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.     

Perception of emergent configurations in humans (Homo sapiens) and chimpanzees (Pan troglodytes)

We examined the perceptions of emergent configurations in humans and chimpanzees using a target-localization task. The stimulus display consisted of a target placed among multiple identical distractors. The target and distractors were presented either solely, within congruent contexts in which salient configurations emerge, or within incongruent contexts in which salient configurations do not emerge. We found that congruent contexts had similar facilitative effects on target localization by humans and chimpanzees, whereas similar disruptive effects emerged when the stimuli were presented within incongruent contexts. When display size was manipulated, targets under the congruent-context condition were localized in a parallel manner, but those under the no-context and incongruent-context conditions were localized in a serial manner by both species. These results suggest that both humans and chimpanzees perceive emergent configurations when targets and distractors are presented within certain congruent contexts and that they process such emergent configurations preattentively.     

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.     

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     

Size matters: impact of item size and quantity on array choice by chimpanzees (Pan troglodytes)

The authors previously reported that chimpanzees (Pan troglodytes) showed a striking bias to select the larger of 2 candy arrays, despite a reversed reward contingency in which the animals received the smaller, nonselected array as a reward, except when Arabic numerals were used as stimuli. A perceptual or incentive-based interference occurred that was overcome by symbolic stimuli. The authors of the present study examined the impact of element size in choice arrays, using 1 to 5 large and small candies. Five test-sophisticated chimpanzees selected an array from the 2 presented during each trial. Their responses were not optimal, as animals generally selected arrays with larger total mass; thus, they received the smaller remaining array as a reward. When choice stimuli differed in size and quantity, element size was more heavily weighted, although choices reflected total candy mass. These results replicate previous findings showing chimpanzees' difficulties with quantity judgments under reverse reward contingencies and also show that individual item size exerts a more powerful interference effect.     

Stacking of irregularly shaped blocks in chimpanzees (Pan troglodytes) and young humans (Homo sapiens)

Stacking blocks provides a way to evaluate cognitive development in humans and other species using the same comparative measures. The present study used regular cubic blocks as well as cubic blocks with bumps on two sides. The bumps changed the physical properties of the blocks and increased the difficulty involved in stacking them. Subjects were required to choose the appropriate orientation for stacking the blocks. Three juvenile chimpanzees and 14 human children (aged 2–3 years) were tested under identical task settings in a face-to-face situation. The goal of a trial was to stack up four blocks (two cubic blocks and two cubic blocks with bumps). The results showed initial difficulty in stacking the blocks with bumps in both chimpanzees and humans. Experienced juvenile chimpanzees and humans older than 3 years became proficient at solving the task. Behavioral strategies adopted to succeed in the task were common to both species. The subjects spontaneously adopted a strategy of stacking as the last block of the tower a block with a bump facing upwards. The subjects also showed active change in the orientation of the blocks when necessary, although correct orientation changes were infrequent especially during the early phases of experiment. The results are discussed in the context of the underlying cognitive development in the domain of physical understanding in both species.     

Human gestures trigger different attentional shifts in chimpanzees (Pan troglodytes) and humans (Homo sapiens)

Brain lateralization has been the matter of extensive research over the last centuries, but it remains an unsolved issue. While hand preferences have been extensively studied, very few studies have investigated laterality of eye use in non-human primates. We examined eye preference in 14 Campbell’s monkeys (Cercopithecus c. campbelli). We assessed eye preference to look at a seed placed inside a tube using monocular vision. Eye use was recorded for 100 independent and non-rewarded trials per individual. All of the 14 monkeys showed very strong preferences in the choice of the eye used to look inside the tube (mean preference: 97.6%). Eight subjects preferred the right eye and six subjects preferred the left eye. The results are discussed in light of previous data on eye preference in primates, and compared to data on hand preference from these subjects. Our findings would support the hypothesis for an early emergence of lateralization for perceptual processes compared to manual motor functions.     

Chimpanzee (Pan troglodytes) intentional communication is not contingent upon food

Studies of great apes have revealed that they use manual gestures and other signals to communicate about distal objects. There is also evidence that chimpanzees modify the types of communicative signals they use depending on the attentional state of a human communicative partner. The majority of previous studies have involved chimpanzees requesting food items from a human experimenter. Here, these same communicative behaviors are reported in chimpanzees requesting a tool from a human observer. In this study, captive chimpanzees were found to gesture, vocalize, and display more often when the experimenter had a tool than when she did not. It was also found that chimpanzees responded differentially based on the attentional state of a human experimenter, and when given the wrong tool persisted in their communicative efforts. Implications for the referential and intentional nature of chimpanzee communicative signaling are discussed.     

Inferences about guessing and knowing by chimpanzees (Pan troglodytes)

The visual perspective-taking ability of 4 chimpanzees (Pan troglodytes) was investigated. The subjects chose between information about the location of hidden food provided by 2 experimenters who randomly alternated between two roles (the guesser and the knower). The knower baited 1 of 4 obscured cups so that the subjects could watch the process but could not see which of the cups contained the reward. The guesser waited outside the room until the food was hidden. Finally, the knower pointed to the correct cup while the guesser pointed to an incorrect one. The chimpanzees quickly learned to respond to the knower. They also showed transfer to a novel variation of the task, in which the guesser remained inside the room and covered his head while the knower stood next to him and watched a third experimenter bait the cups. The results are consistent with the hypothesis that chimpanzees are capable of modeling the visual perspectives of others.     

Spontaneous logicomathematical constructions by chimpanzees (Pan troglodytes, P. paniscus)

Two experiments investigated the spontaneous construction of precursory logicomathematical operations by human-enculturated and language-reared chimpanzees (Pan troglodytes, Pan paniscus) when they were interacting freely with objects. In experiment 1, three chimpanzees ranging in age from 6 to 18 years were presented with sets of six objects. Chimpanzees constructed equivalence, order and reversibility relations within single sets of objects as well as between two or three contemporaneous sets of objects. The chimpanzees’ logicomathematical operations were more advanced, including infrequent and minimal operations on three sets, than those of some previously investigated younger nonenculturated common chimpanzees. In experiment 2, six chimpanzees ranging in age from 6 to 21 years were presented with sets of 12 objects. Chimpanzees constructed more advanced operations on single sets, but not on contemporaneous sets. The results suggest partial convergence and partial divergence between development of logicomathematical cognition in chimpanzees and humans. Received: 20 January 1999 / Accepted after revision: 9 August 1999     

Subjective well-being is heritable and genetically correlated with dominance in chimpanzees (Pan troglodytes)

The hypothesis that subjective well-being (SWB) is heritable and genetically correlated with Dominance was tested using 128 zoo chimpanzees. Dominance was a chimpanzee-specific personality factor including items reflecting Extraversion and low Neuroticism. SWB was measured with a 4-item scale. The best behavior genetic model included additive genetic and nonshared environmental effects for SWB and Dominance, marginal matemal effects for SWB, a high genetic correlation, and a low nonshared environmental correlation. Results indicated that the shared variance between SWB and Dominance was a consequence of common genes and that the unique variance between SWB and Dominance was a consequence of the nonshared environment. These findings indicate that common genes may underlie the correlation between human personality factors and SWB.     

Birth order and hand preference in chimpanzees (Pan troglodytes): implications for pathological models of handedness in humans

The effect of birth order on hand preference was assessed in a sample of 154 captive-born chimpanzees. Subjects were classified as first, middle, or latter born using 2 classification criteria based on their birth order. Hand preference was measured using a task that elicited coordinated bimanual actions. Significant birth-order effects were found for both classification criteria, with first- and latter-born subjects exhibiting a lesser degree of right-handedness compared with middle-born subjects. These data suggest that biological rather than sociological factors play a greater role in explaining the observed birth-order effects on hand preference in humans.     

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.     

Temporal characteristics of visibility in chimpanzees (Pan troglodytes) and humans (Homo sapiens) assessed by a visual-masking paradigm

We used the visual-masking paradigm to compare temporal characteristics of chimpanzee vision with those of humans. Two types of masking experiments were conducted. One type involved masking by noise, in which the visibility of the geometric pattern target was tested with a spatially overlapping noise as the mask stimulus. The other type involved paracontrast and metacontrast masking, in which the mask stimuli flanked but did not spatially overlap the target stimuli. Temporal characteristics regarding the visibility of target stimuli, displayed as functions of temporal asynchrony between target and mask stimuli, differed with the mask type in chimpanzees as in humans. Peak deterioration in visibility occurred at the point of minimum temporal asynchrony both in forward and backward masking by noise, but was not at 0 ms temporal asynchrony when the target and mask stimuli did not spatially overlap. These results suggest that chimpanzees and humans share the underlying mechanisms in two kinds of temporal inhibition caused by spatially overlapping and non-overlapping mask stimuli.     

"Constructive" enumeration by chimpanzees (Pan troglodytes) on a computerized task

Two chimpanzees used a joystick to collect dots, one at a time, on a computer monitor (see video-clip in the electronic supplementary material), and then ended a trial when the number of dots collected was equal to the Arabic numeral presented for the trial. Both chimpanzees performed substantially and reliably above chance 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. Errors that were made were seldom discrepant from the target by more than one dot quantity, and the perceptual process subitization was ruled out as an explanation for the performance. Additionally, analyses of trial duration data indicated that the chimpanzees were responding based on the numerosity of the constructed set rather than on the basis of temporal cues. The chimpanzees' decreasing performance with successively larger target numerals, however, appeared to be based on a continuous representation of magnitude rather than a discrete representation of number. Therefore, chimpanzee counting in this type of experimental task may be a process that represents magnitudes with scalar variability in that the memory for magnitudes associated with each numeral is imperfect and the variability of responses increases as a function of the numeral's value. Accepted after revision: 11 June 2001 Electronic Publication     

Tool use by chimpanzees (Pan troglodytes) of the Arnhem Zoo community

Abstract: Chimpanzees have a large repertoire of tool-use behaviors. This study reports on the variety and the extent of tool use exhibited by the chimpanzees of the Arnhem Zoo community in The Netherlands, living in an enriched captive setting since 1971. The use of tools by 29 chimpanzees aged from 0 to 37 years was observed. We identified 13 types of tool use comparable to those found in the wild. Some of these types of tool use seem to be specific to this community, and can be explained by the ecological characteristics of this captive setting. Chimpanzees started to use tools from the age of 2 years. Young chimpanzees, from 5 to 9 years old, showed a greater repertoire of tool use than infants and adults. All types of tool use in the community have appeared by the age of 10, the age of puberty for chimpanzees. Multivariate analysis was applied for the 29 individuals by 13 types of tool use in a one-zero matrix. The results show two major categories of tool use, one in a practical or substantial context and the other in a nonpractical or play context. The subjects clustered into groups reflecting developmental stages, although there are great individual differences. In conclusion, this captive community provides a unique opportunity to clarify the details of tool use by chimpanzees.     

Urinary cortisol responses to unusual events in captive chimpanzees (Pan troglodytes)

This study investigated the urinary cortisol stress response to one known stressor (anaesthesia) and three unusual events hypothesized to result in increases in cortisol (confinement to one half of an enclosure for several days due to a hurricane, an enrichment exercise, and a change in group composition) in young chimpanzees (Pan troglodytes). Although a cortisol stress response to a variety of laboratory experiences has been documented in captive animals, it is unclear whether other types of atypical events are stressful, including those that are not necessarily negative. Cortisol was measured in 519 urine samples collected from 20 awake, unrestrained chimpanzees; individuals were compared against their own baseline values. A significant increase in urinary cortisol concentration was found as a result of the stress of anaesthesia, but no significant change in urinary cortisol resulted from the three other potential stressors. A lack of a urinary cortisol response to these events may indicate that the events were not actually stressful for the chimpanzees, but may have resulted from the limited temporal resolution of measuring cortisol excretion as an indicator of integrated secretion, or from changes in rates of agonistic behaviors.     

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.