Numerical competence in a chimpanzee (Pan troglodytes)

A chimpanzee (Pan troglodytes), trained to count foods and objects by using Arabic numbers, demonstrated the ability to sum arrays of 0-4 food items placed in 2 of 3 possible sites. To address representational use of numbers, we next baited sites with Arabic numbers as stimuli. In both cases performance was significantly above chance from the first sessions, which suggests that without explicit training in combining arrays, the animal was able to select the correct arithmetic sum for arrays of foods or Arabic numbers under novel test conditions. These findings demonstrate that counting strategies and the representational use of numbers lie within the cognitive domain of the chimpanzee and compare favorably with the spontaneous use of addition algorithms demonstrated in preschool children.     

Prospective memory in a language-trained chimpanzee (Pan troglodytes)

Prospective memory involves the encoding, retention, and implementation of an intended future action. Although humans show many forms of prospective memory, less is known about the future oriented processes of nonhuman animals, or their ability to use prospective memory. In this experiment, a chimpanzee named Panzee, who had learned to associate geometric forms called lexigrams with real-world referents, was given a prospective memory test. Panzee selected between two foods the one she wanted to receive more immediately. That food was scattered in an outdoor yard where she could forage for it. Also outdoors were lexigram tokens, one of which represented the food item that remained indoors throughout a 30 min period, and that could be obtained if Panzee brought in the token that matched that food item. After foraging for the selected food item, Panzee consistently remembered to retrieve and return the correct token when food was available indoors, whereas on control trials involving no indoor food she rarely returned a token. This indicated that Panzee encoded information relevant to the future action of token retrieval after extended delays for one type of food, even when a more immediately preferred food was available.     

Imitation recognition in a captive chimpanzee (Pan troglodytes)

This study investigated the ability of a captive chimpanzee (Pan troglodytes) to recognise when he is being imitated. In the experimental condition of test 1a, an experimenter imitated the postures and behaviours of the chimpanzee as they were being displayed. In three control conditions the same experimenter exhibited (1) actions that were contingent on, but different from, the actions of the chimpanzee, (2) actions that were not contingent on, and different from, the actions of the chimpanzee, or (3) no action at all. The chimpanzee showed more "testing" sequences (i.e., systematically varying his actions while oriented to the imitating experimenter) and more repetitive behaviour when he was being imitated, than when he was not. This finding was replicated 4 months later in test 1b. When the experimenter repeated the same actions she displayed in the experimental condition of test 1a back to the chimpanzee in test 2, these actions now did not elicit those same testing sequences or repetitive behaviours. However, a live imitation condition did. Together these results provide the first evidence of imitation recognition in a nonhuman animal.     

Summation in the chimpanzee (Pan troglodytes)

In this research, we asked whether 2 chimpanzee (Pan troglodytes) subjects could reliably sum across pairs of quantities to select the greater total. Subjects were allowed to choose between two trays of chocolates. Each tray contained two food wells. To select the tray containing the greater number of chocolates, it was necessary to sum the contents of the food wells on each tray. In experiments where food wells contained from zero to four chocolates, the chimpanzees chose the greater value of the summed wells on more than 90% of the trials. In the final experiment, the maximum number of chocolates assigned to a food well was increased to five. Choice of the tray containing the greater sum still remained above 90%. In all experiments, subjects reliably chose the greater sum, even though on many trials a food well on the "incorrect" tray held more chocolates than either single well on the "correct" tray. It was concluded that without any known ability to count, these chimpanzees used some process of summation to combine spatially separated quantities. Speculation regarding the basis for summation includes consideration of perceptual fusion of pairs of quantities and subitization.     

Comparing color-word and picture-word Stroop-like effects: A test of the Glaser and Glaser (1989) model

Summary Glaser and Glaser (1989) assume that the processing of colors and pictures is highly similar in that, compared to words, both kinds of stimulis have privileged access to semantic information. This assumption was tested in the present research. In Experiment 1, the season corresponding to the color or to the word of color-word Stroop stimuli had to be named (e.g., green for spring). In Experiment 2, subjects had to name the season corresponding to the picture or the word of a picture-word stimulus (e. g., flower for spring). According to Glaser and Glaser (1989), privileged semantic processing of colors and pictures should be evidenced by a larger interfering power of color and picture distractors than of word distractors. However, the asymmetric pattern of interference was observed only with picture-word stimuli (Experiment 2), but not with color-word stimuli (Experiment 1), suggesting that, unlike pictures, colors do not have privileged access to semantic information. It was also found that word distractors interfered with the semantic processing of pictures, a result that is incompatible with the dominance rule postulated by Glaser and Glaser (1989). From these results, an adapted version of the Glaser and Glaser model is proposed: colors are assumed to have privileged access to a separate color processing system and the pattern of interference depends upon the relative activation strength of the response alternatives activated by the target and the distractor.     

Post-event misinformation effects in a language-trained chimpanzee (Pan troglodytes)

Animal Cognition - Errors of source monitoring are widespread human memory challenges, and our memories are subject to distortion upon the presentation of subsequent misinformation. Less is known...     

Development of combinatory manipulation in chimpanzee infants (Pan troglodytes)

I made systematic observations of three infant chimpanzees aged 2–4 years, who participated in a series of diagnostic tests of combinatory manipulation. The tasks were stacking blocks, seriating nesting cups, and inserting an object into the corresponding hole in a plate or a box. These tasks were originally devised for developmental diagnosis of human infants. The chimpanzee infants displayed combinatory manipulation comparable to that of 1-year-old human infants. Common motor characteristics were observed across the tasks, namely "repetition" of actions, "adjustment" of actions, "reversal" of actions, and "shifts" of attention. Humans and chimpanzees share these actions when manipulating multiple objects to complete a task. Repetition, adjustment, and reversal of actions and shifts of attention underlie higher levels of cognition common to both species. Accepted after revision: 5 May 2001 Electronic Publication     

Simian scribbles: a reappraisal of drawing in the chimpanzee (Pan troglodytes)

Three chimpanzees (Pan troglodytes) were provided with 18 different stimulus pages for drawing. The resulting 618 drawings were coded for drawn marks, and results were compared with early reports on ape drawing (Morris, 1962; Schiller, 1951) and with more recent systematic studies (Smith, 1973). The findings of the present study confirm Smith's observations of a tendency for the animals to draw closer to the center and toward the bottom of the page. No evidence for perceptual balancing was observed in drawings produced on pages with lateral figures, nor was any evidence for closure apparent with complex figures. The present results agree with earlier findings that chimpanzees will engage in drawing activities without training or reinforcement, and this behavior may reflect their intrinsic interest in exploratory and manipulative play.     

An infant chimpanzee (Pan troglodytes) follows human gaze

The ability of non-human primates to follow the gaze of other individuals has recently received much attention in comparative cognition. The aim of the present study was to investigate the emergence of this ability in a chimpanzee infant. The infant was trained to look at one of two objects, which an experimenter indicated by one of four different cue conditions: (1) tapping on the target object with a finger; (2) pointing to the target object with a finger; (3) gazing at the target object with head orientation; or (4) glancing at the target object without head orientation. The subject was given food rewards independently of its responses under the first three conditions, so that its responses to the objects were not influenced by the rewards. The glancing condition was tested occasionally, without any reinforcement. By the age of 13 months, the subject showed reliable following responses to the object that was indicated by the various cues, including glancing alone. Furthermore, additional tests clearly showed that the subject's performance was controlled by the "social" properties of the experimenter-given cues but not by the non-social, local-enhancing peripheral properties. Electronic Publication     

Tactics to obtain a hidden food item in chimpanzee pairs (Pan troglodytes)

Five dyads of chimpanzees were tested in a competitive situation, as a pilot study to examine chimpanzees' understanding of conspecifics' knowledge. A human experimenter baited one of five containers in an outdoor enclosure. Chimpanzee A (witness) could see where the food was hidden, while chimpanzee B (witness-of-witness) could not see the baited place but could observe the chimpanzee A watching the food being hidden. Then the two were released into the enclosure. This procedure was repeated for a certain number of days along with a control condition in which neither could see the baited location. The witness-of-witness developed tactics to forestall the witness in two pairs. The witness misled the witness-of-witness by taking a route to an empty container in several cases. These episodes might represent examples of deception. Tactics and counter-tactics thus developed through the interaction between the witness and the witness-of-witness, illustrating the high social intelligence of chimpanzees. An examination of the changes in tactics suggests a possibility that the witness-of-witness understands the witness's knowledge of the location of hidden food. Accepted after revision: 22 May 2001 Electronic Publication     

Correlation between menstrual cycle and cognitive performance in a chimpanzee (Pan troglodytes)

Extensive research on human subjects has tried to investigate whether there is a correlation between cognitive performance and the menstrual cycle. Less is known about the relationship between the menstrual cycle and task performance in other cognitive animals. We test whether the secretion of a sex hormone [luteinizing hormone(LH)] influences the performance of cognitive tasks by a female chimpanzee (Pan troglodytes) who is part of a long-term cognition research program. We focus on two cognitive tasks: an "easy task," which consists of simple numerical ordering, and a "difficult task," which combines numerical ordering with memorizing the numerals' spatial location. Data on the performance of these cognitive tasks, urine samples, and sexual swelling over six menstrual cycles showed that the chimpanzee's performance accuracy decreased and that the intertrial interval was longer during the LH-surge of the menstrual cycle, but only for the performance of the difficult task. These performance attributes seem to reflect a decrease in attention or motivation during ovulation. In summary, the cognitive performance of a chimpanzee was disturbed by hormonal changes despite her long-term experience in the tasks.     

Cardiac correlates of individual recognition in the chimpanzee (Pan troglodytes)

The ability of a chimpanzee to recognize individuals depicted in photographs was evaluated through the use of heart rate measures. Heart rate was recorded before and during photographic projections of human caregivers, familiar individuals, strangers, and blank control slides. In the absence of explicit training or reinforcement, the chimpanzee displayed a differential pattern of heart rate response to the stimulus categories. Although heart rate responses to all stimuli were predominantly deceleratory, the photographs of caregivers yielded consistently larger responses than other stimuli. Results indicate that the chimpanzee is able to recognize individual humans from novel photographic representations and that heart rate can provide an objective measure of such recognition.     

Use of gesture sequences in captive chimpanzee (Pan troglodytes) play

This study examined the use of sensory modalities relative to a partner’s behavior in gesture sequences during captive chimpanzee play at the Chimpanzee and Human Communication Institute. We hypothesized that chimpanzees would use visual gestures toward attentive recipients and auditory/tactile gestures toward inattentive recipients. We also hypothesized that gesture sequences would be more prevalent toward unresponsive rather than responsive recipients. The chimpanzees used significantly more auditory/tactile rather than visual gestures first in sequences with both attentive and inattentive recipients. They rarely used visual gestures toward inattentive recipients. Auditory/tactile gestures were effective with and used with both attentive and inattentive recipients. Recipients responded significantly more to single gestures than to first gestures in sequences. Sequences often indicated that recipients did not respond to initial gestures, whereas effective single gestures made more gestures unnecessary. The chimpanzees thus gestured appropriately relative to a recipient’s behavior and modified their interactions according to contextual social cues.     

Unprompted recall and reporting of hidden objects by a chimpanzee (Pan troglodytes) after extended delays

The ability to recall features of environments not present to the senses is important in human thinking, planning, and communication, but to date there are almost no data on recall capabilities in nonverbal animals. In this study, the author used symbol knowledge as a tool to study chimpanzee memory. An 11-year-old female chimpanzee (Pan troglodytes) that had already learned a large number of arbitrarily designated geometric forms (lexigrams) watched as an experimenter hid an object in the woods outside her outdoor enclosure. The type and location of the object varied across trials. After an imposed delay of up to 16 h, the chimpanzee could interact indoors with a person who did not know that an object had been hidden, let alone the type or location of the object. A keyboard in the indoor cage displayed 256 lexigrams. From Trial 1, the chimpanzee seemed to do whatever it took to catch the person's attention and then touched the lexigram corresponding to the type of object hidden, pointed outdoors, went outdoors (if followed), and continued to point manually toward the object and vocalize until the person found the object. The subject indicated nonfood objects as well as more than 20 food types.     

Enumeration of briefly presented items by the chimpanzee (Pan troglodytes) and humans (Homo sapiens)

In this study, we compared the performances on an enumeration task (numerical labeling task) of 1 chimpanzee (Pan troglodytes) and 4 humans. In this task, two types of trials, with different exposure durations of the sample that was to be enumerated, were used. In the unlimited-exposure trials, the sample remained on until the subject made a choice. In the brief-exposure trials, the sample was presented for 100 msec and then was masked. The results show clear differences between the different species. The main differences had to do with accuracy during the unlimited trials and response times during the brief trials. Detailed analyses of the pattern of response times for the chimpanzee and of looking-back behavior during the task suggests that the enumeration process underlying the subject's performance was not counting but estimation.     

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     

Addendum to "Summation in the chimpanzee (Pan troglodytes)"

Rumbaugh, Savage-Rumbaugh, and Hegel (1987) reported that two chimpanzees (Pan troglodytes) could select, with better than 90% accuracy, the greater of two paired quantities of chocolate chips. In that study, no one quantity of chocolates (from 0 through 5) was used in both pairs on a given trial. We investigated the effect of having one quantity in common (CQ) in both pairs. Whether the other quantities (OQs) of chocolates were both less than or greater than the CQ, summation still occurred. Accuracy was primarily a function of the ratios of sums to be differentiated. This finding substantiated the earlier conclusion that summation was based on both quantities of each pair and not on some simpler process such as the avoidance of the tray with the smallest single amount or selection of the tray with the single largest amount.     

Conspecific recognition in the chimpanzee (Pan troglodytes): cardiac responses to significant others

The ability of a chimpanzee (Pan troglodytes) to recognize photographs of conspecifics was evaluated with heart-rate measures. Heart rate was recorded before, during, and after viewing photographs of an aggressive chimpanzee, a friendly companion animal, and an unfamiliar chimpanzee. The subject displayed a differential pattern of heart-rate response to the stimulus animals, without prior experience with the photographs. Responses to the aggressive animal were acceleratory, which suggests a defensive response. Respiratory sinus arrhythmia suggested that this response was associated with sympathetic activation. In contrast, responses to the familiar animal were minimal, whereas cardiac deceleration was observed in response to the strange chimp, which likely reflects an orienting response. Results suggest the chimpanzee is able to recognize photographs of individual conspecifics and that heart-rate change can reflect the nature of established social relationships between chimpanzees.     

Teaching apes to ape language: explaining the imitative and nonimitative signing of a chimpanzee (Pan troglodytes)

Fifteen videotaped conversations of a chimpanzee signing with his trainers were examined in order to determine (a) whether the ape was using imitation to learn about new language forms as some human children do and (b) whether the ape's nonimitative utterances implied knowledge of linguistic structures. The answers to both questions were negative. The evidence suggests that the utterances lacked the semantic and syntactic organization found in the utterances of most children. Instead of learning to use signs as symbols for communicating propositional messages, the ape learned to use gestures as nonsymbolic instrumental responses under the stimulus control of objects in the signing context and verbal and nonverbal cues from the trainers. Other research now underway with chimpanzees may eventually reveal whether this performance is characteristic of chimpanzees in general or is the result of particular training strategies used to teach language to chimpanzees.