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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Handedness for tool use correlates with cerebellar asymmetries in chimpanzees (Pan troglodytes).

Recent studies have shown that great ape species possess patterns of macrostructural neocortical asymmetries that are similar to those found in humans. However, little is known about the asymmetry of subcortical structures in great apes. To address this lack of data, the authors assessed left-right asymmetry of the anterior and posterior aspects of cerebellum from MRI brain scans of 53 chimpanzees (Pan troglodytes). No population-level bias was found for either the anterior or the posterior region of the cerebellum. However, a significant inverse association was found in the asymmetry quotients of the anterior and posterior regions, indicating that the cerebellum was torqued at the individual level. Additionally, handedness for tool use but not other measures was associated with variation in cerebellar asymmetries. Last, older chimpanzees had a smaller cerebellum after brain volume was adjusted for. The results are discussed in the context of brain changes in primate evolution related to tool use. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sequential Responding and Planning in Chimpanzees (Pan troglodytes) and Rhesus Macaques (Macaca mulatta).

Chimpanzees (Pan troglodytes) and rhesus macaques (Macaca mulatta) selected either Arabic numerals or colored squares on a computer monitor in a learned sequence. On shift trials, the locations of 2 stimuli were interchanged at some point. More errors were made when this interchange occurred for the next 2 stimuli to be selected than when the interchange was for stimuli later in the sequence. On mask trials, all remaining stimuli were occluded after the 1st selection. Performance exceeded chance levels for only 1 selection after these masks were applied. There was no difference in performance for either stimulus type (numerals or colors). The data indicated that the animals planned only the next selection during these computerized tasks as opposed to planning the entire response sequence. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Comparative assessment of handedness for a coordinated bimanual task in chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus).

Hand preferences for a coordinated bimanual task were assessed in a sample of 31 captive gorillas (Gorilla gorilla) and 19 captive orangutans (Pongo pygmaeus) and were compared with chimpanzee (Pan Troglodytes) hand preferences in subjects that were matched on the basis of age, sex, and rearing history. The task required that the apes remove food from the inside edges of a symmetrical polyvinyl chloride pipe presented to them in their home cages. The results indicate significant species differences with chimpanzees showing population-level right-handedness and orangutans showing population-level left-handedness. The gorillas showed a nonsignificant trend toward right-handedness. The results are discussed in terms of possible ecological or biomechanical factors that may influence hand preferences in different ape species. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Memory of movies by chimpanzees (Pan troglodytes).

How do animals remember what they see in daily life? The processes involved in remembering such visual information may be similar to those used in interpreting moving images on a monitor. In Experiment 1, 4 adult chimpanzees (Pan troglodytes) were required to discriminate between movies using a movie-to-movie matching-to-sample task. All chimpanzees demonstrated the ability to discriminate movies from the very 1st session onward. In Experiment 2, the ability to retain a movie was investigated through a matching-to-sample task using movie stills. To test which characteristics of movies are relevant to memory, the authors compared 2 conditions. In the continuous condition, the scenes comprising the movie progressed gradually, whereas in the discrete condition, the authors introduced a sudden change from one scene to another. Chimpanzees showed a recency effect only in the discrete condition, suggesting that composition and temporal order of scenes were used to remember the movies. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

New evidence on imitation in an enculturated chimpanzee (Pan troglodytes).

Imitation in the great apes continues to be an active field of research and one that is not free of controversy. Several studies suggest that these species do not tend to match the motor movements of the model they observe, but try to achieve the same results using their own methods (emulation of results). In the studies reviewed, gestures have been used very infrequently outside an intraspecific communicative context to evaluate imitation. In fact, the imitation of gestural actions has been tested only in 4 individual great apes. This study assessed a chimpanzee’s (Pan troglodytes) ability to imitate 52 actions in 4 categories. The levels of accuracy attained by the subject in her imitations exceeded those described in previous studies. Moreover, contrary to the idea defended in some articles, the subject seemed to find it easier to imitate gestures than actions with objects. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Referential Understanding of Videos in Chimpanzees (Pan troglodytes), Orangutans (Pongo pygmaeus), and Children (Homo sapiens).

Performance on identical search tasks based on cues directly perceived or indirectly perceived through video were compared among a group of 4 adult chimpanzees (Pan troglodytes), a group of 2 adult orangutans (Pongo pygmaeus), and a group of 36 children (between 2 and 3 years of age). Children comprehended directly perceived cues but had difficulty with video cues. In contrast, chimpanzees and 1 orangutan were successful in using video to guide their search for a hidden object. Two follow-up studies with 3-year-old children demonstrated the importance of more distinct perceptual and verbal cues in aiding children's understanding of video as referring to real-world events. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Attention to emotional scenes including whole-body expressions in chimpanzees (Pan troglodytes).

Real-life situations provide rich sets of cues that viewers evaluate in terms of their emotional significance. In this study, chimpanzees (Pan troglodytes) viewed a movie depicting naturalistic scenes involving the whole-body expressions of conspecifics to examine how nonhuman primates perceived the combination of these cues and how each cue contributed to the overall perception. Viewing time was measured while the chimpanzees watched movie clips without sound. Among scenes depicting neutrality, general excitement, agonism, and playfulness, chimpanzees looked longest at those depicting agonism. This bias toward agonistic scenes may indicate an attentional sensitivity toward threat/fear-related negative situations among chimpanzees. The effect disappeared when the images were scrambled, ruling out the possible effect of pixel-level properties on the results. In addition, the follow-up analyses revealed that the effect was independent of the presentation order and of the number of individuals in each clip. The manipulation of playback speeds had little effect on the looking times. The elimination of facial cues slightly influenced the looking times but did not change the strong bias toward agonistic scenes. This robustness of the main effect against image manipulations may indicate that the chimpanzees attended directly to the contextual information implied by the cues rather than to the cues per se (e.g., facial expressions, speed of movements). (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Color preferences in gorillas (Gorilla gorilla gorilla) and chimpanzees (Pan troglodytes).

Color plays an important biological role in the lives of many animals, with some species exhibiting preferences for certain colors over others. This study explored the color preferences of two species of ape, which, like humans, possess trichromatic color vision. Six western lowland gorillas, and six chimpanzees, housed in Belfast Zoological Gardens, were exposed to three stimuli (cloths, boxes, sheets of acetate) in red, blue, and green. Six stimuli of the same nature, in each of the three colors, were provided to both species for 5 days per stimulus. The amount of interest that the animals showed toward each stimulus of each color was recorded for 1 hr. Results showed that the apes, both when analyzed as two separate groups, and when assessed collectively, showed significant color preferences, paying significantly less attention to the red-, than to the blue- or green-colored stimuli. The animals' interest in the blue- and green-colored stimuli did not differ significantly. Overall, the findings suggest that gorillas and chimpanzees, our closest living relatives, may harbor color preferences comparable to those of humans and other species. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Do Chimpanzees (Pan troglodytes) and 2-Year-Old Children (Homo sapiens) Understand Double Invisible Displacement?

Chimpanzees (Pan troglodytes) and young children (Homo sapiens) have difficulty with double invisible displacements in which an object is hidden in two nonadjacent boxes in a linear array. Experiment 1 eliminated the possibility that chimpanzees' previous poor performance was due to the hiding direction of the displacement device. As in Call (2001), subjects failed double nonadjacent displacements, showing a tendency to select adjacent boxes. In Experiments 2 and 3, chimpanzees and 24-month-old children were tested on a new adaptation of the task in which four hiding boxes were presented in a diamond-shaped array on a vertical plane. Both species performed above chance on double invisible displacements using this format, suggesting that previous poor performance was due to a response bias or inhibition problem rather than a fundamental limitation in representational capacity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sex and handedness effects on corpus callosum morphology in chimpanzees (Pan troglodytes).

Findings suggest that in humans, sex and hand preference may be associated with the size of the corpus callosum (CC). The authors measured CC morphology from MRIs in 67 chimpanzees (Pan troglodytes) to see whether similar effects were present in this species. Hand preference was assessed by performance on 4 tasks, and chimpanzees were classified as left-handed, right-handed, or ambidextrous. In a subsequent analysis, the chimpanzees were reclassified into 2 groups: right-handed and left-handed. The results revealed no sex difference in CC area, but significant effects of hand preference were found for several CC regions (rostrum body, anterior midbody, posterior midbody, isthmus, and splenium) and for overall CC size, with left-handed chimpanzees exhibiting significantly smaller CC measurements than right-handed chimpanzees. The results indicate that lateralized hand use in chimpanzees, as in humans, is associated with variation in CC size. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Group differences in the mutual gaze of chimpanzees (Pan troglodytes).

A comparative developmental framework was used to determine whether mutual gaze is unique to humans and, if not, whether common mechanisms support the development of mutual gaze in chimpanzees and humans. Mother-infant chimpanzees engaged in approximately 17 instances of mutual gaze per hour. Mutual gaze occurred in positive, nonagonistic contexts. Mother-infant chimpanzees at a Japanese center exhibited significantly more mutual gaze than those at a center in the United States. Cradling and motor stimulation varied across groups. Time spent cradling infants was inversely related to mutual gaze. It is suggested that in primates, mutual engagement is supported via an interchangeability of tactile and visual modalities. The importance of mutual gaze is best understood within a perspective that embraces both cross-species and cross-cultural data. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Moving shadows contribute to the corridor illusion in a chimpanzee (Pan troglodytes).

Previous studies have reported that backgrounds depicting linear perspective and texture gradients influence relative size discrimination in nonhuman animals (known as the “corridor illusion”), but research has not yet identified the other kinds of depth cues contributing to the corridor illusion. This study examined the effects of linear perspective and shadows on the responses of a chimpanzee (Pan troglodytes) to the corridor illusion. The performance of the chimpanzee was worse when a smaller object was presented at the farther position on a background reflecting a linear perspective, implying that the corridor illusion was replicated in the chimpanzee (Imura, Tomonaga, & Yagi, 2008). The extent of the illusion changed as a function of the position of the shadows cast by the objects only when the shadows were moving in synchrony with the objects. These findings suggest that moving shadows and linear perspective contributed to the corridor illusion in a chimpanzee. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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 maternal 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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

How cross-fostered chimpanzees ( Pan troglodytes ) initiate and maintain conversations.

This study systematically sampled typical attention-getting sounds and sign language conversations between each of 4 originally cross-fostered chimpanzees (Pan troglodytes), still living freely, but now in a laboratory setting, and a familiar human interlocutor. Videotape records showed that when they encountered a human interlocutor sitting alone at his desk with his back turned to them, the crossfosterlings either left the scene or made attention-getting sounds. The only signs they made to the interlocutor's back were noisy signs. When the human turned and faced them, the chimpanzees promptly signed to him (98% of the time) and rarely made any sounds during the ensuing signed conversations. Under systematic experimental conditions, the signed responses of the chimpanzees were appropriate to the conversational styles of the human interlocutor, confirming daily field observations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

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. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Summation and numerousness judgments of sequentially presented sets of items by chimpanzees (Pan troglodytes).

Summation and numerousness judgments by 2 chimpanzees (Pan troglodytes) were investigated when 2 quantities of M&Ms were presented sequentially, and the quantities were never viewed in their totality. Each M&M was visible only before placement in 1 of 2 cups. In Experiment 1, sets of 1 to 9 M&Ms were presented. In Experiment 2, the quantities in each cup were presented as 2 smaller sets (e.g., ?+?2 vs. 4?+?1). In Experiment 3, the quantities were presented as 3 smaller sets (e.g., 2?+?2?+?3 vs. 3?+?4?+?1). In Experiment 4, an M&M was removed from 1 set before the chimpanzees' selection. In Experiments 1 and 2, the chimpanzees selected the larger quantity on significantly more trials than would be predicted by chance. In Experiments 3 and 4, 1 chimpanzee performed at a level significantly better than chance. Therefore, chimpanzees mentally represent quantity and successfully combine and compare nonvisible, sequentially presented sets of items. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Object permanence in orangutans (Pongo pygmaeus), chimpanzees (Pan troglodytes), and children (Homo sapiens).

Juvenile and adult orangutans (n?=?5; Pongo pygmaeus), chimpanzees (n?=?7; Pan troglodytes), and 19- and 26-month-old children (n?=?24; Homo sapiens) received visible and invisible displacements. Three containers were presented forming a straight line, and a small box was used to displace a reward under them. Subjects received 3 types of displacement: single (the box visited 1 container), double adjacent (the box visited 2 contiguous containers), and double nonadjacent (the box visited 2 noncontiguous containers). All species performed at comparable levels, solving all problems except the invisible nonadjacent displacements. Visible displacements were easier than invisible, and single were easier than double displacements. In a 2nd experiment, subjects saw the baiting of either 2 adjacent or 2 nonadjacent containers with no displacements. All species selected the empty container more often when the baited containers were nonadjacent than when they were adjacent. It is hypothesized that a response bias and inhibition problem were responsible for the poor performance in nonadjacent displacements. (PsycINFO Database Record (c) 2010 APA, all rights reserved)