Recognizing facial cues: individual discrimination by chimpanzees (Pan troglodytes) and rhesus monkeys (Macaca mulatta)

Faces are one of the most salient classes of stimuli involved in social communication. Three experiments compared face-recognition abilities in chimpanzees (Pan troglodytes) and rhesus monkeys (Macaca mulatta). In the face-matching task, the chimpanzees matched identical photographs of conspecifics' faces on Trial 1, and the rhesus monkeys did the same after 4 generalization trials. In the individual-recognition task, the chimpanzees matched 2 different photographs of the same individual after 2 trials, and the rhesus monkeys generalized in fewer than 6 trials. The feature-masking task showed that the eyes were the most important cue for individual recognition. Thus, chimpanzees and rhesus monkeys are able to use facial cues to discriminate unfamiliar conspecifics. Although the rhesus monkeys required many trials to learn the tasks, this is not evidence that faces are not as important social stimuli for them as for the chimpanzees.     

Note on hand use in the manipulation of joysticks by rhesus monkeys (Macaca mulatta) and chimpanzees (Pan troglodytes)

In a recent article MacNeilage, Studdert-Kennedy, and Lindblom (1987) proposed that nonhuman primate handedness may be contingent on the specific task requirements with visual-spatial tasks yielding left-hand preferences and fine motor tasks producing right-hand preferences. This study reports hand preferences in the manipulation of joysticks by 2 rhesus monkeys (Macaca mulatta) and 3 chimpanzees (Pan troglodytes). Reach data were also collected on these same subjects and served as a basis for comparison with preference data for manipulation of the joystick. The data indicated that all 5 subjects demonstrated significant right-hand preferences in manipulating the joystick. In contrast, no significant hand preferences were found for the reach data. Reaction time data also indicated that the right hand could perform a perceptual-motor task better than the left hand in all 5 subjects. Overall, the data indicate that reach tasks may not be sensitive enough measures to produce reliable hand preferences, whereas tasks that assess fine motor control produce significant hand preferences.     

Matching visual stimuli on the basis of global and local features by chimpanzees (Pan troglodytes) and rhesus monkeys (Macaca mulatta)

This study was designed to examine whether chimpanzees and monkeys exhibit a global-to-local precedence in the processing of hierarchically organized compound stimuli, as has been reported for humans. Subjects were tested using a sequential matching-to-sample paradigm using stimuli that differed on the basis of their global configuration or local elements, or on both perceptual attributes. Although both species were able to discriminate stimuli on the basis of their global configuration or local elements, the chimpanzees exhibited a global-to-local processing strategy, whereas the rhesus monkeys exhibited a local-to-global processing strategy. The results suggest that perceptual and attentional mechanisms underlying information-processing strategies may account for differences in learning by primates. Accepted after revision: 11 September 2001 Electronic Publication     

What meaning means for same and different: Analogical reasoning in humans (Homo sapiens), chimpanzees (Pan troglodytes), and rhesus monkeys (Macaca mulatta)

Thus far, language- and token-trained apes (e.g., D. Premack, 1976; R. K. R. Thompson, D. L. Oden, & S. T. Boysen, 1997) have provided the best evidence that nonhuman animals can solve, complete, and construct analogies, thus implicating symbolic representation as the mechanism enabling the phenomenon. In this study, the authors examined the role of stimulus meaning in the analogical reasoning abilities of three different primate species. Humans (Homo sapiens), chimpanzees (Pan troglodytes), and rhesus monkeys (Macaca mulatta) completed the same relational matching-to-sample (RMTS) tasks with both meaningful and nonmeaningful stimuli. This discrimination of relations-between-relations serves as the basis for analogical reasoning. Meaningfulness facilitated the acquisition of analogical matching for human participants, whereas individual differences among the chimpanzees suggest that meaning can either enable or hinder their ability to complete analogies. Rhesus monkeys did not succeed in the RMTS task regardless of stimulus meaning, suggesting that their ability to reason analogically, if present at all, may be dependent on a dimension other than the representational value of stimuli.     

Retrospective and prospective metacognitive judgments in rhesus macaques (Macaca mulatta)

A growing body of research suggests that some non-human animals are capable of making accurate metacognitive judgments. In previous studies, non-human animals have made either retrospective or prospective judgments (about how they did on a test or how they will do on a test, respectively). These two types of judgments are dissociable in humans. The current study tested the abilities of two rhesus macaque monkeys to make both retrospective and prospective judgments about their performance on the same memory task. Both monkeys had been trained previously to make retrospective confidence judgments. Both monkeys successfully demonstrated transfer of retrospective metacognitive judgments to the new memory task. Furthermore, both monkeys transferred their retrospective judgments to the prospective task (one, immediately, and one, following the elimination of a response bias). This study is the first to demonstrate both retrospective and prospective monitoring abilities in the same monkeys and on the same task, suggesting a greater level of flexibility in animals’ metacognitive monitoring abilities than has been reported previously.     

Affiliative vocalizations in infant rhesus macaques (Macaca mulatta).

In Experiment 1, infant rhesus monkeys (Macaca mulatta) were separated and then reunited with mothers, united with a male, or placed in an empty cage. Infants girned more when with mothers or the male than when alone. Girns declined over time when infants were united with the male. Coo rates were high when the infant was alone or with the male. Shrieks, barks, and fear-related behavior were higher with the male. In Experiment 2 the vocalizations of infants were examined during separation when alone or when mothers or a male were in the same room. Infants cooed more when mothers or a male were present. Cooing increased over time, with a greater increase in the mothers' presence. Girns were given to both mothers and males, but more were given to mothers. Coos and girns are both affiliative vocalizations but are differentially modulated as infants cease cooing when they receive contact comfort.     

Weber's Law influences numerical representations in rhesus macaques (Macaca mulatta)

We present the results of two experiments that probe the ability of rhesus macaques to match visual arrays based on number. Three monkeys were first trained on a delayed match-to-sample paradigm (DMTS) to match stimuli on the basis of number and ignore continuous dimensions such as element size, cumulative surface area, and density. Monkeys were then tested in a numerical bisection experiment that required them to indicate whether a sample numerosity was closer to a small or large anchor value. Results indicated that, for two sets of anchor values with the same ratio, the probability of choosing the larger anchor value systematically increased with the sample number and the psychometric functions superimposed. A second experiment employed a numerical DMTS task in which the choice values contained an exact numerical match to the sample and a distracter that varied in number. Both accuracy and reaction time were modulated by the ratio between the correct numerical match and the distracter, as predicted by Weber's Law.     

Dominance assertion in male chimpanzees (Pan troglodytes)

The social relations of three adult males in a captive chimpanzee group were evaluated for 12 months. The observations encompassed a period of transition in the dominance hierarchy providing an opportunity to assess behavioral patterns involved in the initiation of status changes. This study shows that dominance in the chimpanzee is expressed primarily through ritualized and non-contact behavior rather than by overt aggression. In particular, display behavior is important for indicating dominance rank, in communicating aggressive intent, and as a prelude to status struggles. The functional relationships between status, displays and other socio-sexual behavior patterns are presented.     

Referential communication by chimpanzees (Pan troglodytes)

Two experiments were conducted to assess the referential function of chimpanzee (Pan troglodytes) gestures to obtain food. The chimpanzees received 1 trial per condition. In Experiment 1 (N = 101), in full view of the chimpanzee, a banana was placed on top of 1 of 2 inverted buckets or was hidden underneath 1 of the buckets. In Experiment 2 (N = 35), 4 conditions were presented in constant order: (a) no food, no observer; (b) no food, observer present; (c) food present, no observer; and (d) food present, observer present. Gestures and visual orienting were used socially and referentially. The capacity for nonverbal reference may predate the Hominidae-Pongidae split, and the development of nonverbal reference may be independent of human species-specific adaptations for speech.     

Delay of gratification and delay maintenance by rhesus macaques (Macaca mulatta)

The authors tested the self-control of rhesus macaques by assessing if they could refrain from reaching into a food container to maximize the accumulation of sequentially delivered food items (a delay-maintenance task). Three different versions of the task varied the quantity and quality of available food items. In the first 2 versions, food items accumulated across the length of the trial until a monkey consumed the items. In the 3rd task, a single less-preferred food item preceded a single more-preferred food item. Some monkeys delayed gratification even with relatively long delays between deliveries of items. However, the data suggested that self-control, in the majority of tested individuals, was not significantly different across different task versions and that self-control by macaques was not as prevalent in these tasks as it is in chimpanzees and human children.     

Scale-model comprehension by chimpanzees (Pan troglodytes)

The ability of chimpanzees (Pan troglodytes) to recognize the correspondence between a scale model and its real-world referent was examined. In Experiments 1 and 2, an adult female and a young adult male watched as an experimenter hid a miniature model food in 1 of 4 sites in a scale model. Then, the chimpanzees were given the opportunity to find the real food item that had been hidden in the analogous location in the real room. The female performed significantly above chance, whereas the male performed at chance level. Experiments 3 and 4 tested 5 adult and 2 adolescent chimpanzees in a similar paradigm, using a scale model of the chimpanzees' outdoor area. Results indicate that some adult chimpanzees were able to reliably demonstrate an understanding of the relationship between a scale model and the larger space it represented, whereas other subjects were constrained by inefficient and unsuccessful search patterns.     

Lateral bias in infant chimpanzees (Pan troglodytes)

This study documents the presence, strength, and direction of lateralization in chimpanzees (Pan troglodytes) over the first 3 months of life. Nursery-reared chimpanzees (7 males and 5 females) were repeatedly assessed on a behavioral scale. Lateral bias was measured for 4 behaviors: hand-to-mouth, hand-to-hand, defensive grasp, and first step. Hand-to-mouth was significantly lateralized for the sample. Eight of the 10 chimpanzees that showed hand-to-mouth used the right hand. Lateral bias for defensive grasp was positively related to lateral bias both of first step and of hand-to-mouth. Lateral bias in hand-to-mouth was inversely related to lateral bias in hand-to-hand. Strength of lateralization increased as chimpanzees matured. These laterality effects in infant chimpanzees were expressed under conditions of emotional arousal. Moreover, degree of laterality may be a predictor of responsivity to stress.     

Self-control in rhesus macaques (Macaca mulatta): controlling for differential stimulus exposure

Previous research on self-control using macaques (Macaca fascicularis) showed these animals have a strong bias for a delayed, larger reinforcer (Self-control) over an immediate, smaller reinforcer (Impulsive). Typical studies of self-control have used a discrete trials methodology with a secondary discriminative stimulus during the delay periods. This results in a greater exposure to the stimulus representing the self-controlled option and may account for some of the early exclusive preference for self-control observed. The present experiment examined self-control bias in three rhesus macaques (Macaca mulatta) while controlling for differential durations of stimulus exposure. Subjects were presented stimuli via a computer monitor and made choices by touching the stimulus at which point both stimuli were removed for the delay periods. All three subjects displayed a nearly exclusive bias for the delayed, larger reinforcer self-control). These results are consistent with previous studies, despite the variations in methodology and species.     

Object individuation using property/kind information in rhesus macaques (Macaca mulatta)

Around 1 year of age, infants develop the ability to individuate objects in the absence of spatiotemporal information. Some have proposed that this capacity relies on the emergence of language and, in particular, that comprehending an object's label is required to individuate it as a particular kind. One approach to testing this hypothesis is to conduct experiments on pre-linguistic human infants. A second is to test non-linguistic animals. We followed the second approach, exploring whether semi-free-ranging rhesus macaques can individuate objects using property/kind information. To make the results most directly comparable, we adapted a reaching paradigm used to examine property/kind individuation in infants. Results from three experiments demonstrate that, like 12-month-old infants, adult rhesus macaques can use both spatiotemporal and property/kind information to individuate food objects. In a fourth experiment designed to examine which properties are used to individuate food objects, results revealed that rhesus use color, but not shape. These results, together with experiments involving different procedures, provide support for the conclusion that in the absence of linguistic abilities, some non-human primates spontaneously use property/kind information to individuate objects.     

Multimodal communication by captive chimpanzees (Pan troglodytes)

Many studies have shown that apes and monkeys are adept at cross-modal matching tasks requiring the subject to identify objects in one modality when information regarding those objects has been presented in a different modality. However, much less is known about non-human primates’ production of multimodal signaling in communicative contexts. Here, we present evidence from a study of 110 chimpanzees demonstrating that they select the modality of communication in accordance with variations in the attentional focus of a human interactant, which is consistent with previous research. In each trial, we presented desirable food to one of two chimpanzees, turning mid-way through the trial from facing one chimpanzee to facing the other chimpanzee, and documented their communicative displays, as the experimenter turned towards or away from the subjects. These chimpanzees varied their signals within a context-appropriate modality, displaying a range of different visual signals when a human experimenter was facing them and a range of different auditory or tactile (attention-getting) signals when the human was facing away from them; this finding extends previous research on multimodal signaling in this species. Thus, in the impoverished circumstances characteristic of captivity, complex signaling tactics are nevertheless exhibited by chimpanzees, suggesting continuity in intersubjective psychological processes in humans and apes.     

Spontaneous spatial constructions by chimpanzees (Pan troglodytes,Pan paniscus)

We investigated chimpanzees’ spontaneous spatial constructions with objects. Two were common chimpanzees ages 6 and 18 years and one was an 11‐year‐old bonobo raised in a very enriched human environment from very early in life. Chimpanzees’ products and procedures were quite advanced when constructing functional spatial relations by placing objects in or on each other, but not when constructing nonfunctional spatial relations by placing objects next to each other. This difference was found in all their constructions, including their spatial correspondences and spatial symmetries. Compared to those of younger nonencultured chimpanzees all their spatial constructions were more advanced. Compared to human infants, their functional spatial constructions were also more advanced but their nonfunctional spatial constructions were less advanced.     

A group-specific arbitrary tradition in chimpanzees (Pan troglodytes)

Social learning in chimpanzees has been studied extensively and it is now widely accepted that chimpanzees have the capacity to learn from conspecifics through a multitude of mechanisms. Very few studies, however, have documented the existence of spontaneously emerged traditions in chimpanzee communities. While the rigour of experimental studies is helpful to investigate social learning mechanisms, documentation of naturally occurring traditions is necessary to understand the relevance of social learning in the real lives of animals. In this study, we report on chimpanzees spontaneously copying a seemingly non-adaptive behaviour (“grass-in-ear behaviour”). The behaviour entailed chimpanzees selecting a stiff, straw-like blade of grass, inserting the grass into one of their own ears, adjusting the position, and then leaving it in their ear during subsequent activities. Using a daily focal follow procedure, over the course of 1 year, we observed 8 (out of 12) group members engaging in this peculiar behaviour. Importantly, in the three neighbouring groups of chimpanzees (n = 82), this behaviour was only observed once, indicating that ecological factors were not determiners of the prevalence of this behaviour. These observations show that chimpanzees have a tendency to copy each other’s behaviour, even when the adaptive value of the behaviour is presumably absent.     

Representation of the numerosities 1-9 by rhesus macaques (Macaca mulatta)

Three rhesus monkeys (Macaca mulatta) were trained to respond to exemplars of 1, 2, 3, and 4 in an ascending, descending, or a nonmonotonic numerical order (1-->2-->3-->4, 4-->3-->2--1, 3-->1-->4-->2). The monkeys were then tested on their ability to order pairs of the novel numerosities 5-9. In Experiment 1, all 3 monkeys ordered novel exemplars of the numerosities 1-4 in ascending or descending order. The attempt to train a nonmonotonic order (3-->1-->4-->2) failed. In Experiment 2A, the 2 monkeys who learned the ascending numerical rule ordered pairs of the novel numerosities 5-9 on unreinforced trials. The monkey who learned the descending numerical rule failed to extrapolate the descending rule to new numerosities. In Experiment 2B all 3 monkeys ordered novel exemplars of pairs of the numerosities 5-9. Accuracy and latency of responding revealed distance and magnitude effects analogous to previous findings with human participants (R. S. Moyer & T. K. Landaeur, 1967). Collectively these studies show that monkeys represent the numerosities 1-9 on at least an ordinal scale.     

Sequential responding to arabic numerals with wild cards by the chimpanzee (Pan troglodytes)

One adult female chimpanzee (Pan troglodytes) was trained to respond serially to three arabic numerals between 1 and 9, presented on a cathode-ray-tube (CRT) screen. To examine the factors affecting her sequential responding behavior, wild-card items were added to the three-item sequences. When this wild-card item remained until the subject responded to the last numeral (i.e., the terminator condition), her response to the terminator at each point of the sequence was controlled by the ordinal distance between numerals. Thus, the number of responses to the terminator increased as the ordinal distance between numerals increased. When the wild-card item was eliminated by the subject’s response (wild-card conditions), the probability of responses to the wild card before the first numeral increased as a function of the serial position of the first numeral. These results were consistent with previous studies of response time and suggest both serial position and symbolic distance effects. It is suggested that the subject might form the integrated 9-item linear representations by training of possible subsets of three-item sequences. Knowledge concerning the ordinal position of each numeral was established through this training. Received: 27 October 1999 / Accepted: 22 November 1999