Chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) quantify split solid objects

Recent research suggests that gorillas’ and orangutans’ object representations survive cohesion violations (e.g., a split of a solid object into two halves), but that their processing of quantities may be affected by them. We assessed chimpanzees’ (Pan troglodytes) and bonobos’ (Pan paniscus) reactions to various fission events in the same series of action tasks modelled after infant studies previously run on gorillas and orangutans (Cacchione and Call in Cognition 116:193–203, 2010b). Results showed that all four non-human great ape species managed to quantify split objects but that their performance varied as a function of the non-cohesiveness produced in the splitting event. Spatial ambiguity and shape invariance had the greatest impact on apes’ ability to represent and quantify objects. Further, we observed species differences with gorillas performing lower than other species. Finally, we detected a substantial age effect, with ape infants below 6 years of age being outperformed by both juvenile/adolescent and adult apes.     

Chimpanzees (Pan troglodytes) accurately compare poured liquid quantities

Although many studies have shown that nonhuman animals can choose the larger of two discrete quantities of items, less emphasis has been given to discrimination of continuous quantity. These studies are necessary to discern the similarities and differences in discrimination performance as a function of the type of quantities that are compared. Chimpanzees made judgments between continuous quantities (liquids) in a series of three experiments. In the first experiment, chimpanzees first chose between two clear containers holding differing amounts of juice. Next, they watched as two liquid quantities were dispensed from opaque syringes held above opaque containers. In the second experiment, one liquid amount was presented by pouring it into an opaque container from an opaque syringe, whereas the other quantity was visible the entire time in a clear container. In the third experiment, the heights at which the opaque syringes were held above opaque containers differed for each set, so that sometimes sets with smaller amounts of juice were dropped from a greater height, providing a possible visual illusion as to the total amount. Chimpanzees succeeded in all tasks and showed many similarities in their continuous quantity estimation to how they performed previously in similar tasks with discrete quantities (for example, performance was constrained by the ratio between sets). Chimpanzees could compare visible sets to nonvisible sets, and they were not distracted by perceptual illusions created through various presentation styles that were not relevant to the actual amount of juice dispensed. This performance demonstrated a similarity in the quantitative discrimination skills of chimpanzees for continuous quantities that matches that previously shown for discrete quantities.     

Choice Between Two Discrimination Tasks in Chimpanzees (Pan troglodytes)

The purpose of the present study was to investigate the effects of cognitive effort on a chimpanzee's choice between discrimination tasks. The tasks were of the matching-to-sample type. In Experiment 1, the chimpanzee was required to choose between a task providing a high reinforcement rate with high cognitive effort and a task providing a low reinforcement rate with low cognitive effort. Her choice depended on the reinforcement rate more than the cognitive effort. Experiments 2, 3, and 4 examined whether the kind of cognitive effort influenced the chimpanzee's choice. Experiment 2 manipulated the sample-choice relationship, so-called symmetry in the matching-to-sample tasks. Experiment 3 manipulated the number of comparison stimuli. Experiment 4 manipulated the delay interval between the sample and the comparison stimuli. In Experiments 2 and 3, the effects of the cognitive effort were confounded with those of the reinforcement rates. However, in one condition of Experiment 4, the chimpanzee preferred the simultaneous task to the delayed task, despite the lack of a significant difference in reinforcement rates between the two tasks. The present study demonstrated that the chimpanzee's choice of discrimination task depended on the cognitive effort involved, in addition to the reinforcement rate.     

Chimpanzees (Pan troglodytes) conceal visual and auditory information from others

Chimpanzees (Pan troglodytes) competed with a human for food. The human sat inside a booth, with 1 piece of food to her left and 1 to her right, which she could retract from her chimpanzee competitor's reach as needed. In Experiment 1, chimpanzees could approach either side of the booth unseen but then had to reach through 1 of 2 tunnels (1 clear, 1 opaque) for the food. In Experiment 2, both tunnels were clear and the human was looking away, but 1 of the tunnels made a loud noise when it was opened. Chimpanzees preferentially reached through the opaque tunnel in the first study and the silent tunnel in the second, successfully concealing their taking of the food from the human competitor in both cases. These results suggest that chimpanzees can, in some circumstances, actively manipulate the visual and auditory perception of others by concealing information from them.     

Chimpanzees (Pan troglodytes) know what can be seen from where

Visual perspective taking research has established that chimpanzees recognize what others can or cannot see in the presence of occluding barriers. Less is known about chimpanzees’ appreciation of what they themselves can or cannot see in similar situations. Additionally, it is unclear whether chimpanzees must rely on others’ gaze cues to solve such tasks or whether they have a more general appreciation of what can be seen from where. Hence, we investigated chimpanzees’ ability to anticipate what they would or would not be able to see from different visual perspectives. Food was hidden among arrays of open containers, with different containers providing visual access from unique viewing perspectives. Chimpanzees immediately adopted the correct perspective for each container type. Follow-up experiments showed that they were not simply moving to align themselves with visible openings. Our study thus suggests that chimpanzees have good visual perspective taking abilities with regard to themselves as well as others, and that both likely reflect a more general knowledge, at least implicit, of what can be seen from where.

Chimpanzees (Pan troglodytes) remember the location of a hidden food item after altering their orientation to a spatial array

Two chimpanzees (Pan troglodytes) had a direct view of an experimenter placing a food item beneath one of several cups within a horizontal spatial array. The chimpanzees then were required to move around the spatial array, shifting their orientation to the array by 180 degrees . Both chimpanzees remembered the location of the food item. In the next experiment, a visual barrier was placed between the chimpanzees and the spatial array after the food item had been hidden to prevent visual tracking of the location of the object during the chimpanzees' movement. One chimpanzee remembered the location of the hidden item in this variation. These results demonstrate another capacity for spatial memory in this species that complements data indicating chimpanzee spatial memory for invisible displacements, array rotations, and array transpositions.     

Chimpanzees (Pan troglodytes) detect strange body parts: an eye-tracking study

Animal Cognition - This study investigated chimpanzee body representation by testing whether chimpanzees detect strangeness in body parts. We tested six chimpanzees with edited chimpanzee body...     

Erratum to: Chimpanzees (Pan troglodytes) accurately compare poured liquid quantities

Although many studies have shown that nonhuman animals can choose the larger of two discrete quantities of items, less emphasis has been given to discrimination of continuous quantity. These studies are necessary to discern the similarities and differences in discrimination performance as a function of the type of quantities that are compared. Chimpanzees made judgments between continuous quantities (liquids) in a series of three experiments. In the first experiment, chimpanzees first chose between two clear containers holding differing amounts of juice. Next, they watched as two liquid quantities were dispensed from opaque syringes held above opaque containers. In the second experiment, one liquid amount was presented by pouring it into an opaque container from an opaque syringe, whereas the other quantity was visible the entire time in a clear container. In the third experiment, the heights at which the opaque syringes were held above opaque containers differed for each set, so that sometimes sets with smaller amounts of juice were dropped from a greater height, providing a possible visual illusion as to the total amount. Chimpanzees succeeded in all tasks and showed many similarities in their continuous quantity estimation to how they performed previously in similar tasks with discrete quantities (for example, performance was constrained by the ratio between sets). Chimpanzees could compare visible sets to nonvisible sets, and they were not distracted by perceptual illusions created through various presentation styles that were not relevant to the actual amount of juice dispensed. This performance demonstrated a similarity in the quantitative discrimination skills of chimpanzees for continuous quantities that matches that previously shown for discrete quantities.     

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.     

Chimpanzees (Pan troglodytes) and the question of cumulative culture: an experimental approach

There is increasing evidence for cultural variations in behaviour among non-human species, but human societies additionally display elaborate cumulative cultural evolution, with successive generations building on earlier achievements. Evidence for cumulative culture in non-human species remains minimal and controversial. Relevant experiments are also lacking. Here we present a first experiment designed to examine chimpanzees' capacity for cumulative social learning. Eleven young chimpanzees were presented with a foraging device, which afforded both a relatively simple and a more complex tool-use technique for extracting honey. The more complex 'probing' technique incorporated the core actions of the simpler 'dipping' one and was also much more productive. In a baseline, exploration condition only two subjects discovered the dipping technique and a solitary instance of probing occurred. Demonstrations of dipping by a familiar human were followed by acquisition of this technique by the five subjects aged three years or above, whilst younger subjects showed a significant increase only in the elements of the dipping technique. By contrast, subsequent demonstrations of the probing task were not followed by acquisition of this more productive technique. Subjects stuck to their habitual dipping method despite an escalating series of demonstrations eventually exceeding 200. Supplementary tests showed this technique is within the capability of chimpanzees of this age. We therefore tentatively conclude that young chimpanzees exhibit a tendency to become 'stuck' on a technique they initially learn, inhibiting cumulative social learning and possibly constraining the species' capacity for cumulative cultural evolution.     

Chimpanzees (Pan troglodytes) do not develop contingent reciprocity in an experimental task

Chimpanzees provide help to unrelated individuals in a broad range of situations. The pattern of helping within pairs suggests that contingent reciprocity may have been an important mechanism in the evolution of altruism in chimpanzees. However, correlational analyses of the cumulative pattern of interactions over time do not demonstrate that helping is contingent upon previous acts of altruism, as required by the theory of reciprocal altruism. Experimental studies provide a controlled approach to examine the importance of contingency in helping interactions. In this study, we evaluated whether chimpanzees would be more likely to provide food to a social partner from their home group if their partner had previously provided food for them. The chimpanzees manipulated a barpull apparatus in which actors could deliver rewards either to themselves and their partners or only to themselves. Our findings indicate that the chimpanzees’ responses were not consistently influenced by the behavior of their partners in previous rounds. Only one of the 11 dyads that we tested demonstrated positive reciprocity. We conclude that contingent reciprocity does not spontaneously arise in experimental settings, despite the fact that patterns of behavior in the field indicate that individuals cooperate preferentially with reciprocating partners.     

Chimpanzees (Pan troglodytes) transfer tokens repeatedly with a partner to accumulate rewards in a self-control task

There has been extensive research investigating self-control in humans and nonhuman animals, yet we know surprisingly little about how one’s social environment influences self-control. The present study examined the self-control of chimpanzees in a task that required active engagement with conspecifics. The task consisted of transferring a token back and forth with a partner animal in order to accumulate food rewards, one item per token transfer. Self-control was required because at any point in the trial, either chimpanzee could obtain their accumulated rewards, but doing so discontinued the food accumulation and ended the trial for both individuals. Chimpanzees readily engaged the task and accumulated the majority of available rewards before ending each trial, and they did so across a number of conditions that varied the identity of the partner, the presence/absence of the experimenter, and the means by which they could obtain rewards. A second experiment examined chimpanzees’ self-control when given the choice between immediately available food items and a potentially larger amount of rewards that could be obtained by engaging the token transfer task with a partner. Chimpanzees were flexible in their decision-making in this test, typically choosing the option representing the largest amount of food, even if it involved delayed accumulation of the rewards via the token transfer task. These results demonstrate that chimpanzees can exhibit self-control in situations involving social interactions, and they encourage further research into this important aspect of the self-control scenario.     

Chimpanzees (Pan troglodytes) respond to nonvisible sets after one-by-one addition and removal of items

Two chimpanzees (Pan troglodytes) made numerousness judgments of nonvisible sets of items. In Experiment 1, 1-10 items were dropped 1 at a time into an opaque cup, and then an additional 1-10 items were dropped 1 at a time into another opaque cup. The chimpanzees' performance levels were high and were more dependent on factors indicative of an analogue-magnitude mechanism for representation of set size than on an object file mechanism. In Experiment 2, a 3rd visible set was made available after the sequential presentation of the first 2 sets. The chimpanzees again performed at high levels in selecting the largest of the 3 sets. In Experiment 3, 1 of the 2 initially presented sets was reduced in number by the sequential removal of 1, 2, or 3 items. Both chimpanzees performed above chance levels for the removal of 1, but not more than 1, item.     

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.     

Maintenance of Self-Imposed Delay of Gratification by Four Chimpanzees (Pan troglodytes) and an Orangutan (Pongo pygmaeus)

Delay maintenance, which is the continuance over time of the choice to forgo an immediate, less preferred reward for a future, more preferred reward, was examined in 4 chimpanzees (Pan troglodytes) and 1 orangutan (Pongo pygmaeus). In the 1st experiment, the apes were presented with 20 chocolate pieces that were placed, one at a time, into a bowl that was within their reach. The apes could consume the available chocolate pieces at any time during a trial, but no additional pieces would be given. The total length of time taken to place the 20 items into the bowl ranged from 60 s to 180 s. All 5 apes delayed gratification on a majority of trials until all 20 chocolate pieces were presented. Unlike in most experiments with human children using this test situation, attention by the apes to the reward was not detrimental to delay maintenance. In a 2nd experiment with the chimpanzees only, 4 foods of differing incentive value were presented in different trials in the same manner as in Experiment 1. The chimpanzees were highly successful in obtaining all food pieces, and there was no difference in performance as a function of food type.     

Recognizing impossible object relations: intuitions about support in chimpanzees (Pan troglodytes)

Using looking-time measures, the authors examined untrained chimpanzees' (Pan troglodytes) ability to distinguish between adequate and inadequate support. In 3 experiments, the chimpanzees' sensitivity to different support relations between 2 objects was assessed. In each experiment, the chimpanzees saw a possible and an impossible test event, presented as digital video clips. Looking times in the 3 experiments suggest that chimpanzees use amount of contact between 2 objects, but not type of contact, to distinguish between adequate and inadequate support relations. These results indicate that chimpanzees have some intuition about support phenomena but their sensitivity to relational object properties may differ from that of human infants (Homo sapiens) in this domain.     

Acquisition and Transmission of Tool Making and Use for Drinking Juice in a Group of Captive Chimpanzees (Pan troglodytes)

The present study examined the acquisition and transmission of tool making and use in a group of chimpanzees. We set up a piece of apparatus that provided orange juice in an outdoor compound for a group of nine chimpanzees. Although they could reach the juice with their hands, eight of the nine subjects used tools. Fifteen kinds of tools in total were used, such as straw, twigs, and some kinds of leaves. The chimpanzees showed high selectivity with regard to tool type. They preferred to use Thuja occidentalis as a tool although there were 28 species of tree and several kinds of grass available in the compound. Two females initiated the use of the Thuja tool. Since then, five other individuals have begun to use it selectively. Before making the tools by themselves, these five chimpanzees first watched others using the Thuja tool for drinking juice, and then used the Thuja tool which had been used and left by another chimpanzee.     

Executive Function in Young Children and Chimpanzees (Pan troglodytes): Evidence From a Nonverbal Dimensional Change Card Sort Task

In this article the authors compared chimpanzees’ executive function with that of children. They developed a nonverbal dimensional change card sorting task, which indexed the development of executive function. Three pairs of mother and offspring chimpanzees and 30 typically developed 5-year-old children were presented with 2 target stimuli and a test stimulus comprising 2 dimensions (size and shape) on a display; they were required to sort the test stimulus according to 1 dimension (e.g., shape). After 5 consecutive correct trials, the participants had to sort the test stimulus according to the other dimension (e.g., size). The results showed that the chimpanzees often failed to sort the test stimuli according to the first and reversed dimensions. On the other hand, the children were correctly able to use both dimensions. These results indicate that chimpanzees may have less developed executive skills than children.     

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.     

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.