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Abstract: We examined growth changes in concentrations of plasma insulin-like growth factor-1 (IGF-1) and testosterone, and somatometric parameters in two captive male agile gibbons from birth to about 4 years of age, to examine the evolution of growth patterns in primates. Plasma IGF-1 concentrations in agile gibbons generally increased with age with values ranging from 200 to 1,100 ng/ml. The growth profiles in plasma IGF-1 in the gibbons were similar to those reported for chimpanzees. The highest concentrations of plasma testosterone (230 and 296 ng/dl) were observed within the first 0.3 years from birth, then the concentrations rapidly decreased and fluctuated below 100 ng/dl. Continuously higher IGF-1 concentrations were observed after 2.6 and 3.5 years of age. The profiles of plasma testosterone in these gibbons also resembled those of other primates including humans. However, their plasma testosterone levels in both neonate and adult stages (60 ng/dl) were lower than those reported for macaques and chimpanzees of respective stages. The obtained growth profiles of plasma IGF-1 and testosterone suggest that the adolescent phase starts around 2.6 or 3.5 years of age in male agile gibbons. The growth trend in many morphological parameters including body weight showed a linear increase without a significant growth spurt at approximately the onset of puberty. Head length and first digit length had reached a plateau during the study period. Brachial index, which indicates the relative length of forearm to upper arm, significantly increased gradually through the growth period. This result indicates that forearm becomes relatively longer than the upper arm with growth, which may be an evolutionary adaptation for brachiation.

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.

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.

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.

An adult female chimpanzee with previous training in the use of Arabic numerals 1–9 was introduced to the meaning of "zero" in the context of three different numerical tasks. The first two were cardinal tasks where the subject was required either to select numerals corresponding to the number of items presented on a computer screen (productive use of numerals) or to match sets of the appropriate size to numerals presented as samples (receptive use). The third task addressed the ordinal meaning of the same symbols where the subject was required to respond to numerals sequentially, arranging them into an ascending series. The subject mastered the recognition of the meaning of zero in all three tasks. However, details of her usage of the symbol revealed that transfer of the meaning between different kinds of tasks was incomplete, suggesting that the level of abstraction characteristic of human numerical ability was not attained in the chimpanzee. Over the course of acquisition leading to the high levels of accuracy eventually observed, the newly introduced zero appeared to shift along the length of a continuous numerical scale toward the lower end, while confusions with 1 remained the most frequently encountered mistakes. Such patterns of error thus suggest that Ai’s understanding of the meaning of zero in relation to the rest of the number symbols was not consistent with an "absence of items versus presence of items" scheme.

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.

This study examines the effectiveness of cues of visual depth and distance in the course of development and how this process depends on visuo-motor development. In the visual pitfall situation, i.e. a modification of Gibson's visual cliff, eight Japanese monkeys (macaca fuscata) were observed with respect to their depth avoidance and visuo-motor activity. The tests were run once a week from the first until the sixteenth week after birth. Binocular parallax, motion parallax and texture density rates were manipulated to examine their effectiveness as cues. It was shown that for the first two months depth perception depended exclusively on motion parallax, whereas in the third month cues of motion and texture were added. Binocular cues did not have any effect in this age range. Three items of behaviour, i.e. visual regard of depth, head movement, and body movement, were checked and measured to obtain information which could explain the process of development of the cue function. The three items showed different developmental curves. During the first month, visual regard closely concurred with head and body movements, then visual activity suppressed motor behaviour and, after the end of the second month, the two became almost independent of each other. These analyses demonstrated that at a later stage pictorial cues produced an effect additional to the primary motion cues and that the effective cue function was based on the development of visuo-motor activity.

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Abstract: Two captive, female chimpanzees (Pan troglodytes) were taught to draw straight lines with a method of electronic finger painting. Subjects faced a touch‐sensitive monitor. Because a touch instantaneously generated electronic ink in the form of a graphic symbol at the touched location, movement of the finger over the monitor surface produced a visible trace. Drawing was taught in several steps, beginning with pressing separate small circles one at a time. As the circles were moved closer and overlapped, the subjects began to connect them without lifting the finger. After smooth drawing was established, two small dots, 12 cm apart, appeared on the monitor in different orientations. Each subject drew a line connecting the dots by placing a finger on one dot and moving the finger, without lifting, to the next dot. Thereby the dot orientations guided the drawing. The subjects were not drawing by tracking a moving object but were truly free drawing. The fully automated training and recording methods generated highly accurate drawing behavior that could be measured quantitatively. Our results provide clear evidence that with training the chimpanzee is capable of structured drawing guided by visual commands.

The present study examined the heart-rate changes which occurred in the visual-depth situation of Japanese monkeys (Macaca fuscata) Four animals were tested over the first four weeks of age in the visual pitfall designed as a modification of the visual cliff. The infant monkeys showed heart-rate reduction in the depth condition. This reduction was observed from the first week and became remarkable at the third week. There were no differences in heart-rate change between monocular and binocular vision. These findings suggest that the Japanese monkey can discriminate visual depth shortly after birth and that the heart-rate reduction depends upon the fear of visual depth, not upon the novelty of the situation.

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.

Two female chimpanzees (Pan troglodytes) had been taught previously to draw straight lines on a touch‐sensitive monitor with a method of electronic finger painting. Both subjects could accurately connect two guide dots on the monitor by placing the finger at one dot (startdot) and moving the finger over to the other dot (stopdot), thereby leaving a trace of “electronic ink” on the monitor that followed the orientation of the dots (horizontal, vertical, or diagonal). In Experiment 1, the subjects were connecting the two guide dots while a model (a 12‐cm bar) was placed next to them. Probe trials tested whether this model would guide drawing when one or both of the dots were removed. One subject was able to draw parallel to the model on some of the test trials without the stopdot while the other subject showed no such behavior. In Experiment 2, the latter subject was explicitly trained to draw parallel to the model while the stopdot was gradually faded away. In an attempt to improve accuracy of drawing parallel to the model for both subjects, Experiment 3 presented only three models (vertical and the two diagonals) and one startdot location; in addition, the subjects could make multiple strokes on each trial. Both subjects, especially the chimpanzee given extra training in Experiment 2, were able eventually to draw a trace that was guided by and therefore parallel to the model. The results provide evidence that the chimpanzee can be taught a simple form of structured drawing guided by a model. The fully automated recording and teaching method induced quite accurate elementary copying behavior in chimpanzees without the use of verbal instruction, demonstration, or manual assistance.

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 occidentalisas 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 Thujatool. 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 Thujatool for drinking juice, and then used the Thujatool which had been used and left by another chimpanzee.

In Exp. 1, three young chimpanzees were trained to match red to a cross and green to a circle in an arbitrary matching-to-sample task. After acquisition of this task, they were tested for the emergence of associative symmetry of these conditional relations using the trials on which shapes were presented as samples and colors as comparisons. One of the three chimpanzees showed statistically significant accuracy on these test trials. This successful subject served in Exp. 2, in which an auditory-visual stimulus appeared contingent upon red and a cross while another auditory-visual stimulus was contingent upon green and a circle. This subject showed higher accuracies in symmetry tests than in Exp. 1, which suggested the facilitative effects of these events common to sample and correct comparison on the development of symmetry. In Exp. 3, subsequent tests in which only these stimuli were presented as samples indicated that these stimuli might have become the members of equivalence classes.

A 6.5-year-old female chimpanzee learned to distinguish perfectly every letter of the alphabet in a matching-to-sample task with 26 letters as choice alternatives. Confusion of letters during the initial training was used to scale them in a multidimensional similarity space and to associate them in hierarchical clusters. The results resembled those obtained from similarity judgements by humans. Using letters of various sizes, a visual acuity test revealed that the chimpanzee’s acuity was about 1.5, comparable to that in normal humans. The chimpanzee also readily learned to use letters as names of individual humans and chimpanzees. A species-typical feature was identified in the perceptual processes associated with complex forms, such as those involved in individual recognition.

Japanese monkeys consumed two kinds of food, the novel almonds and the familiar sweet potatoes, simultaneously, and then received a cyclophosphamide injection (20 mg/kg) intravenously. As the food-poison pairing was repeated, they first avoided the novel food completely, and then came to suppress eating the familiar one. During the subsequent extinction tests, the aversion to the familiar food was extinguished rapidly, whereas the aversion to the novel one was retained more than 2 months. In food aversion learning in a mixed situation, monkeys have a strategy to avoid a noxious food on the basis of the novelty of the food.

Abstract: Monkeys and pigeons were trained to discriminate between normally oriented full frontal pictures of humans and upside-down reversals of the same pictures as stimuli. Monkeys displayed a high level of transfer to the new pictures of full frontal and rear views of humans and silhouettes, but failed to transfer to the close-up and far human faces. Pigeons showed poorer transfer to the silhouettes and higher transfer to the far human faces than did monkeys. Further transfer tests were performed with non-human pictures, including monkeys, birds, mammals, and man-made objects. Pigeons failed to transfer to the non-human pictures. This indicates that the pigeons had learned to classify the pictures based on some concrete features specific to the humans and that the transfer to the new versions of human pictures could be explained by simple stimulus generalization based on perceptual similarity. Two out of four monkeys did transfer fairly well to the non-human pictures, except for the man-made objects. High levels of transfer to the non-human natural pictures suggested that the monkeys classified the pictures on the basis of the orientation of objects represented by the pictorial displays.

Abstract: The hand preference of chimpanzees in their natural habitat was studied at Bossou, Republic of Guinea, West Africa. The quantitative difference in left/right hand use was small in food picking and carrying. In contrast, the chimpanzees employed either the right or left hand in nutcracking behavior using a pair of stones. All adults and many adolescents and juveniles utilized one hand exclusively for holding a hammer stone. Left hand preference was more prevalent among adults. However, when adolescents and juveniles were included, there was no significant bias in the ratio of left/right handers. Nut-cracking behavior requires long-term learning of the fine manipulation of stones and nuts by both hands. Each hand has a separate role, and the hands work together in nut cracking. The differential and complementary use of both hands may be a prime factor promoting exclusive hand preference in chimpanzees comparable to that of humans.

Abstract: Changes in auditory event related potentials (ERPs) as a function of frequency of tones ranging from 500 Hz to 8,000 Hz were examined under various hypobaric hypoxia conditions of simulated high altitudes up to 6,000 m. A sequence of P1-N1-P2 components, corresponding to those of the human subject, was observed with the monkey's ERPs. These component amplitudes changed as a function of frequency of tone. The relationship revealed an inverted U curve with a maximum amplitude at a tone of 2,000 Hz. The following results under hypoxia were noted: marked reduction of amplitude in ERPs induced by hypoxia linked only to a tone's particular frequency, especially for 2,000 Hz. The delay of latency in the respective components of the ERPs was dependent on the degree of hypoxia. These findings seem to suggest the anatomical bases of the neural generators contributing to each component of the ERPs.