitated the manifestation of these behaviors. Future study should reveal what are the most effective experimental conditions, including mirror configurations and the time required to develop the contingency. The mark test [1], therefore, is an inadequate measure of self-recognition for rhesus monkeys. A similar argument can be made for the results of studies of other species that rely heavily on audition or olfaction, as the mark test relies solely on vision, because they may reveal some form of self-recognition if tested differently [15], [29]. More fundamentally, the mark test may not be enough to reveal that members of a given species are self-aware [14]. These observations, taken together, demonstrate that rhesus monkeys do recognize themselves in the mirror and, therefore, have the fundamental elements to have the capacity to be self-aware. Accordingly, we conclude that behavioral differences between hominoids and lower primates are not the result of cognitive deficits in the latter, but rather of a different position on the underlying evolutionary continuity of mental functions [6], [15], [30].

Materials and Methods Five male rhesus monkeys (Macaca mulatta) 5–13 years of age that had been exposed to mirrors as part of their enrichment throughout most of their lives were studied. A sixth monkey, who had also received a head implant, showed no interactions with the mirror and thus was not included in the study. The mirror was two-sided, set in a plastic frame measuring 3×4.75 inches, hung outside the cage, and could be swiveled. All five subjects had been prepared for behavioral/electrophysiological experiments with a head implant, the area around which was cleaned before experiments with a dry cotton swab. The implant consisted of a block of acrylic (Ortho Resin, Justi Products, Oxnard, CA) ranging in size ∼(40 mm–100 mm×40–80 mm). The acrylic was blue in color and held (1) a lightweight titanium head post used for holding a water spout in front of the animal's lips during head-unrestrained oculomotor experiments and to restrain the head to clean and care for the area surrounding the implant [31], (2) connectors for the eye coils [32] used to record eye movements with the scleral search coil technique [33], and (3) a cylinder to insert microelectrodes into the brain for physiological recordings. Two implants, one with and one without a recording cylinder are shown in Figure S1. The implants were attached to the skull of the subjects with human grade titanium screws. The animals were housed individually and provided double the space, 12.4 cu ft, typically provided for rhesus monkeys. Data were acquired in the room where the animals were housed. Video recordings followed the initial observations using a webcam without humans present. The animals continued to participate in their assigned experiments. Five one-hour sessions were videotaped for the two mirror sizes, small and large (12×24 inches set in a metal frame), and corresponding controls in which the mirrors were covered with black non-reflective plastic. The non-reflective controls were used to determine if similar behaviors took place when the monkeys could not see themselves. The data were scored offline for self-directed and social behaviors in front of the mirror according to the categories outlined in Table 2. All behaviors scored were active and purposeful, that is the monkey either positioned the mirror with his hand to look at himself or moved to attain the appropriate angle for viewing himself. These are similar to the criteria used to characterize the behavior of chimpanzees [20]. Of particular importance were self-exploration, defined as manipulation of areas not visible without use of the mirror (e.g., the anal-genital area) used to classify animals as showing positive evidence of self-recognition, and social behaviors, aggressive or appeasing gestures suggesting that the monkey sees a conspecific [20]. Three observers, aware of the hypothesis being tested, viewed and scored the first group of data collected according to the behavioral categories listed in Table 2. The formula used by Povinelli et al. [20] was used to calculate reliability where the percentage of agreement between observers = total instances of agreement/total opportunities for agreement with L.C. Populin used as the standard for comparison. A congruency between the scoring of three observers exceeded 95% in the first group of video data obtained thus only one observer scored the remaining data. The small proportion of inconsistencies among the three observers primarily comprised the length of brief behaviors such as glances into the mirror; they were resolved by consensus after frame-by-frame review of the pertinent sections of the video record. All efforts were made to ameliorate suffering of the animals. Specifically, all procedures were approved by the University of Wisconsin Animal Care Committee and were in accordance with the National Institutes of Health Guide for the Care and User of Laboratory Animals.

Supporting Information Figure S1. Head implants. (A) Basic head implant used for behavioral experiments. The acrylic holds a titanium head post and two connectors for eye coils. (B) Head implant used for physiological experiments. A recording cylinder, 19 mm in diameter, has been added to the basic implant to allow the insertion of microelectrodes. https://doi.org/10.1371/journal.pone.0012865.s001 (1.35 MB TIF) Movie S1. Self-directed behavior in front of the mirror. This movie shows a monkey waking up from a nap, then reaching for the small mirror outside his cage, positioning it to view himself, and grooming the area around the implant while looking at himself. A green mark used for the mark test, which he failed, is still visible on his left cheek. The view of the head implant has been blocked for discretion. https://doi.org/10.1371/journal.pone.0012865.s002 (0.33 MB MOV) Movie S2. Typical use of the large mirror by monkeys. This movie shows a monkey using the large mirror inside his cage to view his neighbor and to examine himself. Note the position of his right leg, which is elevated thereby exposing his genital area. For nearly one minute the monkey observes himself without signs of social behaviors directed at the mirror. https://doi.org/10.1371/journal.pone.0012865.s003 (1.10 MB MOV) Movie S3. This movie shows a monkey inspecting the lower part of his body and genitals using the small mirror. He looks over his shoulder to view his backside and genitals. Note that toward the end of the movie he reaches with his hand between his legs and pushes his genitals forward into view, confirming, therefore, that he is examining them in the mirror. This movie was recorded after the implant had been removed from this monkey. https://doi.org/10.1371/journal.pone.0012865.s004 (1.12 MB MOV) Movie S4. Use of the big mirror to inspect genitals; two clips are shown in succession. First the monkey positions the mirror, orients and lifts his left, then grabs his genitals while looking attentively. Second, the monkey directly looks between his legs, then turns toward the mirror to view the same part of his body. https://doi.org/10.1371/journal.pone.0012865.s005 (0.89 MB MOV) Movie S5. Monkey performing acrobatics in front of the mirror to view his backside and genitals. First the monkey looks between his legs while pushing his genitals with his hand. Second, he hangs upside down from the top of his cage while attempting to view his genital area from this angle. https://doi.org/10.1371/journal.pone.0012865.s006 (3.12 MB MOV)

Acknowledgments We are grateful to Chris Coe, John Harting, Steve Shelton, and Charles Snowdon for comments on an earlier version of this manuscript. We also thank C Daly, P Esser, M Gallardo, K Sayles, and K Tegt for outstanding animal care.

Author Contributions Conceived and designed the experiments: AZR KRR KML LCP. Performed the experiments: AZR KRR KML LCP. Analyzed the data: AZR KRR KML LCP. Contributed reagents/materials/analysis tools: AZR KRR KML LCP. Wrote the paper: AZR KRR KML LCP.