Long-term retention of the differential values of Arabic numerals by chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>)

As previously reported (Beran and Rumbaugh, 2001), two chimpanzees used a joystick to collect dots, one-at-a-time, on a computer monitor, and then ended a trial when the number of dots collected was equal to the Arabic numeral presented for the trial. Here, the chimpanzees were presented with the task again after an interval of 6 months and then again after an additional interval of 3.25 years. During each interval, the chimpanzees were not presented with the task, and this allowed an assessment of the extent to which both animals retained the values of each Arabic numeral. Despite lower performance at each retention interval compared to the original study, both chimpanzees performed above chance levels in collecting a quantity of dots equal to the target numeral, one chimpanzee for the numerals 1-7, and the second chimpanzee for the numerals 1-6. For the 3.25-year retention, errors were more dispersed around each target numeral than in the original study, but the chimpanzees' performances again appeared to be based on a continuous representation of magnitude rather than a discrete representation of number. These data provide an experimental demonstration of long-term retention of the differential values of Arabic numerals by chimpanzees.     

Constructive and deconstructive tool modification by chimpanzees (Pan troglodytes)

Nine chimpanzees (Pan troglodytes) were tested for their ability to assemble or disassemble the appropriate tool to obtain a food reward from two different apparatus. In its deconstructed form, the tool functioned as a probe for one apparatus. In its constructed form, the tool functioned as a hook, appropriate for a second apparatus. Each subject completed four test trials with each apparatus type. Tool types were randomized and counter-balanced between the two forms. Results demonstrated that adult and juvenile chimpanzees (N = 7) were successful with both tool types, while two infant chimpanzees performed near chance. Off-line video analyses revealed that tool modifications followed by attempted solutions by the adults and juveniles were typically correct on the first attempt. Neither infant was successful in modifying tools correctly on the first attempt over all eight trials. The older chimpanzees’ ability to modify the appropriate tool consistently prior to use indicates an immediate recognition of the functional attributes necessary for the successful use of tool types on each apparatus, and represents a non-replication of a previously reported study by Povinelli.

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.     

Spontaneous class grouping behavior by bonobos (Pan paniscus) and common chimpanzees (P. troglodytes)

Two experiments investigated spontaneous class grouping behavior by human-enculturated and language-reared bonobos (Pan paniscus) and common chimpanzees (P. troglodytes). In experiment 1, three chimpanzees ranging in age from 6 to 18 years were presented with six objects. The objects embodied three conditions: additive, multiplicative and disjoint classes. All chimpanzees spontaneously produced single- and two-category classifying. In experiment 2, six chimpanzees ranging in age from 6 to 21 years were presented with 12 objects in the same class conditions. Chimpanzees mainly produced single-category classifying. Their two-category classifying was more rudimentary than that found in experiment 1. Chimpanzees did not produce any three-category classifying which would be necessary to construct the hierarchies that humans begin to construct during early childhood. Received: 1 February 1999 / Accepted after revision: 9 August 1999     

Acquisition of navigation by chimpanzees (Pan troglodytes) in an automated fingermaze task

These experiments investigated how chimpanzees learn to navigate visual fingermazes presented on a touch monitor. The aim was to determine whether training the subjects to solve several different mazes would establish a generalized map-reading skill such that they would solve new mazes correctly on the first presentation. In experiment 1, two captive adult female chimpanzees were trained to move a visual object (a ball) with a finger over the monitor surface toward a target through a grid of obstacles that formed a maze. The task was fully automated with storage of movement paths on individual trials. Training progressed from very simple mazes with one obstacle to complex mazes with several obstacles. The subjects learned to move the ball to the target in a curved path so as to avoid obstacles and blind alleys. After training on several mazes, both subjects developed a high level of efficiency in moving the ball to the target in a path that closely approached the ideal shortest path. New mazes were then presented to determine whether the subjects had acquired a more generalized maze-solving performance. The subjects solved 65–100% of the new mazes the first time they were presented by moving the ball around obstacles to the target without making detours into blind alleys. In experiment 2, one of the chimpanzees was trained using mazes with two routes to the target. One of the routes was blocked at one of many possible locations. After training to avoid the blind alley in different mazes, new mazes were presented that also had one route blocked. The subject correctly solved 90.7% of the novel mazes. When the mazes had one short and one long open route to the target the subject preferred the shorter route. When the short route was blocked, the subject solved only 53.3% of the mazes because of the preference for the shorter route even when blocked. The overall results suggest that with the training methods used the subjects learned to solve specific mazes with a trial-and-error method. Although both subjects were able to solve many of the novel mazes they did not fully develop a more general "map-reading" skill. Accepted after revision: 30 July 2001 Electronic Publication     

The generalization of deferred imitation in enculturated chimpanzees (Pan troglodytes)

Deferred imitation of object-related actions and generalization of imitation to similar but not identical tasks was assessed in three human-reared (enculturated) chimpanzees, ranging in age from 5 to 9 years. Each ape displayed high levels of deferred imitation and only slightly lower levels of generalization of imitation. The youngest two chimpanzees were more apt to generalize the model's actions when they had displayed portions of the target behaviors at baseline, consistent with the idea that learning is more likely to occur when working within the "zone of proximal development." We argue that generalization of imitation is the best evidence to date of imitative learning in chimpanzees. Electronic Publication     

Discrimination and categorization of photographs of natural objects by chimpanzees (Pan troglodytes)

Two experiments assessed the ability of four adult female chimpanzees to categorize natural objects. Chimpanzees were initially trained to match different color photographs of familiar objects from four possible categories. In training, all the comparison stimuli were from the same category in one condition, and from different categories in another condition. For all subjects, training performance was consistently better for the "different category" than for the "same category" trials. Probe trials were shown after training. In probe trials, the sample and positive comparison stimuli were different items from the same category, and the foils were selected from among the three other test categories. Individual performance was above chance in probe trials, suggesting that categorization by chimpanzees may transcend perceptual resemblance. These results were later replicated with novel stimulus items from the same four categories (experiment 2). Altogether, this research demonstrates that chimpanzees grouped perceptually different exemplars within the same category, and further suggests that these animals formed conceptual representations of the categories. Accepted after revision: 10 August 2001 Electronic Publication     

The use of tokens as rewards and tools by chimpanzees (Pan troglodytes)

This paper explores the effectiveness of token rewards in maintaining chimpanzees (Pan troglodytes) in working at intellectually costly tasks, and studies the "saving" behavior of the subjects, investigating the factors that can condition it. Two experiments were run. Tokens were introduced as rewards in a matching-to-sample task and used as exchange tools for food by three adult female chimpanzees. Subjects' performances were maintained at constant high levels of accuracy, suggesting that the tokens were almost equivalent to direct food rewards. The results also showed the emergence of saving behavior. The subjects spontaneously saved the tokens during the matching-to-sample task before exchanging them for food. The chimpanzees also learned a new symbolic discrimination task, with tokens as the reward. During this learning process a rarely reported phenomenon emerged: one of the subjects showed symmetry, a form of stimulus equivalence. Accepted after revision: 7 August 2001 Electronic Publication     

Structural analysis of tool-use by tufted capuchins (Cebus apella) and chimpanzees (Pan troglodytes)

Using Matsuzawa’s hierarchical system of classification, I compared tool-use patterns of tufted capuchins (Cebus apella) to those of chimpanzees (Pan troglodytes). The results indicated that wild C. apella exhibit fewer and less complex tool-use patterns than do captive C. apella and wild and captive P. troglodytes. Although most patterns of tool-use observed among P. troglodytes occur in captive C. apella, there are some notable exceptions, including tool-use in communicative contexts and the use ¶of three-tool combinations. I conclude that C. apella are unique among monkeys in their demonstrated propensities for higher-order combinatorial behavior and are likely capable of using symbolic combinations, although not at the level of complexity that has been demonstrated in ¶P. troglodytes.     

Perception of shape from shading in chimpanzees (Pan troglodytes) and humans (Homo sapiens)

The perception of shape from shading was tested in two chimpanzees (Pan troglodytes) and five humans (Homo sapiens), using visual search tasks. Subjects were required to select and touch an odd item (target) from among uniform distractors. Humans found the target faster when shading was vertical than when it was horizontal, consistent with results of previous research. Both chimpanzees showed the opposite pattern: they found the target faster when shading was horizontal. The same difference in response was found in texture segregation tasks. This difference between the species could not be explained by head rotation or head shift parallel to the surface of the monitor. Furthermore, when the shaded shape was changed from a circle to a square, or the shading type was changed from gradual to stepwise, the difference in performance between vertical and horizontal shading disappeared in chimpanzees, but persisted in humans. These results suggest that chimpanzees process shading information in a different way from humans. Received: 20 January 1998 / Accepted after revision: 30 March 1998     

Causal knowledge and imitation/emulation switching in chimpanzees (Pan troglodytes) and children (Homo sapiens)

This study explored whether the tendency of chimpanzees and children to use emulation or imitation to solve a tool-using task was a response to the availability of causal information. Young wild-born chimpanzees from an African sanctuary and 3- to 4-year-old children observed a human demonstrator use a tool to retrieve a reward from a puzzle-box. The demonstration involved both causally relevant and irrelevant actions, and the box was presented in each of two conditions: opaque and clear. In the opaque condition, causal information about the effect of the tool inside the box was not available, and hence it was impossible to differentiate between the relevant and irrelevant parts of the demonstration. However, in the clear condition causal information was available, and subjects could potentially determine which actions were necessary. When chimpanzees were presented with the opaque box, they reproduced both the relevant and irrelevant actions, thus imitating the overall structure of the task. When the box was presented in the clear condition they instead ignored the irrelevant actions in favour of a more efficient, emulative technique. These results suggest that emulation is the favoured strategy of chimpanzees when sufficient causal information is available. However, if such information is not available, chimpanzees are prone to employ a more comprehensive copy of an observed action. In contrast to the chimpanzees, children employed imitation to solve the task in both conditions, at the expense of efficiency. We suggest that the difference in performance of chimpanzees and children may be due to a greater susceptibility of children to cultural conventions, perhaps combined with a differential focus on the results, actions and goals of the demonstrator.     

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.     

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.     

Wild chimpanzees (Pan troglodytes schweinfurthii) distinguish between different scream types: evidence from a playback study

When experiencing aggression from group members, chimpanzees commonly produce screams. These agonistic screams are graded signals and vary acoustically as a function of the severity of aggression the caller is facing. We conducted a series of field playback experiments with a community of wild chimpanzees in the Budongo Forest, Uganda, to determine whether individuals could meaningfully distinguish between screams given in different agonistic contexts. We compared six subjects’ responses to screams given in response to severe and mild aggression. Subjects consistently discriminated between the two scream types. To address the possibility that the response differences were driven directly by the screams’ peripheral acoustic features, rather than any attached social meaning, we also tested the subjects’ responses to tantrum screams. These screams are given by individuals that experienced social frustration, but no physical threat, yet acoustically they are very similar to screams of victims of severe aggression. We found chimpanzees looked longer at severe victim screams than either mild victim screams or tantrum screams. Our results indicate that chimpanzees attend to the informational content of screams and are able to distinguish between different scream variants, which form part of a graded continuum.     

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     

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     

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.     

The effect of response contingencies on scale model task performance by chimpanzees (Pan troglodytes)

The effects of modified procedures on chimpanzees' (Pan troglodytes) performance in a scale model comprehension task were examined. Seven chimpanzees that previously participated in a task in which they searched an enclosure for a hidden item after watching an experimenter hide a miniature item in the analogous location in a scale model were retested under procedures incorporating response costs. In Experiment 1, chimpanzees were trained under procedures that rewarded only item retrievals occurring on the 1st search attempt. During test trials, 6 chimpanzees performed above chance, including 4 that were previously unsuccessful under the original procedures (V. A. Kuhlmeier, S. T. Boysen, & K. L. Mukobi, 1999). Experiment 2 compared performance under the new and original procedures. Results indicated that for some chimpanzees, performance depended on procedures that decreased the use of competing search strategies and encouraged strategies based on information from the scale model.     

A reversed-reward contingency task reveals causal knowledge in chimpanzees (Pan troglodytes)

In the reversed-reward contingency task, subjects are required to choose the less preferred of two options in order to obtain the more preferred one. Usually, this task is used to measure inhibitory skills, but it could also be used to measure how strong the subjects’ preferences are. We presented chimpanzees with support tasks where only one of two paper strips could physically bring food into reach. Subjects were rewarded for choosing the non-functional strip. In Experiment 1, subjects failed to pick the non-baited strip. In Experiment 2, subjects failed to pick the broken strip. Chimpanzees performed worse in these tasks than in other similar tasks where instead of paper strips, there were similar shapes painted on a platform. The fact that subjects found the reversed-reward contingency task based on causality more difficult to solve than a perceptually similar task with no causality involved (i.e., arbitrary) suggests that they did not treat real strips as an arbitrary task. Instead, they must have had some causal knowledge of the support problem that made them prefer functional over non-functional strips despite the contrary reward regime.     

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.